Human Anatomy for Physician Assistant Students - Unit 1 - Thorax and Back

Unit 1 - Thorax and Back

Karen Krabbenhoft, PhD and Sarah Traynor, PhD

Human Anatomy for Physician Assistant Students - Unit 1 - Thorax and Back Copyright © by Karen Krabbenhoft, PhD and Sarah Traynor, PhD. All Rights Reserved.

This book was produced with Pressbooks.

Contents

- Welcome!

- Dissection 01 - Skinning the Trunk

- Anatomical Terminology (Independent Study)

- Skin and Superficial Fascia (Independent Study)

- Mammary Gland (Independent Study)

- Lymphatic System (Independent Study)

- Dissection 02 - Trunk Musculature, Vertebral Column and Spinal Cord

- Bones of the Trunk

- Ligaments of the Vertebral Column and Intervertebral Discs (Independent Study)

- Muscles of the Trunk

- Collateral Circulation and Blood Supply to the Trunk Muscles

- Overview of the Nervous System

- Spinal Meninges and Cord

- Spinal Reflexes and Tracts

- Dissection 03 - Heart

- Mediastinum (Independent Study)

- Heart

- Dissection 04 - Lungs

- Lungs and Base of Neck

- Dissection 05 - Posterior Mediastinum

- Autonomic Nervous System

- Thoracic Duct

- Venous Drainage of the Thorax

- Branches of the Thoracic Aorta

1

Welcome!

Welcome!

Congratulations on your admission to the University of Wisconsin Physician Assistant Program and welcome to anatomy!  The course is demanding, fast paced and fascinating, and will lay the groundwork for the remainder of your curriculum.  Through learning anatomy you will have an understanding of the body’s structure and relationships, and why clinical tasks are performed in particular ways. We are excited to work and learn together this summer.

Anatomical Terminology (Independent Study)

Key Concept

Anatomists and clinicians use specific terms to describe the spatial relationships between parts of the body. These terms are essential tools for accurately discussing anatomy with colleagues.

Learning Objectives

- Demonstrate the anatomical position.

- Define and cite examples of the fundamental anatomical terms.

- Explain the three anatomical planes and axes, and the associated movements.

I.  The Anatomical Position

All terms describing direction and position of body parts assume that the body is in the anatomical position. In this position, the body is erect, with the feet together and the toes pointing straight ahead. The arms hang at the sides with palms facing forward and the thumbs pointing away from the body. This is not the position where your arms hang naturally–try it. All descriptions of the body during this course will refer to the anatomical position.

Anatomical Position, Image from Open Stax Anatomy and Physiology text.

II.  Directional Terms

As you learn to identify individual structures, you must also be aware of their positions relative to other structures. The following terms are used commonly when describing the relative positions of body structures.

Directional Terms, Image from Open Stax Anatomy and Physiology text.

Term

Definition

Example

medial

closer to the midline of the body

the nose is medial to the ears

lateral

farther from the midline of the body

the ears are lateral to the eyes

anterior/ventral

toward or closer to the front surface of the body

the eyes are ventral (anterior) to the ears

posterior/dorsal

toward or closer to the back of the body

the spine is dorsal (posterior) to the sternum

superior/cranial

toward or closer to the top of the head

the eyes are cranial to the nose

inferior/caudal

toward or closer to the soles of the feet

the abdomen is inferior to the thorax

proximal

toward or closer to the attachment of an extremity to the trunk

the thigh is proximal to the knee

distal

away from or farther from the attachment of an extremity to the trunk

the foot is distal to the knee

internal/deep

away from the surface of the body, toward the inside

The heart is deep to the sternum

external/superficial

close to the surface of the body, toward the outside

the skin is superficial to the muscles

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III. Anatomical Planes and Axes

It is often useful to imagine the body cut in various planes of section. For example, when we describe muscle actions, we define both an axis and plane for each movement. To understand the relationship between planes and axes, envision a bicycle wheel and its axle. The axle is perpendicular to the wheel itself. If you hold onto the ends of the axle and spin the wheel, it will rotate in a plane that is perpendicular to the axis. The axle of the wheel is analogous to the axis of movement. The wheel is analogous to the plane. For example, if we drove an axis through the shoulder joint from anterior to posterior, we would be creating an anteroposterior (AP) axis. Movements around the AP axis through the shoulder joint occur in the frontal plane and are called abduction and adduction.

Anatomical Planes

Planes of Section, Pearson Education, Inc.

Plane

Description

Related Axis

Movements

sagittal

a vertical plane that divides the body into right and left portions

transverse

flexion/extension

transverse/horizontal

a plane that divides the body into superior and inferior portions

vertical

medial/lateral rotation

frontal/coronal

a vertical plane that divides the body into dorsal and ventral portions

anteroposterior

aBduction/aDduction

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Skin and Superficial Fascia (Independent Study)

Key Concept

Skin forms a protective barrier for the body and the superficial fascia bridges the skin to deeper structures.

Learning Objective

- Describe the major layers in the skin and superficial fascia.

- List and explain the functions of these structures within the skin and superficial fascia.

- hair follicles and arrector pili muscles

- sweat and sebaceous glands

- blood vessels

- cutaneous nerves

- lymphatic vessels and lymph nodes (subsequent chapter)

- Explain how cleavage/Langer’s lines inform decision making for surgical incisions.

I. Makeup and Contents of the Skin and Superficial Fascia

Components of the Skin and Superficial Fascia

We will spend substantial time removing the skin and subcutaneous tissue to expose deeper structures.  Although this tissue looks somewhat homogeneous grossly, it is important to appreciate the variety of important structures that the skin and superficial fascia contain.

The skin includes two layers.  The more superficial epidermis, is an epithelium that provides a physical barrier to pathogens and mechanical stress, and the deeper dermis, is a layer of connective tissue that lends additional support and transmits the neurovascular supply to the skin.  The underlying subcutaneous tissue (“deep to the skin”) is called the hypodermis or superficial fascia.  It contains varying amounts of fat (adipose tissue) and collagen fibers.  The fat lies in irregular lobules between fibrous septa that have a honeycomb arrangement.  (The septa do not lengthen appreciably with increased fat deposition within the lobules; this can lead to the lumpy appearance characteristic of cellulite.)  Collagen fibers of the superficial fascia attach to the deep surface of the dermis and blend at a deeper level with the investing fascia, the outermost layer of connective tissue that surrounds the skeletal muscles. In some regions (e.g., palm of the hand), the superficial fascia is tightly bound to the deep fascia; in others (e.g., back of the hand), it is loosely bound.

Skin and Superficial Fascia, Adapted from Pearson Education.

The skin and superficial fascia are not simply layers of epithelium and connective tissues.  They contain hair follicles, glands, blood vessels, lymphatic vessels and cutaneous nerves.   Most of these structures are involved in our ability detect and respond to changes in our external environment.  For example, thermoregulation is accomplished through coordination of the sweat glands and hair follicles derived from the epidermis and the arrector pili muscles (smooth muscle) and blood vessels of the dermis.  In elevated temperatures, dilation of blood vessels and evaporation from sweat glands helps to cool us.  Alternatively, in decreased temperatures, constriction of the cutaneous vessels directs the blood to deeper vascular networks, and contraction of the arrector pili muscles causes the hairs to stand up and trap warm air against the body surface to help preserve body temperature.  The cutaneous nerves that course through the skin transmit the necessary sensory and motor impulses to control these actions.  Innervation will be discussed in great detail during the next dissection.

As illustrated below, these skin specializations are not found uniformly across the entire surface of the body.  They are abundant in so-called thin skin, which is located everywhere except for the palms of the hands and soles of the feet.  In these locations, the thick skin lacks hairs, arrector pili muscles and sebaceous glands because it is specialized to withstand the physical stresses unique to the palms and soles.

Layers of the Skin, Image from Wikipedia.org

Cleavage/Langer’s Lines

When examined histologically, the collagen fibers in the dermis appear to be arranged randomly.  The tissue is commonly called “irregular” connective tissue based on this observation.  However, although the collagen is positioned in multiple directions, there is an overall tendency for the fibers to lie in a particular orientation based on the physical stresses of the region.  This is the basis of cleavage/Langer’s lines.  It is important for providers to be aware of Langer’s lines when planning incisions, as it will influence the healing process and scar formation.  Cutting parallel to the cleavage lines tends to produce minimal scars, while cutting perpendicular to the cleavage lines tends to produce more prominent scars because tension on the tissue forces the collagen fibers to gape apart.

Langer’s/cleavage lines should be considered when making incisions to minimize the scarring.  Image from https://healthjade.com/dermis your textbox content here.

Stretch Marks

Sudden stretching of the skin, due to rapid weight gain during puberty or pregnancy, or due to hypertrophy of underlying muscles,  can tear the collagen fibers of the dermis.   If the damage cannot be repaired by the healing process, there may be permanent scars called stretch marks.

Stretch marks following pregnancy and weight lifting

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Mammary Gland (Independent Study)

Key Concept

The mammary gland is a modified sweat gland that grows during puberty and differentiates during pregnancy to provide nutrition for the infant.

Learning Objective

- Describe the structure and blood supply of the breast.

I. Position and Gross Structure of the Breast

The breast is a fibroglandular organ. Its base extends from the second rib to the sixth rib in the midclavicular line, and from the sternum to the midaxillary line. The base of the breast, for the most part, overlies the pectoralis major muscle. The base is not circular because a lateral extension follows the tendon of the muscle into the axilla. This lateral extension is called the axillary tail. The base of the breast is separated from the pectoralis major muscle by a space that contains loose connective tissue, so the organ is easily separated from the underlying muscle. Breast tissue receives its blood supply from the internal mammary/thoracic artery (branch of the subclavian artery) and branches of the axillary artery.

Gross Features of the Breast

The pigmented areola and nipple are at the apex of the breast. On the surface of the areola, small elevations contain sebaceous glands that secrete sebum to protect the areola and nipple during suckling. The glands can form sebaceous cysts, which can become infected. Internally, the mammary gland is divided into 15-20 lobes (functional units of the gland). Each lobe is connected to the nipple by a lactiferous duct. Surrounding each lobe is abundant adipose tissue. The lobes are separated by dense connective tissue septa called suspensory (or Cooper’s) ligaments. Suspensory ligaments extend between the investing fascia of the pectoralis major muscle and the dermis of the overlying skin.

The diagrams below illustrate lactating breasts with enlarged glands. (In non-lactating breast tissue, the glandular tissue is largely absent; lactiferous ducts are present in all breasts post-puberty.)

Anatomy of the breast

Internal structure of the breast

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Lymphatic System (Independent Study)

Key Concepts

The lymphatic system is a vascular system that is critical for recovery of substances lost from the blood vessels to the interstitial space. In addition to transmitting desirable substances, the lymphatics may conduct metastasizing tumor cells and other antigens that must be destroyed.

Learning Objectives

- State the general function of the lymphatic system.

- Define lymph and explain its source.

- Explain how lymph is recovered and how it is routed to the venous system.

- Iterate the function of lymph nodes.

- Specify the nodes that receive lymph from superficial and deep tissues.

- Trace drainage pathways from the mammary gland.

- Describe consequences of lymphatic system blockage.

I.  Lymphatic System

This is an anatomically discrete system of vessels that recovers fluid from the interstitial space and returns it to the blood circulation.  By definition, lymph is the fluid within lymphatic vessels. It is derived from the interstitial fluid that has leaked out of the blood capillaries. As a filtrate of the blood, the lymph contains varying amounts of all the non-cellular components of the blood, as well as lymphocytes that can move freely into and out of the vessels. Lymph varies in composition depending on its source. For example, it is clear and opalescent in the extremities but is milky in vessels leading from the digestive tract. Lymph from the liver is rich in plasma proteins. Lymph from glandular tissue generally contains secretion products; lymph from the testis is rich in testosterone and lymph from the ovary is rich in estrogen.

II.  Lymphatic Vessels

Lymphatic Capillaries

It is critical to recover excess interstitial fluid and return it to the circulation.  This recovery begins with lymphatic capillaries, blind-ended vessels interposed among the blood capillaries. They anastomose extensively and unite to form increasingly larger ducts, which ultimately deliver their lymph into veins. Lymphatic capillaries are much more permeable than blood capillaries. Gaps between adjacent endothelial cells open when interstitial fluid pressure increases. This allows particles up to 24 mm in diameter to enter the lymphatic capillary.

Relationship between blood and lymphatic systems

Lymphatic Ducts

Lymphatic capillaries converge to form lymphatic ducts.  Because there is no pump to drive the flow of lymph, it is dependent on other factors. Valves, movement of adjacent structures, and negative intra-thoracic pressure are the three most important mechanisms in propelling lymph within its channels.

1. Numerous valves in the lymphatic ducts ensure that all lymph flows in one direction, toward the veins.

2. Contraction of voluntary muscles, pulsations of adjacent arteries, and contraction of smooth muscle in organs of the digestive, respiratory and urinary tracts can all compress lymphatic ducts and “milk” the lymph toward the veins.

3. The negative intra-thoracic pressure created during inspiration also plays a role in lymphatic flow.

Animation of Valves Preventing Backflow

The larger lymphatic ducts are dissectible. The ducts deliver the lymph to one or more lymph nodes before returning it to a vein. The lymph nodes are responsible for removing antigens from the lymph as it flows through, and for producing lymphocytes to aid the immune system. These larger ducts eventually form two major lymphatic ducts, the right lymphatic duct and the thoracic duct. The right lymphatic duct and thoracic duct are the terminal vessels in the lymphatic system. They deliver their lymph to the blood at the junctions between the jugular and subclavian veins at the base of the neck. As the diagram below shows, the regions of the body drained by these terminal vessels are not symmetrical.

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Lymphatic Drainage

III.  Lymph Nodes

As the lymph flows toward the venous system, it passes through at least one lymph node.  The nodes filter the lymph and screen for and attempt to destroy foreign substances/antigens before the lymph is delivered to the blood and potentially distributed systemically.

Superficial Lymph

All lymph from the skin and subcutaneous tissue below the head and neck drains into either axillary or inguinal nodes before entering the deep lymphatic vessels. The region below the head and neck and above the umbilicus drains primarily into the axillary nodes. Below the umbilicus, lymph typically drains to the inguinal nodes. There are connections so it is possible for lymph to follow alternative drainage pathways. For example, lymph from the mammary gland could drain to the inguinal lymph nodes, but this is rare.

Because cancer cells can enter the lymphatic vessels and spread throughout the body, it is clinically important to know which nodes filter the lymph from different parts of the body. As lymph passes through a lymph node, the cancer cells tend to lodge within the node.

Major Lymphatic Pathways

Deep Lymph

Lymph from the abdominal and pelvic organs drains through iliac and intestinal nodes, or through nodes associated with individual organs such as the spleen, liver and pancreas. Eventually, abdominal and pelvic lymph drains to the thoracic duct.

Lymph from the thoracic organs drains superiorly along the trachea, then through either the right lymphatic or thoracic duct. The lymph passes through a final set of lymph nodes, called supraclavicular nodes. This is the last opportunity to remove antigens from the lymph before it enters the blood. These are also called sentinel nodes because their enlargement indicates pathology in the region of the body drained through the nodes. Enlargement of the supraclavicular nodes on the right side may indicate pathology in the thorax. Enlargement of supraclavicular nodes on the left may indicate pathology in the thorax, abdomen, or pelvis.

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IV.  Lymphatic Drainage of the Mammary Gland

As one pertinent example of cutaneous lymphatic drainage, you should know the pattern of lymphatic drainage from the mammary gland. Most of the lymphatic channels pass laterally to a series of axillary lymph nodes (pectoral, subscapular, central, and apical nodes). Secondary routes pass medially to the parasternal nodes, superiorly to the supraclavicular nodes, and inferiorly to the abdominal wall and to connections with the deeper liver lymphatic channels. This secondary drainage pattern explains metastases to the liver or upper abdominal wall. Anastomotic connections across the front of the chest may result in metastases to the opposite breast.

Lymphatic Drainage of the Breast

V. Obstruction of the Lymphatic System

General Obstruction

The primary function of the lymphatic vessels is to recover plasma proteins that have escaped from the blood capillaries into the interstitial space and return them to the blood. This is essential to maintain osmotic equilibrium. If the proteins are not returned to the blood, fluid accumulates in the extracellular space, a condition called edema. Lymphatic capillary networks are very widely distributed, particularly within the loose connective tissues. They are particularly abundant in the dermis (skin) and in the mucosa and submucosa of the digestive, respiratory, and genitourinary tracts.

Edema in Left Foot, Getty Images

Obstruction of Drainage from the Breast

When the breast contains a tumor, or is substantially infected, or inflamed, the skin can take on a distinctive appearance.  This is due to obstruction of its lymphatic drainage, which leads to edema within the gland.  The swelling is localized between the sturdy suspensory (Cooper’s) ligaments, which do not stretch in accordance with the generalized swelling.  The ligaments are anchored firmly to the skin and cause shallow indentations.  The appearance of the skin resembles that of an orange peel, and the condition is called peau d’orange.

Classic peau d’orange appearance with lymphatic congestion in the breast. Image from: http://healthitis.blogspot.com/2014/07/brlu.html

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Bones of the Trunk

Sarah Traynor

Key concepts

Students will learn the bones of the trunk including the vertebral column and the thoracic cage.

Learning Objectives

- Differentiate between the bones of the axial and appendicular skeletons.

- Describe the types of vertebral curvature.

- Describe generic structures across all vertebrae and contrast the unique features of cervical, thoracic, lumbar, and sacral vertebrae that relate to the function.

- Describe the movements of the vertebral column.

- Describe the bones of the thoracic cage and the features of the sternum.

I.  Axial and Appendicular Skeletons

The skeleton consists of 206 bones that are grouped into two major divisions: the axial skeleton and the appendicular skeleton. The axial skeleton includes the bones of the head and trunk – the skull, vertebral column, ribs, and sternum. The appendicular skeleton includes the bones of the limbs – both the limb girdles and the bones within the free limbs.

Axial Skeleton (green) and Appendicular Skeleton (yellow). Image from Marieb, Human Anatomy, 7th ed, 2014.

II. Vertebral Curvatures

The vertebrae surround and protect the spinal cord within the vertebral canal. Typically there are 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused), and 4 coccygeal (more or less fused) vertebrae.

The vertebrae between C2 and S1 are separated by intervertebral discs that contribute to the curvatures at different spinal levels and the mobility of the vertebral column. As illustrated in the figure below, there are four curvatures in adults, two primary curvatures in the thoracic and sacral areas, and two secondary curvatures in the cervical and lumbar regions. The primary curvatures reflect those that developed in the fetus and the secondary curvatures develop after birth to accommodate from changing weight load distribution and centering the line of gravity in an upright, bipedal stance.

Primary and secondary vertebral curvatures

Normal Spine

Rotating Spine – click on image to start rotation

In addition to the typical curvatures, there are atypical curvatures that can occur due to muscle weakness, arthritis, structural abnormality, pregnancy, and genetic defects. An exaggerated primary curvature in the thoracic region is referred to as kyphosis, while an exaggerated secondary curvature in the lumbar region is lordosis. Finally, the atypical lateral curvature of the vertebral column is called scoliosis. Lateral bending of the spine in the lumbar region is typical of acquired or degenerative scoliosis that occurs in older adults. Idiopathic scoliosis occurs more in the thoracic region during childhood development (more common in females) and if left untreated, it can result in adult idiopathic scoliosis.

Spinal Curvatures

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III. Typical Vertebrae

Body – This weight-bearing, anterior portion of the vertebra increases in size from the cervical region to the lumbar region to support the increasing weight that is carried.

Arch – This posterior portion of the vertebra is composed of two pedicles and two laminae. The pedicle has superior and inferior vertebral notches that align with the notches of adjacent vertebrae to form intervertebral foramina where the spinal nerves exit the vertebral column.

Vertebral canal – The arch and body of each vertebra form a vertebral foramen that aligns with those of the other vertebrae to form the vertebral canal, which contains and protects the spinal cord.

Processes:

- Spinous process – This arises where the two laminae meet and project posteriorly. It is an attachment site for ligaments and muscles.

- Transverse processes – These arise at the lamina/pedicle junctions. They are also attachment sites for muscles and ligaments.

- Superior and inferior articular processes(facets) – These form zygapophyseal (facet) joints with adjacent vertebrae.

Features of Typical Vertebrae

Superior and inferior vertebral notches of the pedicles form the intervertebral foramen. Spinal nerves emerge through the intervertebral foramina and then divide into the dorsal and ventral rami and innervate the body.

IV. Distinguishing Vertebrae from Different Levels

Although vertebrae share certain common features, this figure illustrates the distinctive features of the cervical, thoracic, and lumbar vertebrae.

Distinguishing Features of the Vertebrae

Cervical vertebrae are characterized by:

- a small body.

- articular processes in the horizontal plane.

- a foramen in each transverse process (aka transverse foramen)

- bifid spinous processes.

Unique Features of the Atlas and Axis

The first two cervical vertebrae are highly modified. The first cervical vertebra (C1) is the atlas. It looks very different from a typical vertebra because it has neither a spinous process nor a body. The second cervical vertebra (C2), the axis, has a large tooth-like vertical projection called the odontoid process, or dens. The atlas rotates around the odontoid process of the axis.

The 7th cervical vertebra has a large spinous process that may be quite long and almost horizontal; it causes the first visible elevation in the upper back.

Thoracic vertebrae are characterized by:

- spinous processes that point inferiorly.

- articular processes in the coronal plane.

- facets on the transverse processes and bodies for articulating with the ribs.

Lumbar vertebrae are characterized by:

- a large body.

- articular processes in the sagittal plane.

- large squarish spinous processes.

The sacrum is a fusion of five vertebrae that articulates with the os coxa bones of the pelvis. The sacrum has anterior and posterior foramina for the roots of the cauda equina to travel through (rather than intervertebral foramina) to exit the vertebral column.

From Radiopedia

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V. Movements of the Vertebral Column

There are three cardinal axes, the horizontal/transverse, vertical and anteroposterior axes (which correspond to the x, y, and z axes of geometry). Each of these axes is associated with a single plane of movement.  The axes and their associated planes of movement can be superimposed on any joint in the body, including the intervertebral joints of the vertebral column. At every joint, movements occur around an axis and within a plane. A joint can be uniaxial (moves around only one axis), biaxial (moves around two axes) or multiaxial (moves around all three axes).

Anatomical Planes and Their Respective Axes. Adapted from Marieb, Human Anatomy, 7th ed. 2014.

Only two movements can occur around a single axis. The movements of the trunk around each of the three axes are as follows:

- Flexion and extension occur within the sagittal plane around the transverse axis.

- ABduction and aDduction occur within the frontal/coronal plane around the anteroposterior (AP) axis.  Abduction refers to the movement away from the midline (in anatomical position, the midline is the vertebral column of the trunk), whereas adduction refers to movement towards the midline. We use these terms when thinking about the movement of the limbs away from or towards the midline of the trunk.

- Movement of the vertebral column is different than the limbs because the vertebral column of the trunk is the midline of the body in anatomical position. So when discussing the movement of the vertebral column around the AP axis we refer to it as lateral bending because the head/trunk is bending laterally and away from its anatomical position of midline.

- Medial and lateral rotation occur within the transverse/horizontal plane around the vertical axis. Medial (Internal) rotation  is the rotation of a limb segment so that the anterior aspect of the joint is facing the midline. Lateral (External) rotation is the rotation of a limb segment so that the anterior aspect of the joint is facing away from midline.

- Movement of the vertebral column is different than the limbs because the vertebral column of the trunk is the midline of the body in anatomical position.  When referring to the head/trunk, we classify movement around the vertical axis as rotation to same side or opposite side with ‘side’ referring to the side (left or right) of the muscle that is moving the head/trunk. For example, if the right splenius capitus muscle rotates the head to the right, we would say that movement is ‘rotation of head to same side’.

Movement at each intervertebral joint is minimal, but when combined, the range of movement of the entire vertebral column is quite extensive. Mobility varies at different levels depending on a variety of features including the thickness of the intervertebral discs, the position and orientation of the intervertebral joints, and the looseness of the joint capsule.

Movements of the Vertebral Column

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A. Cervical Joints

The atlantooccipitaljoint is specialized for flexion/extension and is often called the “yes” joint because it allows you to nod your head ‘yes’. The atlantoaxialjoint is specialized for rotation around the odontoid process and is often call the “no” joint because it allows you to shake your head ‘no’. Among the remaining cervical vertebrae, all three types of movement are permitted because of the relatively thick intervertebral discs and somewhat horizontal orientation of the articular processes.

B. Thoracic Joints

In the thoracic region, movement is very restricted because of the thin discs, the frontal direction of the zygapophyseal joint spaces, the articulation of the ribs to the vertebral column and sternum, and the overlapping spinous processes. As a result, flexion and extension are restricted, but some rotation and lateral bending are possible.

C. Lumbar Joints

In the lumbar region, the discs are thick and the articular processes forming the zygapophyseal joints are in the sagittal plane, so flexion and extension are extensive. Laxity of the articular facets also permits a small amount of lateral flexion. Rotation is severely limited.

In summary, free movements of the vertebral column become more limited from cervical to thoracic to lumbar. The number of movements permitted in each region from cervical to lumbar, are summarized as 3-2-1. Cervical movements occur in all three planes; thoracic movements occur in two planes (lateral bending and rotation); lumbar movements occur in one plane (flexion/extension).

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VI. The thoracic cage

The thoracic cage is composed of the thoracic vertebrae, the ribs, costal cartilage and the sternum. The true and false ribs attach to the sternum via the costal cartilage. The floating ribs do not attach to the sternum. The cage protects the thoracic organs and structures. Anteriorly, the sternum is composed of the manubrium, body and xiphoid process. The cranial notch of the manubrium is the jugular notch. The fusion between the manubrium and the body is the manubriosternal joint, or the sternal angle. This sternal angle is a critical surface landmark for the thorax. It is located at the level of the second costal cartilage. Clinically, if you palpate the sternal angle, you are at the level of the second costal cartilage and you can find the second intercostal space and the location for auscultating the pulmonic and aortic valves of the heart. From there, if you count down to the fifth intercostal space and auscultate the mitral and tricuspid valves. We will discuss this further when we discuss the heart.

The sternal angle also provides orientation to the internal organs of the thorax. A horizontal plane through the sternal angle intersects with the:

- origin and termination of the aortic arch

- intervertebral disc between the fourth and fifth thoracic vertebrae

- boundary between the superior and inferior mediastinum

- and the bifurcation of the trachea.

Thoracic cage

Ligaments of the Vertebral Column and Intervertebral Discs (Independent Study)

Key concept

The ligaments of the vertebral column support the joints of the vertebral column and limit movement of the vertebral column.  The intervertebral discs are composed of the anulus fibrosus and the nucleus pulposus.

Learning objectives

- Map the location of the ligaments of the vertebral column and their function.

- Describe the composition of the intervertebral discs and their function.

Ligaments of the Vertebral Column

Ligaments work when they become taut. When a ligament is stretched, it is pulled tight and limits whatever movement is pulling it tighter. When thinking about which movement a ligament limits, think about which movement (in this case, flexion or extension) is going to make it taut.

Several ligaments limit the amount of movement at the intervertebral joints. The first two longitudinal ligaments join all of the vertebral bodies.

1. Anterior longitudinal ligament: This strong, broad ligament extends along the entire length of the vertebral column covering the anterior 1/3 of each body. It limits extension (prevents hyperextension) and reinforces the anterior portion of the intervertebral disc.

2. Posterior longitudinal ligament: It is narrower and weaker than the anterior longitudinal ligament. It lies on the anterior wall of the vertebral canal (posterior surface of the vertebral body). It is narrow as it crosses the posterior surfaces of the vertebral bodies but flares out to blend with the anulus fibrosus of the intervertebral discs. However, it incompletely reinforces the anulus fibrosus posteriorly. Consequently, intervertebral disc herniation is usually posterolateral, where the anulus is not reinforced.  This ligament limits flexion.

3. Ligamentum flavum: This ligament links the laminae of adjacent vertebrae along the posterior wall of the vertebral canal. It is unique because of the high content of yellow elastic tissue. The ligamentum flavum is stretched, similar to a rubber band, during flexion of the vertebral column and its elastic recoil assists in extension of the column.

4. Supra- and interspinous ligaments: These attach spinous processes of adjacent vertebrae and prevent hyperflexion. They are usually very weak in humans. During a midline approach for a spinal tap, the needle traverses these ligaments.

Major Vertebral Ligaments

Ligaments of vertebral column. Notice the width of the posterior longitudinal ligament versus the anterior longitudinal ligament.

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VI. Intervertebral Disc Structure

Intervertebral discs separate the bodies of all adjacent vertebrae with one exception. There is not an intervertebral disc between the atlas and axis. The discs are composed of a peripheral area of fibrocartilage called the anulus fibrosus, and a core of gelatinous tissue called the nucleus pulposus. Both change shape when compressed; the nucleus pulposus serves as a gel-like structure that absorbs forces impacting the vertebrae. The thickness of the disc affects the amount of movement that is possible between vertebrae. The thicker the disc, the greater the movement. Discs are thickest in the cervical and lumbar regions and contribute to the greater degree of movement at these levels.

Disc compression under weight

Intervertebral Discs

Muscles of the Trunk

Sarah Traynor

Key Concepts

Students will understand the musculoskeletal support and movement of the trunk.

Learning Objectives

- Describe the three types of intercostal muscles and the function of this muscle group.

- Explain the difference between extrinsic and intrinsic back muscles.

- Distinguish the three groups of intrinsic back muscles and give example(s) of each:

- Spinotransversae group

- Splenius capitis

- Erector spinae group

- Iliocostalis, longissimus, spinalis

- Transversospinae group

- Semispinalis, multifidus, rotatores

- Demonstrate the actions of intrinsic back muscles.

- Describe the four abdominal wall muscles and their contributions to the rectus sheath:

- External abdominal oblique

- Internal abdominal oblique

- Transversus abdominis

- Rectus abdominis

- Demonstrate the actions of abdominal wall muscles.

- Describe a surgical approach to the abdomen

- Describe the nerves that innervate the trunk musculature.

I. Trunk Muscle Introduction

The musculature in different regions of the trunk follows a repeating pattern. There are typically three layers of muscle, with the fibers of each layer oriented in a unique direction. This is true for the intercostal, intrinsic back and abdominal wall muscles.

II. Intercostal Muscles

The three muscle layers of the thoracic wall are the external, internal, and innermost intercostal muscles. As their names suggest, these muscles are located between the ribs.  The muscle fibers of the external and internal intercostal muscles are perpendicular to each other, while the innermost intercostal fibers are more irregularly arranged.  None of the layers fills the entire intercostal space from anterior to posterior, so it is possible to see one, two or all three layers at any given point along the intercostal space.

Rather than playing an active role in expansion and contraction of the thoracic wall during respiration, the intercostal muscles appear to maintain the spacing between the ribs.  They are considered to be accessory respiratory muscles, while the thoracoabdominal diaphragm is the essential muscle of inspiration.

Intercostal Muscles

III. Back Muscles

The most superficial muscles of the back (trapezius, rhomboids, latissimus dorsi, etc.) are extrinsic back muscles and they work to move the shoulder and arm. We will reflect these muscles in this dissection and study them carefully in Unit 4.  Deep to these muscles are the true back muscles or intrinsic back muscles that move the trunk, neck and head and are crucial for our bipedal, erect posture. Each of the three intrinsic layers may or may not run the entire length of the axial skeleton.

Extrinsic Back Muscles

Extrinsic Back Muscles and Thoracolumbar Aponeurosis (Fascia)

Intrinsic Back Muscles

The most superficial layer of the intrinsic back muscles is the spinotransverse group.  Muscle fibers from this group originate on the midline on the spinous processes of the vertebrae, then pass obliquely in a superolateral orientation before inserting on either transverse processes of cervical vertebrae in the case of splenius cervicis or the base of the skull in the case of splenius capitis. When looking at the right and left splenius muscles, their fibers are in a “V” formation.  Spinotransverse muscles are located only in the upper thoracic and cervical regions.

The intermediate layer, the erector spinae group, includes muscles whose fibers pass vertically between its attachment points. This group is located along the entire length of the vertebral column and includes three muscles that can be differentiated based on their bony attachments and positions relative to the midline.  The most lateral muscle is the iliocostalis, which originates from the iliac crest and passes superiorly to insert on the ribs.  The longissimus is medial to the iliocostalis and originates from the sacrum and thoracolumbar fascia and inserts on the transverse processes of thoracic and cervical vertebrae. The most medial muscle is called spinalis.  Spinalis fibers span between spinous processes of the vertebrae.

Spinotransverse and Erector Spinae Muscles; Adapted from Gilroy, Atlas of Anatomy, 2nd ed, 2012.

The deepest layer, the transversospinae group, includes muscles whose fibers travel from their origin on the transverse processes (laterally) to their insertion on spinous processes (medially). This group can also be further subdivided based on the number of intervertebral joints that are crossed between the origin and insertion.  The semispinalis muscle fibers cross 4-6 joints, the multifidus muscles cross 2-4 joints and the rotatores cross 1-2 joints.  The transversospinae muscles are found along the entire length of the spinal column.

Transversospinae Muscles; Adapted from Gilroy, Atlas of Anatomy, 2nd ed, 2012.

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IV. muscle analysis and Actions of the Intrinsic Back Muscles

The actions of the intrinsic back muscles are not listed here because we will work through these during our muscle analysis activity. Muscle analysis is a way to think about the action of a muscle at specific joint around the transverse, anteroposterior (AP), and vertical axes. If a muscle crosses a joint, it moves that joint. Where that muscle is located relative to an axis will determine how it moves around that axis.

For example, all of the intrinsic back muscles cross posterior to the transverse axis of the intervertebral joints so they extend the vertebral column. These muscles allow the body to maintain an upright posture. These muscles (particularly erector spinae) are considered ‘postural’ muscles because they are mainly composed of slow-twitch muscle fibers that require minimal ATP (energy) to function over a long period of time. It is up to you and your group members during the muscle activity to determine the additional movements of the intrinsic back muscles (and abdominal wall muscles) around the AP and vertical axes based on their attachments and fiber orientation.  Also consider if the action is unilateral (only one side is contracting) or bilateral (both sides are contracting) as well.

V. Abdominal Wall Muscles

The four abdominal wall muscles are important for support and compression of the abdominal contents, and movement of the intervertebral joints/trunk.  This group includes three layers of muscles laterally and one muscle anteriorly. The external abdominal oblique muscle is the most superficial of the lateral wall muscles. Its fibers run inferomedially, like the orientation of your fingers when you place your hands in your front pockets. The fleshy portion of the muscle is attached to the lower 7 ribs and to the iliac crest. The anterior half of the external abdominal oblique is an aponeurosis that travels to the midline (linea alba) and attaches at the anterior midline to the aponeurosis from the opposite side. The inferior edge of the aponeurosis runs between the anterior superior iliac spine (ASIS) and the pubic tubercle to form the inguinal ligament. The fibers of the internal abdominal oblique muscle run at a right angle to those of the external abdominal oblique, particularly in its upper 2/3. They originate from the inguinal ligament and iliac crest, and pass superomedially to insert into the lower ribs and the anterior midline. The fibers of the transversus abdominis are horizontal. They originate from several ribs, the thoracolumbar aponeurosis, and iliac crest. Like the oblique muscles, the transversus abdominis aponeurosis contributes to the rectus sheath on its approach to the anterior midline. Anteriorly, the rectus abdominis extends from the pubic symphysis to the ribs. It is subdivided into four “heads” by tendinous bands (tendinous inscriptions) along its length. As a result, when the muscle hypertrophies with exercise, it produces the six pack appearance. This is the only muscle layer of the anterior abdomen.

Abdominal Wall Muscles

Surrounding the rectus abdominis anteriorly and posteriorly is the rectus sheath. It is composed of the aponeuroses (flat tendons) of the lateral abdominal muscles as they travel from a lateral position and fuse at the linea alba, an area of dense connective tissue at the midline. The linea alba is a common entry site for abdominal surgery. The area holds sutures very well and is relatively nonvascular. For most of the rectus sheath, the aponeuroses form both anterior and posterior layers (Top of Figure below). The anterior rectus sheath is composed of the aponeuroses of external and internal oblique; the posterior rectus sheath is composed of aponeuroses of internal oblique and transversus abdominis (yes, internal oblique contributes to both the anterior and posterior rectus sheaths). About midway between the umbilicus and the pubic symphysis, the posterior rectus sheath ends to allow the inferior epigastric arteries to access the posterior surface of rectus abdominis. The inferior edge of the posterior layer is called the arcuate line. Below the level of the arcuate line, only the anterior rectus sheath persists (Bottom of Figure below), and it is now composed of the aponeuroses of all three lateral abdominal wall muscles.

Structure of the Rectus Sheath

This is a link to a video that describes the rectus sheath and arcuate line in more detail.

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VI. Actions of abdominal wall muscles

Working as a group the rectus abdominis, external oblique and internal oblique flex the trunk because they cross anterior to the transverse axis of the intervertebral joints. Rotational movements vary among the abdominal wall muscles. We will determine the additional movements external and internal oblique during the muscle analysis activity. All of the abdominal wall muscles can compress the abdominal cavity (particularly transversus abdominis). Compression of the abdominal cavity is important in activities such as urination, defecation and childbirth.

Abdominal Wall Muscle Actions

The abdominal wall is the main protection of many underlying organs. During a physical exam, the reflexes of the abdominal wall muscles can help determine if there is abdominal disease. If a physician palpates the abdomen while there is infection in the abdomen, the abdominal musculature will reflexively spasm. This is called the guarding response. Abdominal organs are sensitive to pressure, which will be exceedingly painful when an infection is present.

VII. Surgical Approach to the Abdomen

It is important to consider the anatomy of the abdominal wall muscles when planning abdominal surgery. In the midline approach, the linea alba, the site where the aponeuroses of the abdominal muscles converge, is incised. The thick connective tissue in this area holds sutures well. In the paramedian approach, the anterior rectus sheath is opened, the rectus abdominis muscle is displaced laterally, and the posterior sheath and peritoneum are incised. Finally, in the gridiron approach, the directions of the muscles are taken into consideration. For example, to approach the appendix after a skin incision has been made, the fibers of the external abdominal oblique layer are spread parallel to the fiber direction (so few fibers are actually transected) and the muscle layer is spread open. The internal abdominal oblique muscle (which runs 90 degrees to the external oblique) is now spread parallel to its fiber direction. After spreading the internal oblique fibers, the transversus abdominis is spread parallel to the direction of its fibers. The advantage is that since the fibers (and the incisions) run orthogonally, the abdominal muscles “snap together” and the wall remains quite strong after the operation with a minimum of suturing.

VIII. Nerves to trunk muscles

The muscles of the trunk are innervated by ventral and dorsal rami depending on their location. The intrinsic back muscles are innervated by dorsal rami. Ventral rami innervate all other trunk muscles (and the limbs as well). The ventral rami travel with an artery and a vein as a neurovascular bundle between the second the third layers of the trunk muscles (in intercostal spaces between the inner and innermost layers; in the lateral abdominal wall between the internal abdominal oblique and the transversus abdominis).

The ventral and dorsal rami are branches of a spinal nerve and so carry different types of neurons. ALL ventral and dorsal rami carry somatomotor, general sensory and postganglionic sympathetic fibers. Somatomotor fibers go to the skeletal muscle of the trunk; general sensory and postganglionic sympathetic fibers go to the skin (as the cutaneous nerves distributing to a dermatome).

Nerves of the trunk

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The intercostal neurovascular bundle is located in a slight depression on the underside of the upper rib. The vein is closest to the rib, followed by the artery and nerve (use the mnemonic VAN). In addition, there are usually smaller collateral vessels and nerves on the upper side of the lower rib. Knowing the location of the main neurovascular bundle is important clinically. In a procedure called thoracocentesis, where a needle is inserted through an intercostal space to obtain a sample of pleural fluid, or to remove blood, the needle is usually introduced just above the rib to avoid injuring the main neurovascular bundle  that  is  located  along  the  inferior  aspect  of  each  rib within  the  ‘costal  groove’.

Collateral Circulation and Blood Supply to the Trunk Muscles

Sarah Traynor

Key Concept

Many body parts receive dual blood supplies. This is a valuable back up system so that if one of the arteries is obstructed, the tissue will more likely receive adequate blood supply from the other vessel.

Learning Objectives

- Define the terms anastomosis and collateral circulation.

- Describe the blood supply and collateral flow to the thoracic body wall.

- internal thoracic/mammary

- anterior and posterior intercostal arteries

- Describe the blood supply and collateral flow to the abdominal body wall.

- superior and inferior epigastric arteries

I.  Collateral Circulation

The circulatory system is commonly represented as a series of arteries that carry blood away from the heart toward capillary beds, where exchange of gasses, nutrients and waste products occurs.  Blood then drains through a series of veins, which return blood to the heart.  This pattern is accurate, but it is also common that blood vessels anastomose, or connect, with other vessels of the same dimension. These connections may occur between arteries, within capillary beds or between veins.  Anastomoses are the anatomical basis for collateral circulation, so that there is more than one route for delivering blood to some part of the body.

II. Blood supply to the thoracic wall

The arteries in the intercostal space are the anterior and posterior intercostal arteries, branches of the internal thoracic and thoracic aorta, respectively. Each has a corresponding vein, and is accompanied by the ventral ramus of a thoracic spinal nerve. The intercostal veins drain into the azygos system of veins.  The intercostal vein, artery and nerve form the neurovascular bundle that travels along the costal groove at the inferior aspect of each rib.

Intercostal Neurovascular Bundle

The anterior and posterior intercostal vessels anastomose within the intercostal space. The posterior intercostal arteries are segmental branches from the thoracic aorta. The anterior intercostal arteries are segmental branches from the internal thoracic arteries. The anastomosis of similarly sized vessels (with no intervening capillary bed) creates collateral circulation of blood, and this ability to detour blood around a blockage is extremely important.

In instances where there is a slow occlusion of the thoracic aorta (as in the case of a tumor that presses on the aorta), these vessels provide alternative routes for the blood to bypass the occlusion. For example, if there were an occlusion in the upper part of the thoracic aorta, blood could flow through the aorta to the site of the blockage, continue through a posterior intercostal artery, then through an anterior intercostal artery to reach the internal thoracic artery. The blood could descend in the internal thoracic artery to a level below the blockage, and then go back to the aorta using the reverse intercostal route. In such cases, the intercostal arteries can enlarge to the extent that the costal grooves erode and become very deep and prominent. The condition is easily recognized by differences in the pulses in the upper and lower extremities, and can be confirmed by chest x-ray.

III. Blood Supply to the Abdominal Wall

The lateral wall muscles (external and internal oblique and transversus abdominis) are supplied by the continued segmental branches from the descending aorta in the lower thoracic and lumbar regions that are equivalent to the intercostal arteries that supplied the intercostal muscles. These ‘intercostal’ and lumbar arteries (and veins) run with the ventral rami within the neurovascular plane between the internal oblique and transversus abdominis muscles.

The next example of collateral circulation is with the arteries that supply the fourth abdominal wall muscle, the rectus abdominis. The internal thoracic artery and vein lie directly deep to the ribs along the lateral edge of the sternum. After the internal thoracic artery supplies the intercostal muscles via the anterior intercostal arteries, it continues inferiorly towards the abdomen. As the internal thoracic vessels cross the costal margin, their names change and they become the superior epigastric vessels, which anastomose with the inferior epigastric vessels on the deep surface of the rectus abdominis muscle between the umbilicus and xiphoid process. The internal thoracic artery arises from the subclavian artery in the base of the neck and the inferior epigastric artery arises from the external iliac artery in the groin. Because of the superior and inferior epigastric connections, blood can be shunted around a blockage of the aorta. Such collateral circulation can also be valuable during surgery, allowing some vessels to be clamped without causing a problem, because blood can still reach the ligated ends from both directions.

Superior and inferior epigastric Vessels anastomosing within or deep to rectus abdominis.

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Overview of the Nervous System

Karen Krabbenhoft

Key Concept

As complicated as the nervous system is, it is also highly organized and predictable. Certain cell types and information pathways are always found in the same locations.

Learning Objective

- Outline the general organization of the nervous system.

- Describe the components of the CNS and PNS.

- Differentiate the general functions of neurons and glial cells.

- Elucidate the functions of sensory, motor and interneurons.

- List the major components of the PNS.

I.  Basics of the Nervous System

Anatomical Divisions

The nervous system is subdivided anatomically into the Central Nervous System (CNS — the brain and spinal cord), and the Peripheral Nervous System (PNS — the nerves and ganglia). The first components of the nervous system that you identified in the cadaver were the cutaneous nerves to the skin of the trunk.  These are branches of spinal nerves, which are part of the PNS.  In this dissection you will see the continuity between the CNS and PNS by examining the spinal cord, the origin of the spinal nerves.

Overview of the Nervous System

The central and peripheral nervous systems.

Like much of the rest of the human body, the CNS and PNS are bilaterally symmetric; there are right and left versions of most structures.  In addition, the nervous system is arranged segmentally. A series of structures repeats down the length of the body in a regular pattern.  (Think of an earthworm. Humans are organized like that, but with limbs stuck on the sides.) There is a pair of nerves associated with each segment of the body. You can’t see the segments of the human body from the outside, but the segmental relationship between one set of nerves and one set of structures exists in humans as much as it does in earthworms.

This segmentation is particularly evident when you look at the spinal cord and its associated nerves (the spinal nerves). Nerves arise from the spinal cord at regular intervals along the entire length of the cord.  These spinal nerves supply all of the muscles and skin associated with a particular body segment. (Each segment of the body is innervated by a right and a left nerve.)

The segmental arrangement of the human body is evident in the organization of the spinal nerves of the peripheral nervous system.

Cells of the Nervous System

Despite the organizational complexity of the nervous system, it consists of only two principal kinds of cells – neurons and supporting or glial cells. Neurons have electrophysiological properties, very long cellular processes, and are specialized for transmitting impulses. The glial cells have short processes, and support the neurons in a variety of ways, including maintenance of the chemical environment around neurons and production of insulating material called myelin that surrounds the neuronal processes.  Supporting cells do not transmit impulses.  This chapter will describe neurons in greater detail but will not discuss glial cells.

Neurons:

Typical neurons have three major parts, a cell body, dendrites and a single axon.  The  cell body contains the nucleus and most organelles of the cell.  Of the processes that extend from the cell body, dendrites vary highly in number but always receive input and transmit that information to the cell body.  By contrast there is a single axon that transmits an electrical signal away from the cell body and to a target.  Axons of multiple neurons are bundled together to form the nerves that you will see throughout the body in gross anatomy dissections. When we talk about axons that are within nerves, we usually call them ‘fibers.’

Generic Neuron.  Dendrites transmit information to the soma and the axon transmits information away from the soma.

Functional types of neurons:

There are three fundamental types of neurons: motor, sensory and interneurons.  Depending on the type of muscle innervated, or the type of sensory information conveyed, we an describe more specific types of neurons.

- Motor neurons send signals away from the CNS to cause contraction of muscles and secretion by glands.

- Somatomotor neurons carry information from the CNS to skeletal muscles of the trunk and limbs.

- Sympathetic neurons carry information from the CNS to smooth muscle, cardiac muscle, and glands.

- In a later dissection, you will learn that sympathetic neurons area type of visceromotorneuron.(Viscero– means ‘relating to organs (viscera)’.)  All visceromotorneurons send motor signals to organs and glands. There are two types of visceromotorneurons: sympathetic and parasympathetic. For now, you just need to know that spinal nerves contain sympathetic fibers that innervate smooth muscle and glands.

- Sensory neurons take information from sensory receptors in the body’s tissues toward the CNS.

- Somatosensory neurons carry information from sensory receptors in the skin to the CNS (pain, touch, and temperature sensations).

- Viscerosensory neurons carry information from sensory receptors in the organs to the CNS.

- Interneurons are located entirely within the CNS and mediate communication among other neurons.

Overview of the Nervous System

Morphological types of neurons:

Based on their shapes/morphology there are three general types of neurons, as shown here:

Morphological types of neurons.

- Multipolar neurons have multiple dendrites leading to the cell body and a single axon carrying impulses away from the cell body.  Most motor neurons are multipolar.

- Bipolar neurons have a single dendrite leading into the cell body and a single axon leading away from the cell body.  These neurons are exclusively found in special sensory systems (olfaction, vision).

- Pseudounipolar neurons have a single process emerging from their cell bodies.  That single process is short and divides into two axons: one axon brings information into the cell body and one axon takes information away from the cell body.   Sensory neurons of the PNS are pseudounipolar. Both of their axons are considered sensory.

- The axons of pseudounipolar, sensory neurons of the PNS are usually bundled with motor axons from other neurons to form nerves throughout the body.  These are the nerves that you we see in gross anatomy dissections.

Components of the PNS

In gross anatomy we study the axons in detail because they are bundled to form nerves of the PNS. The PNS includes:

1. spinal nerves that innervate the body wall of the trunk and the limbs.

2. cranial nerves that innervate the head in the adult and the structures that developed in the embryonic head and then migrated into the neck, shoulder, thorax, or abdomen. These nerves will be covered mainly in the unit on the head and neck.

3. splanchnic nerves that innervate smooth muscle, cardiac muscle, and internal glands.

4. ganglia that are collections of sensory or motor nerve cell bodies.

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Spinal Meninges and Cord

Karen Krabbenhoft

Key Concepts

Understand the protective tissues around and segmental nature of the spinal cord.

Learning Objectives

Spinal Meninges

- Describe the spinal dura mater, arachnoid and pia mater.

- State the positions and contents of the epidural, subdural, and subarachnoid spaces.

Spinal Cord Gross Anatomy

- Explain the disparity between the lengths of the spinal cord and the vertebral column and how spinal nerves are numbered relative to the vertebrae.

- State the locations of these structures:

- conus medullaris

- cauda equina

Spinal Cord Cellular Organization

- Diagram and label a typical thoracic spinal cord segment including the:

- dorsal, lateral and ventral horns of the gray matter

- dorsal, lateral and ventral columns/funiculi of the white matter

Spinal Nerves

- Diagram and label a typical thoracic spinal nerve including the:

- dorsal and ventral roots

- dorsal root ganglion

- dorsal and ventral rami

- posterior, lateral and anterior cutaneous branches

- Define the components found in spinal nerves and identify the locations of their respective nerve cell bodies.

- somatomotor

- postsynaptic sympathetic

- general sensory

- Define a dermatome. Locate the landmark dermatomes T4, T10, and T12 on the trunk.

- Describe the anatomy relevant to disc herniation and the most common type of herniation.

- Describe the anatomy relevant to lumbar puncture and sacral block.

I. Spinal Meninges

The meninges are a series of protective wrappings around the CNS. They include the dura mater, arachnoid, and pia mater. The meninges help protect the CNS by surrounding it with hydraulic cushions filled with CSF and/or blood.

Spinal Meninges and Associated Spaces

Dura mater:

The outermost layer of the meninges, the dura mater (L. tough mother), is a tough covering composed of a dense felt work of connective tissue. The dura mater surrounding the spinal cord is separated from the inner surface of the vertebral canal by the epidural space. It contains a plexus of thin-walled veins, loose connective tissue, and fat. This space is essential to allow movement between the spinal dura and the vertebral column. Between the dura mater and the arachnoid is the subdural space. This is a potential space that contains a thin film of serous fluid; it is present only in pathological conditions.

Arachnoid:

The middle layer of the meninges, the arachnoid (Gr. arachnoeides, cobweb like), is a delicate, continuous membrane that has two components: a layer in contact with the dura mater, and a system of trabeculae connecting it with the pia mater. The portion of the arachnoid in contact with the dura is composed of layers of flattened cells bound together by extensive tight junctions. This portion of the arachnoid forms a selective barrier called the arachnoid barrier that prevents harmful substances from migrating from the dura or extraspinal capillaries and entering the space deep to the arachnoid called the subarachnoid space. The subarachnoid space is filled with cerebrospinal fluid (CSF) and communicates with the ventricles of the brain. There are no attachments between the dura mater and the arachnoid, so only pressure of the CSF keeps the arachnoid against the dura. There are approximately 150-200 cc of CSF in the subarachnoid space. It is produced within the ventricles of the brain (lumen of the neural tube) from the interstitial fluid of the brain and by secretions of the choroid plexuses.

The second component of the arachnoid is formed by the delicate trabeculae that interconnect and pass through the subarachnoid space to attach to the pia mater. The hundreds of delicate trabeculae give the arachnoid its web-like appearance that accounts for its name (spider). Blood vessels pass through the subarachnoid space and are supported by the arachnoid trabeculae.

Because the contour of the central nervous system is irregular, the subarachnoid space varies widely in depth. The largest subarachnoid space in the vertebral column is found below vertebral level L2, and contains the cauda equina. The clinical procedure of lumbar puncture makes use of this enlarged subarachnoid space in the caudal vertebral levels.

Pia Mater:

The innermost layer of the meninges, the pia mater (L. pia, tender; mater, mother), covers the CNS like a skin. In contrast to the dura mater and arachnoid, which ignore the contours of the CNS, the pia mater is closely adherent to every bump and groove. Unlike the arachnoid, there is no barrier function associated with the pia. Substances in the cerebrospinal fluid of the subarachnoid space can move freely through the pia into the CNS and vice versa. The pia mater is a vascular, loose connective tissue that differs in thickness in the spinal and cranial regions. Over the brain it is a delicate layer, but over the spinal cord it is substantially thicker and forms about 21 pairs of denticulate ligaments, scalloped tooth-like structures that anchor the spinal cord laterally to the dura mater. This lateral “shelf” of pia mater lies between the dorsal and ventral nerve rootlets. The filum terminale, a continuation of the pia mater, extends from the end of the spinal cord at the level of the second lumbar vertebra to the coccyx.

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Dissection of the Spinal Meninges

Drag the slider to review the layers of back muscles and meninges.

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II. Length of the Spinal Cord and Spinal Nerve Numbering

After removing the vertebral laminae and meninges you can examine the spinal cord and its nerves. One feature that should be obvious is the disparity in length between the spinal cord and vertebral column. The spinal cord ends at vertebral level L2. Despite this difference in length, the spinal cord has approximately the same number of segments, as the vertebral column. There are 8 cervical, 12 thoracic, 5 lumbar and 5 sacral spinal cord segments.  These functional segments are evident due to the spinal nerve rootlets attached to the spinal cord. The spinal rootlets combine to form spinal nerves that are numbered in relation to the vertebrae that border the intervertebral foramen through which they pass. The cervical nerves have the same number as the vertebra below the intervertebral foramen. All othernerves have the same number as the vertebra above the intervertebral foramen. (Note that the difference exists because there are seven cervical vertebrae, but eight cervical nerves)

Spinal Cord Segments

Because the spinal cord is shorter than the vertebral column (inferior tip of the spinal cord is at vertebral level L2), the spinal cord segments do not correspond to the adjacent vertebral body. As shown in the figure, as you progress to more caudal levels, the spinal cord and vertebral segments become more and more out of register. Thus, the spinal nerve rootlets must descend within the column to reach the appropriate vertebral level before exiting the intervertebral foramen. Below the L2 vertebral level, the collection of these descending rootlets forms the cauda equina.

Do not learn the exact relations of spinal cord segments to vertebral levels, but appreciate the fact that this disparity has important clinical significance during injury to the vertebral column. Consider the spinal cord level that could be damaged following injury to various vertebral column levels. If the cervical vertebrae are damaged, it is possible to compress the cord and to disrupt neural signals from all lower spinal cord levels. On the other hand, if the sacral vertebrae are damaged, the damage is restricted to the exiting nerve roots.

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Animation by Karen Krabbenhoft

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II. Cellular Organization of the Spinal Cord

Gray and White Matter

The nervous system is highly organized in many ways. Grossly it is possible to see a color difference within the nervous tissue. The major distinction between gray and white matter is the fact that gray matter contains nerve cell bodies, but white matter does not. The gray matter contains not only cell bodies, but many other elements as well. Among the cell bodies, the gray matter contains axons, dendrites, axon terminals, glial cells (astrocytes, oligodendrocytes, microglia) and capillaries. The white matter is composed of axons, glial cells, and capillaries. The large number of myelinated axons gives the white matter its distinctive color.

Different regions within the gray and white matter are named according to their anatomical positions. The gray matter includes the dorsal horn, lateral horn and ventral horn.  Similarly, the white matter includes dorsal, lateral and ventral columns/funiculi.  Each region contains specific types of cell bodies or axons as detailed below.

Organization of Gray and White Matter

White matter and gray matter are organized in different arrangements throughout the CNS. In the spinal cord, we have seen that the white matter is superficial and the gray matter is deep and has a distinct “butterfly” configuration. In the cerebrum, there are deep clusters of cell bodies called the basal nuclei/ganglia. They are surrounded by a thick layer of white matter and another layer of gray matter at the brain’s surface. Many different configurations of gray and white matter exist in other parts of the brain.

IV. Spinal Nerves

Each spinal cord segment gives rise to a pair of spinal nerves (right and left). All spinal nerves are alike in their basic construction and may carry any combination of five components. Cell bodies for each component are always located in certain sites.

Component                                        Cell Body Location

1. general sensory                                 dorsal root ganglia

2. somatomotor                                     ventral horn of the spinal cord gray matter

3. preganglionic sympathetic                lateral horn of the spinal cord gray matter (T1-L2)

4. postganglionic sympathetic              sympathetic ganglion in the PNS

5. preganglionic parasympathetic         lateral horn of the spinal cord gray matter (S2-S4)

In this diagram of a thoracic spinal nerve, note the components and the sites of their cell bodies.

Spinal Nerve Schematic

Spinal Nerve in situ

Whenever you dissect a nerve in the cadaver, you should get in the habit of determining which of these five components it contains. For example, the cutaneous nerves that you saw on the first dissection are terminal branches of spinal nerves, and contain components needed to innervate the skin:

– general sensory and

– postsynaptic sympathetic fibers (to sweat glands and smooth muscle).

At about the level of the intervertebral foramen, the spinal nerves divide into dorsal and ventralrami. The smaller dorsal rami innervate the intrinsic muscles and skin of the back. The larger ventral rami innervate the skeletal muscles of the thoracic and abdominal walls and the skin of the lateral and anterior body wall. Because the dorsal and ventral rami innervate muscles as well as the skin, they contain somatomotor axons as well as the general sensory and sympathetic axons, which mainly supply the skin.

This figure illustrates the branches of a typical spinal nerve in the thoracic region.

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Thoracic Spinal Nerves

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V. Dermatomes

Cutaneous branches of each spinal nerve innervate a dermatome, a strip of skin extending from the dorsal midline to the ventral midline. Rather than learning all the dermatomes, you should know the locations of landmark dermatomes that will help in overall orientation. The T4 dermatome is located at the level of the nipple, the T10 dermatome at the umbilicus, and the T12 dermatome at the suprapubic region. Using the diagrams below, pick out the landmark dermatomes, and note the general location of the dermatomes between these levels.

Although a dermatomal map gives the impression that there are very distinct boundaries between the areas of skin innervated by sequential spinal nerves, in reality there is overlap. So the T3, T4, and T5 spinal nerves innervate the T4 dermatome. Thus, destruction of the T4 spinal nerve would simply reduce the sensation from its dermatome. On the other hand, destruction of the T3, T4, and T5 spinal nerves would result in complete loss of sensation from the area of overlap among these nerves.

Clinically, it is useful to know where particular dermatomes are when trying to localize the level of a herniated disc. When a disc herniates and compresses a spinal nerve, the pain tends to follow a dermatomal distribution. Shingles, a herpes zoster infection of a dorsal root ganglion, causes formation of painful vesicles in the dermatome innervated by the infected nerve.

Dermatome Map from Marieb, Human Anatomy, 7th ed., 2014.

Dermatomes

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VI. Disc herniations

The combination of the high water content of the nucleus pulposus, and the stresses on the disc due to poor posture and aging, account for frequent herniations of the central nucleus pulposus through the anulus fibrosus. Although discs can herniate in any direction, they usually herniate posterolaterally because the anulus fibrosus is thinner posteriorly and because the posterior longitudinal ligament incompletely reinforces the anulus fibrosus posteriorly.

Rupture of the Anulus Fibrosis

The most common sites for herniations are the L4-5 disc or the L5-S1 disc, because those lumbar discs bear more weight. When a lumbar disc herniates, it usually impinges upon the nerve that is exiting at the next level down. The drawing illustrates herniation of the L4-5 disc. Note how the L4 roots escape above the herniation, but the L5 roots are compressed as they descend to their intervertebral foramen. Thus, the rule is that when there is a herniation of a disc, the nerve roots involved have the same number as the vertebra below the ruptured disc. What symptoms would you expect following this disc herniation? Would there be any sensory losses? Would there be any motor problems?

Herniated Lumbar Disc

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VII. Lumbar Puncture

CSF is removed frequently for diagnostic tests by doing a lumbar puncture or spinal tap. To obtain cerebrospinal fluid, the patient is asked to grab his/her knees to maximize the distance between the spinous processes. The puncture is usually made in the midline between L3/4 or L4/5. The L4/L5 interspace is at the level of the iliac crests. To get CSF from the subarachnoid space, the needle will penetrate the supraspinous ligament, interspinous ligament, ligamentum flavum, epidural space, dura mater, subdural space, arachnoid, and finally the subarachnoid space. This is a relatively safe place to puncture in adults because you are entering the subarachnoid space where the cauda equina is floating. Why is this procedure contraindicated in very young children?

Spinal punctures allow for CSF pressure and composition measurements and injection of anesthetics that will bathe the spinal cord and its roots, producing spinal anesthesia.

To produce anesthesia of only the lower nerve roots, the anesthetic may be introduced into the epidural space where it can diffuse through the connective tissue of the space. This is called a “sacral or caudal block.” The needle is introduced through the sacral hiatus into the epidural space near the ending of the dural sac. This procedure may be used to access CSF in very young children.

Lumbar Puncture Procedure

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Spinal Reflexes and Tracts

Karen Krabbenhoft

Key Concepts

Understand the neuronal networks that form at local and distant levels of the central nervous system.

Learning Objectives

- Explain the major functions of the spinal cord.

- Diagram the type of circuit that occurs at a single spinal cord segment.

- Describe the components and advantages of withdrawal and stretch reflexes.

Spinal Tracts

- Define the term tract and differentiate the general functions of ascending and descending tracts.

- Label the major tracts in a spinal cord section.

- Define the terms ipsilateral and contralateral.

- Describe the kind of information that travels in each of these tracts:

- dorsal columns

- spinothalamic

- dorsal spinocerebellar

- lateral corticospinal

- Draw the entire pathway and list the locations of all cell bodies and synapses for each of these tracts.

- Describe the deficits/symptoms following injury to the tracts.

I. Spinal Cord Functions

As a major component of the central nervous system, and due to its anatomical position between with the peripheral nervous system and brain, the spinal cord serves many functions such as:

1. providing sensory and motor innervation of the trunk and limbs via spinal nerves.

2. mediating rapid responses to stimuli via simple reflex circuits.

3. conducting information to and from the brain for processing via tracts.

Spinal Cord Functions

II. Spinal Nerves

Each spinal cord segment gives rise to a pair of spinal nerves (right and left) that innervate both skeletal muscles and a dermatome. Because they all innervate comparable targets, spinal nerves all carry the same combination of components. This diagram of a thoracic spinal nerve illustrates how the dorsal and ventral roots converge to form the spinal nerve, which quickly branches to dorsal and ventral rami.  Note the components in each of these structures, and the particular sites of their cell bodies.

Schematic Spinal Nerve

The next diagram places the schematic spinal nerve in the context of the body.  As it traverses the intervertebral foramen, the spinal nerve divides into dorsal and ventralrami. The smaller dorsal ramus innervates the intrinsic muscles and skin of the back, while the larger ventral ramus innervates the skeletal muscles of the thoracic and abdominal walls and the skin of the lateral and anterior body wall.  These peripheral branches of spinal nerves contain the 3 S’s – general somatosensory, somatomotor and sympathetic fibers.

Spinal Nerve in situ

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III.  Spinal Reflexes

We have discussed the structure of a typical spinal nerve to understand how each spinal cord level is related to its segment of the body. We can use these neurons to form the simplest possible circuit in the form of a reflex. In this situation, incoming sensory information causes a rapid motor response. There are many types of reflexes. For example, in the withdrawal reflex we withdraw our body from a painful stimulus. Imagine a scenario of touching an iron or a hot stove, or stepping on a sharp object. This reflex is advantageous because the rapid response helps minimize injury. If either the sensory or motor component of the reflex were damaged, this could lead to substantial injury.

Withdrawal Reflex

The myotactic/stretch reflex is commonly used in clinical practice to test for the integrity of particular nerves. When the provider taps a tendon with a reflex hammer there is a small stretch in the muscle. The sensory axon enters the spinal cord, synapses on a motor neuron that stimulates contraction to return the muscle to its resting length. The patellar reflex is probably the most familiar, but many reflex arcs can be tested in a similar way.

Myotactic Reflex

IV. Spinal Cord Tracts

While reflexes are important for quick responses to stimuli, it is also necessary to send signals from the spinal cord up to the brain for analysis, and to send signals from the brain down to the spinal cord to generate the response to that stimulus. This vertical communication depends on massive bundles of axons that ascend or descend within the white matter.  These pathways that carry information to and from the brain are called tracts, and each one is located in a particular region of the white matter.  Similar to how we describe the gray matter as including dorsal, lateral and ventral horns, the white matter includes the dorsal, lateral and ventral funiculi (rope).

Subdivisions of the Gray and White Matter

Some of the tracts ascend, carrying sensory information to the brain. Other tracts descend carrying motor information from the brain to the spinal cord. When learning the different tracts, notice that their names commonly indicate the position and/or connections of the axons they contain.  We will study the subset of the tracts that is labeled on the diagram below.

Major Spinal Cord Tracts

**Note that the following descriptions portray the major features of each tract.  In depth analysis has revealed exceptions and variations on these descriptions.  For the purposes of this course’s learning objectives and assessments, use the information below**

Sensory/Ascending Tracts

All sensory axons enter the spinal cord through the dorsal root and their cell bodies are in the dorsal root ganglia. After entering the CNS, some sensory axons participate in reflexes, but they also follow specific ascending pathways depending upon the particular kind of sensory information that they transmit. We will discuss three major sensory pathways: the dorsal columns, the spinothalamic tracts and the dorsal spinocerebellar tract. As the information ascends in sensory pathways, multiple neurons and synapses are involved. The pathways terminate in different regions of the brain, and may travel entirely on the ipsilateral (same) side of the CNS or may cross to the contralateral (opposite) side of the nervous system somewhere along the way. As we will see it is very common that sensory information from one side of the body terminates in the contralateral side of the brain, and that motor information from one side of the brain causes contraction of muscles on the contralateral side of the body.  As one might imagine, there are exceptions to this rule.

Overview of the Major Sensory Tracts

1. Dorsal columns

These tracts are located in the dorsal funiculus of the white matter and carry sensory information concerning fine touch, vibration and conscious proprioception. This information originates in highly specialized receptors in the skin or in joints. The fine touch sensation allows you to discriminate the texture that you are feeling. Is it glass, sandpaper, or fleece? The vibration information helps you sense a train rumbling by on nearby tracks. The conscious proprioception provides awareness of your body position in space without seeing it. For example, you can close your eyes and successfully touch your nose with your finger.

The dorsal columns contain two parallel tracts called fasciculus cuneatus and fasciculus gracilis. They both carry the same kinds of sensory information, just from different levels of the body. Specifically, the fasciculus cuneatus transits information from dermatomes C2-T6, while the fasciculus gracilis transmits information from dermatomes T7-S5.  Refer to the dermatome map if you need a refresher.  This is an example of somatotopy, the extraordinary organization of the nervous system that organizes information related to different body parts in specific locations.  All spinal cord tracts exhibit somatotopy in the arrangement of their fibers.

Dermatome Map. Image from Fotosearch.com

Somatotopy of the Spinal Cord Tracts. Image form Waxman, Clinical Neuroanatomy on www.accessmedicine.com

To be conscious of these sensations, the pathway must ultimately ascend to the cerebral area of the brain. Stimulation of a receptor in the periphery initiates an action potential in a first order neuron. The information streams centrally, passes through the dorsal root ganglion and enters the spinal cord. The axon enters the dorsal columns and immediately ascends toward higher levels of the CNS. When the axon reaches the medulla of the brainstem, it synapses on the second order neuron within the nucleus gracilis/cuneatus. The axon from this neuron decussates (crosses) to the contralateral side of the brainstem and ascends within a tract called the medial lemniscus to the thalamus, where it synapses on the third order neuron. The axon from the thalamic neuron ascends even further to synapse on a neuron in the postcentral gyrus of the parietal lobe. The fine touch, vibration or conscious proprioception information has now reached consciousness and can be processed.

By integrating input regarding fine touch, we can reach into a bag and selectively remove a pencil, a sandwich or set of keys, an ability called stereognosis.  Integration of fine touch input is also used for an ability called graphesthesia, the ability to interpret a shape that is being drawn on the skin.

Details of the Dorsal Columns System.  Fasciculus gracilis shown in red, and fasciculus cuneatus shown in blue. Image from http://what-when-how.com

2.  Spinothalamic Tracts

This system carries sensory information concerning pain, temperature and crude touch that originates in naked nerve endings in the periphery. It is involved in withdrawal reflexes and ascends to consciousness.

How does this information reach consciousness? Pain, temperature or touch stimulation initiates an action potential in a first order neuron. The information streams centrally, passes through the dorsal root ganglion and enters the spinal cord. The axon enters the dorsal horn of the gray matter and synapses on the second order neuron located in one of several nuclei of the dorsal horn, most commonly in the substantia gelatinosa or nucleus proprius. The axon from this neuron decussates (crosses) to the contralateral side of the spinal cord and ascends in one of two tracts.  Pain and temperature information specifically ascends in the lateral funiculus of the white matter, while crude touch information specifically ascends in the anterior funiculus of the white matter.  Due to their locations and connections, the tracts are called the lateral and anteriorspinothalamic tracts.  Axons in both of these tracts reach the thalamus then synapse on the third order neurons. The axon from this neuron ascends and synapses on a neuron in the cerebral cortex. The pain, temperature and crude touch information has now reached consciousness.

Spinothalamic Tract Details from http://what-when-how.com/wp-content/uploads/2012/04/tmp14102.jpg

Somatotopy

If we mapped the projections of the dorsal columns and lateral spinothalamic tract to the cerebral cortex we would see somatotopy.  Sensory information from particular regions of the body terminate in predictable regions of the cortex.  Interestingly, the projections are not proportional as one might expect.  Instead, the amount of cortex dedicated to a particular body part is proportional to the sensitivity of that body part.  If the body were constructed accordingly, the face, lips, hands, and genitals would be much larger than they are in reality.  The distorted represented is called a homunculus.

Sensory Homunculus from http://www.scratchtape.it/illustration.html

Motor and Sensory Homunculus

3. Dorsal Spinocerebellar Tract (DSCT)

This sensory pathway is unique in several ways.  It involves only two neurons, is entirely ipsilateral, and it terminates in the cerebellum. The DSCT carries information from sensory receptors within skeletal muscles that measure the state and rate of contraction, so called unconscious proprioception. This is the sensory input for myotactic stretch reflexes that also helps coordinate motor activity due to its projection to the brain.

When a muscle changes in length, receptors in muscle spindles or the tendon are stimulated. This information enters the spinal cord via the first order neuron, whose cell body is in the dorsal root ganglion.  The location of the second order neuron varies depending on the entry point of the first order neuron.  We will simplify the details and focus on the major pathway for the purposes of this course! The first order neurons enter the dorsal horn of the gray matter and synapse on second order neurons in a region called Clarke’s column.  The second order axons pass into the lateral funiculus of the white matter and ascend to the medulla in the ipsilateral dorsal spinocerebellar tract.  These axons enter the cerebellum through the inferior cerebellar peduncle where they synapse on neurons that monitor and modulate muscular contraction.

Simplified pathway of the DSCT from what-when-how.com

Motor/Descending Tract

Somatomotor neurons of the ventral horn exit the spinal cord and follow nerves to their target skeletal muscles. These neurons may be stimulated either via reflexes or because we consciously want to execute a movement. That voluntary contraction begins with a neuron in the brain that travels in the major descending spinal cord tract.

Lateral Corticospinal Tract (LCST)

This descending tract allows us to voluntarily initiate contraction. The signal begins in an upper motor neuron that is located in the precentral gyrus of the frontal lobe of the cerebral cortex. Specifically, the signal initiates from a pyramidal neuron, which is diagnostic of the cerebrum.  Its axon descends through the brain and brainstem before it enters the upper spinal cord. Collectively, the corticospinal fibers form an enormous bundle of axons that can easily be traced through all levels of the CNS.  The fibers descend from the cortex within the internal capsule and enter the midbrain through the cerebral peduncle.  The fibers intermingle somewhat with nuclei of the pons then coalesce in the medulla as the pyramids.  This name relates to the origin of the axons. Axons of the pyramidal neurons form the pyramidal tracts/pyramids.  The fibers cross to the contralateral side in the decussation of the pyramids as they transition from the medulla to the spinal cord, then continue to descend in the lateral funiculus of the white matter within the lateral corticospinal tract. After reaching the appropriate spinal cord segment the axon turns medially into the ventral horn and synapses on the lower motor neuron. The axon of the lower motor neuron exits via the ventral root then follows a spinal nerve branch to reach its target. This is the same somatomotor neuron that we discussed with our typical spinal nerve.

This motor pathway reiterates a theme that we have seen with two of the three ascending tracts. That is, the brain commonly relates to the contralateral side of the body. Practice drawing the four motor and sensory pathways, including the positions of all axons, cell bodies, tracts, synapses, etc.

Overview of the Lateral Corticospinal Tract; Image modified from Pearson Education

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Challenge yourself to predict the deficits that will result from injuries in these pathways. These are some questions to consider:

Following damage to the ascending tracts, what are the deficits and the laterality of the deficits (is the deficit ipsi or contra to the injury)?

Following damage to the major descending tract what are the deficits? Differentiate upper and lower motor neuron syndromes. What are the signs? Is the deficit ipsi or contra to the injury?

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Mediastinum (Independent Study)

Karen Krabbenhoft

Key Concept

The mediastinum is the region in the thorax between the two pleural sacs. It is subdivided into different regions relative to the pericardial sac.

Learning Objectives

- Describe the boundaries of the mediastinum and its subdivisions.

- List the contents of the superior mediastinum.

- List the contents of the anterior, middle and posterior regions of the inferior mediastinum.

I. The Mediastinum

The mediastinum is the region in the thorax between the two pleural sacs.

Boundaries:

Superior –        Thoracic inlet (T2)

Anterior –        Sternum

Inferior –          Diaphragm (T9)

Posterior –       Vertebral column

Lateral –           Mediastinal pleura

The mediastinum is divided into the superior mediastinum and inferior mediastinum by a horizontal plane that intersects the T4-5 vertebra posteriorly and the sternal angle anteriorly. This plane is at the level of the carina (tracheal bifurcation). The inferior mediastinum is divided into three regions:

- The middle mediastinum includes the pericardium and all its contents.

- The anterior mediastinum is between the pericardium and the sternum.

- The posteriormediastinum is between the pericardium and the vertebral column.

Subdivisions of the mediastinum

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II. Contents of the Superior Mediastinum

The structures in the superior mediastinum may be considered in layers. As illustrated in the images below, from anterior to posterior the layers are:

1. Glandular layer – the thymus.

2. Venous layer- the superior vena cava and its tributaries: right and left brachiocephalic veins (5). The union of the internal jugular and subclavian veins forms the brachiocephalic veins. Note that the right brachiocephalic vein has a vertical course in line with the superior vena cava. The left brachiocephalic vein has an oblique course downward and to the right across the midline.

3. Arterial layer – the aortic arch and its branches:

- brachiocephalic artery (4)

- left common carotid artery (10)

- left subclavian artery (12)

4. Respiratory layer – the trachea (20). Note that the trachea bifurcates at about T4-5 and therefore does not descend below the superior mediastinum.

5. Digestive layer– the esophagus (7).

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Inferior View of the Superior Mediastinum from Gray’s Anatomy for Students

CT scan through the Superior Mediastinum

Line drawing key of CT scan

Note:  The leftvagus nerve enters the superior mediastinum, passes anterior to the aortic arch and gives off the leftrecurrent laryngeal nerve, which passes under the aortic arch inferior to the ligamentum arteriosum. The left vagus nerve continues inferiorly to reach the anterior surface of the esophagus. The right vagus nerve gives off the right recurrent laryngeal nerve in the neck before it enters the superior mediastinum. Therefore, although the left recurrent laryngeal nerve is within the mediastinum, the right recurrent laryngeal nerve is never in the mediastinum. The right vagus nerve continues downward to reach the posterior surface of the esophagus. The recurrent nerves supply the muscles of the larynx. It is possible to have a lesion (tumor, aortic aneurysm) in the thorax that manifests itself through difficulty in vocalization, but only on the left (because the right recurrent branch is never in the mediastinum).

III. Contents of the Inferior Mediastinum

The CT scan and labeled line drawing below illustrate many of the major structures in the inferior mediastinum.

1. Anterior Mediastinum

The anterior mediastinum is a small area between the body of the sternum and the pericardium. It contains mostly fat, lymph nodes, and branches of the internal thoracic artery. The most clinically important of these structures are the retrosternal lymph nodes that are situated along the internal thoracic vessels. In the female, these nodes receive lymphatic drainage from the medial quadrant of the breast and, as a consequence, may be sites of metastases.

2. Middle Mediastinum

Contains the heart within the pericardial sac (8, 13, 18).

3. Posterior Mediastinum

Contains the:

- Descending aorta (6)

- Thoracic duct

- Azygos vein (3)

- Hemiazygos vein

- Esophagus (7)

- Vagus nerves

- Sympathetic trunks

CT scan through the Inferior Mediastinum

Line drawing of the CT scan through the inferior mediastinum

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Heart

Karen Krabbenhoft

Key Concepts

This chapter will address relationships, anatomy and major disease of the adult heart.

Learning Objectives

- State the basic function of the heart and specify the chamber involved in the two major circuits.

- Describe the surface projections of the heart.

- Explain the relationship of the pericardial sac to the heart and to cardiac tamponade.

- Describe the purpose and technique for performing pericardiocentesis.

- Describe the functions of the phrenic nerves and their course through the mediastinum.

- Locate the distinguishing features of each chamber and state the function/significance of each :

- superior and inferior venae cavae

- fossa ovalis

- tricuspid valve

- chordae tendineae

- papillary muscles

- pulmonic valve

- pulmonary trunk and arteries

- pulmonary veins

- bicuspid/mitral valve

- aortic valve

- Describe the locations and functions of the semilunar and atrioventricular valves.

- Practice surface anatomy of the thoracic wall and listening to heart sounds.

- Describe the anatomy and location of the conducting system components (the parts cannot be observed grossly):

- S‑A node

- A‑V node

- atrioventricular bundle

- right and left crura

- Purkinje fibers

- Discuss vagal (parasympathetic) and sympathetic innervation of the heart (cardiac plexus).

- Describe the positions and distributions of the major coronary arteries and cardiac veins on the heart.

- right coronary artery and its right marginal and posterior interventricular branches

- left coronary artery and its anterior interventricular/left anterior descending and circumflex branches

- great, middle, small cardiac veins

- coronary sinus

- Discuss implications of coronary collateral circulation and the consequences if collateral flow cannot compensate for obstruction of a coronary artery.

- Describe the common treatments for atherosclerosis of coronary arteries.

- angioplasty

- coronary artery bypass grafting

I. Basic Function and Anatomy of the Heart

The heart is responsible to pump blood throughout the body from embryonic stages until death.  It is functionally divided into four chambers that receive blood from and deliver blood to two major circulations, the pulmonary and systemic circuits.  The pulmonary circulation carries blood to and from the lungs and the systemic circulation carries blood to and form the rest of the body, from head to toes.

Pulmonary and Systemic Circuits

The heart contains four chambers.  The right and left atria are the chambers that receive blood, and the right and left ventricles are the chambers that eject blood into the arteries of the circulatory system. Chambers on the right side of the heart contain deoxygenated blood that must be delivered to the lung through the pulmonary circulation, while chambers on the left side of the heart contain oxygen-rich blood that will be delivered to the body through the systemic circulation.

Chambers and Valves of the Heart

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II. Surface Projections of the Heart

It is important to know the surface projections of the heart to prepare for listening to heart sounds and assessing possible injury to the heart after penetrating wounds of the chest, etc. You will see in the lab that hearts come in a variety of sizes so the following description is an estimate for the normal, average-sized male or female. The superior margin of the heart can be visualized by drawing a line from the left second intercostal space to the right third costal cartilage.

A line between the right third and sixth costal cartilages represents the right border of the heart. This line extends about ½ inch to the right of the sternum.

The inferior border of the heart is represented by a line between the right sixth costal cartilage and the fifth intercostal space about 3½ inches to the left of the sternum.

A line connecting the second and fifth intercostal spaces represents the left border of the heart. That line is 2-3 inches from the left border of the sternum.

Heart Surface Anatomy

Being able to imagine the outline of the heart as a surface projection is important in instances where you have to assess whether a penetrating wound of the chest might involve the heart. For example, would you be concerned that a penetrating wound of the chest located two inches lateral to the left border of the sternum at the fifth intercostal space might involve the heart? The answer is yes! You wound be less concerned with a wound the same distance from the right side of the edge of the sternum.

Not only is it clinically important to be able to predict if the heart would be involved in a particular penetrating wound, but it is equally important to know which chamber would most likely be involved. To visualize this, look at the figures below and note which chambers of the heart form the right, left and inferior borders.

Surface Anatomy of the Heart

For the illustrations below, imagine that the chest has been cut in an axial section and you are looking up from below, the same orientation as a chest CT.  Right and left are reversed such that structures on the left sides of the images are from the right side of the body, and vice versa.  The right border of the heart is formed entirely by the right atrium. Likewise, the left ventricle forms the left border of the heart, with a small contribution from the left atrium. The right ventricle forms the surface in contact with the sternum, the anterior surface. The left atrium forms the posterior surface. Because the apex of the heart points somewhat anteriorly, the left ventricle forms the diaphragmatic surface of the heart.

Inferior View of an Axial Section through the Thorax

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III. Pericardial Sac and Pericardiocentesis

The heart is in constant motion and must be protected from friction as it moves against adjacent structures.  This is accomplished by the pericardial sac that separates and protects the heart. To visualize the relationship between the pericardium and the heart, imagine a balloon shrink wrapped around the organ.  The collapsed sac has two layers, the inner, visceral pericardium that is fused to the surface of the heart, and the outer parietal pericardium.  The visceral and parietal layers of the sac are normally separated by a thin film of fluid that lubricates the heart as it moves.

Relationship Between the Heart and Pericardial Sac

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While the visceral pericardium is merely a layer of simple squamous epithelium (mesothelium) fused to the surface of the heart, the parietal pericardium includes both a mesothelium and a substantial layer of dense connective tissue called the fibrous pericardium. Because the fibrous pericardium is so dense and does not stretch, if a significant amount of fluid accumulates within the pericardial sac the heart cannot expand adequately to fill with blood. Delivery of blood can be significantly compromised. This condition is called cardiac tamponade.

Cardiac Tamponade Restricts Expansion of the Heart

Pericardiocentesis is performed to remove excess fluid from the pericardial sac to prevent impairment of cardiac function. The fluid can accumulate due to inflammation of the pericardial sac (pericarditis) or due to bleeding from rupture of the heart wall or a coronary vessel. The accumulation of any fluid in the sac has the potential to strangulate the heart due to the indistensible nature of the fibrous pericardium.

If time permits, local anesthetic is used. The physician palpates the xiphoid process and the left costal margin, then inserts the needle between the xiphoid process and left costal margin in a superior, posterior direction, angled slightly towards the left shoulder. Next s/he pushes the needle through the chest wall and penetrates the central tendon of the diaphragm entering the pericardial sac. The diaphragmatic surface of the pericardium is bound to the diaphragm. Now it is possible to remove air or fluid from the pericardial cavity.

There is a danger of entering the myocardium if the needle is too deep, but in a living patient, ECG leads are attached to the needle to verify position and it is possible to feel the contraction of the ventricle through the needle.

Common sites for pericardiocentesis

Common sites for pericardiocentesis

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IV. The Phrenic Nerve

The phrenic nerves arise from cervical spinal cord levels C3-5 and enter the superior mediastinum between the subclavian artery and the forming brachiocephalic veins. They descend between the pericardial and pleural sacs on their course to the diaphragm. Care must be taken during dissection, and particularly during surgery, to preserve the nerves. The right phrenic nerve is found on the right side of the right brachiocephalic vein, the superior vena cava, and the right atrium. At the level of the right atrium, it lies between pleura and pericardial sacs. The left phrenic nerve crosses the arch of the aorta, then travels inferiorly along the pericardium over the surface of the left atrium and left ventricle. It is also located between the mediastinal pleura and the fibrous pericardium. Both phrenic nerves pass anterior to the root of their respective lung.

Course of the Phrenic Nerves

In addition to somatomotor fibers to the diaphragm, the phrenic nerve has sympathetic and many sensory fibers that transmit information from the mediastinal pleura, pericardium and diaphragm. Sensory information from these areas is conveyed to the same region of the spinal cord that receives sensory input from the base of the neck and the upper limbs. As a result, irritation of the diaphragm may be perceived as pain in the base of the neck and the shoulder region. This phenomenon is called referred pain (also see Topic VIII for another example of referred pain).

Referred Pain from the Diaphragm

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V. Internal Anatomy of the Heart

After removing and sectioning the heart, distinguishing features of each chamber are visible.  Each structure is present because of its function during fetal circulation, adult circulation, or both.

Right Atrium

This chamber receives deoxygenated blood from the systemic circulation through the superior and inferior vena cavae.  There is a depression called the fossa ovalis in its medial wall, the interatrial septum.  This landmark was patent during fetal circulation and allowed blood to bypass the pulmonary circulation by flowing directly from the right atrium into the left atrium.  It normally seals shut at birth, but may remain patent in some individuals.

Right Ventricle

This chamber receives blood from the right atrium.  Its three most prominent features are the tricuspid valve, with attached chordae tendineae and papillary muscles.  Contraction of the right ventricle pushes blood into the pulmonary trunk.

Left Atrium

This chamber receives oxygenated blood from the pulmonary circulation through the four pulmonary veins.

Left Ventricle

This chamber receives blood from the left atrium.  Its three most prominent features are the bicuspid valve, with attached chordae tendineae and papillary muscles.  The muscular wall is substantially thicker than that of the right ventricle as it must contract with greater force to push blood into the aorta and systemic circulation.

Internal Features of the Heart Chambers

VI. Valves of the Heart

Efficient, one way flow of blood through the heart depends on the presence of valves.  There are four valves in the heart that control the direction of blood flow.

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The two atrioventricular valves are found between the atria and ventricles; the two semilunar valves are found at the bases of the pulmonary trunk and aorta. All the valves attach to the dense connective of the cardiac skeleton.

Heart Valves

The major structural difference between the valves is the fact that the atrioventricular valves are attached to chordae tendineae and papillary muscles, projections of the myocardium. When the ventricles contract, the papillary muscles also contract to prevent the valve cusps from everting into the atria.

Function of the Atrioventricular Valves

The semilunar valves function without the help of chordae tendineae and papillary muscles. They are simply semilunar flaps of tissue that protrude into the lumen of the aorta and pulmonary trunk. As illustrated below, when the ventricles relax, blood fills the cusps and the lumen of the vessel is closed. Regardless of the type of valve, all of its components including the valve cusps, chordae tendineae, and papillary muscles are covered with endothelium because they are directly in contact with blood.

Function of the Semilunar Valves

A major clinical problem in patients with recurring bouts of rheumatic fever is gradual but progressive narrowing of the mitral valve, known as mitral stenosis. The leaflets undergo an initial phase of inflammation, which is followed by a more protracted phase of scarring (fibrosis). Extensive fibrosis not only thickens and distorts the valve leaflets, but also commonly shortens and thickens the chordae tendineae. Subsequent rupture or fusion of the chordae tendineae alters the range of motion of the leaflets and leads to mitral insufficiency. These chronic destructive changes lead to formation of the narrow, crescent shaped orifice that is characteristic of mitral stenosis.

The aortic and tricuspid valves may be similarly involved in patients with rheumatic heart disease. The pulmonary valve is the least likely to be damaged.

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VII. Surface Anatomy – Heart Sounds

As future physician assistants, you will be examining patients and must become comfortable with, and skilled at the art of palpation. Throughout this course we will set aside some time during particular dissections so that you can practice palpating certain body parts on yourself and on a partner. We do not claim clinical relevance for all of the structures you will palpate. The major objective of these exercises is to give you some practice and confidence in your ability to relate the anatomy you will learn from your cadaver to a living human being, including yourself.

Select a partner with whom you are comfortable. Remove your lab coat and any outer garment that might interfere with palpation (men can take their shirts off; the women should wear an appropriate top).

A. Bony landmarks of the thorax

Practice palpating the following on your partner:

- clavicle

- jugular (suprasternal) notch

- manubrium of sternum

- sternal angle (of Louis)

- body of sternum

- xiphoid process

- subcostal angle

Landmarks for listening to heart sounds

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Begin where the second rib articulates with the sternum at the sternal angle and count ribs (costal cartilages) until you reach the subcostal angle. The last costal cartilage you feel just lateral to the xiphoid process is from rib seven. Ribs 8 through 12 are called false ribs because their costal cartilages do not reach the sternum. Ribs 11 and 12 are also called floating ribs because their tips end in the soft tissues of the body wall. Run your fingers posteriorly along the lower margin of the rib cage until you feel the free tip of rib 11. Proceed a little further along the inferior margin of rib 11 to see if you can feel the tip of rib 12. Use one of the skeletons for reference.

Because this may be your first opportunity to use a stethoscope, we would like to orient you to its proper use. The stethoscope’s earpieces should conform to the direction of your external auditory meatus. They should point anteriorly as you insert them into your ear canals to prevent any leakage of sound. The chest piece consists of a diaphragm that transmits high-pitched sounds optimally, and a bell that uses the chest wall as a diaphragm and transmits low-pitched sounds. If the bell is pressed too tightly against the chest wall, the skin becomes a tense diaphragm and the low-pitched sound is damped out. Avoid pressing too hard with the bell to reduce damping the chest wall. For now, do not expect to hear all the subtle variations of heart sounds. The recognition of heart sounds requires training and experience, a quiet room, and unencumbered access to the skin of the area being examined.

B. Heart sounds

Normal contraction of the heart produces two sounds, lub and dup. These sounds are produced by the closure of the heart valves. The first sound is produced by the closure of the atrioventricular valves at the start of systole. The second sound is produced by the closure of the aortic and pulmonic valves at the end of systole. Each valve sound is heard best at a particular location on the chest wall, but these sites do not correspond exactly to the locations (surface projections) of the valves. The figure above shows the rib cage with an outline of the heart behind it and letters denoting the locations on the chest where each of the valves is heard best (on average!). The aortic (A) valve is heard best over the second intercostal space to the right of the sternum. Try it there. Now place the stethoscope over the second intercostal space to the left of the sternum to hear the sound produced by the pulmonic (P) valve. Next, place it to the left of the sternum at the level of the 5th costal cartilage to listen to the sound produced by closure of the tricuspid (T) valve. Lastly, the mitral (M) valve is heard best over the apex of the heart. Place the diaphragm of the stethoscope over the apex and count down to the fifth intercostal space where the apex is located. Now listen. Note that the best position to listen to the heart sounds will vary from person to person, depending on the size and position of the heart. This exercise is meant as an introduction to the use of the stethoscope. Enjoy yourselves!

https://www.youtube.com/watch?v=h8B3JBW_mX8

VI. Conducting System of the Heart

Contraction of the heart can occur without innervation, due to the conducting system, a system of specialized cardiac myocytes that can stimulate contraction. The conducting system of the heart cannot be seen grossly, but it is important to realize where its components are located. Contraction is initiated at an average rate of 100 depolarizations per minute by a mass of cells located at the junction of the superior vena cava and the right atrium. This sinoatrial (SA) node sends fibers to three locations; the right atrium, the left atrium, and the atrioventricular (AV) node, which lies in the interatrial septum, above and in front of the opening of the coronary sinus. One function of the AV node is to momentarily delay the conduction of the electrical impulse from the atria into the ventricles. This allows the atria to complete their contraction and fill the ventricles completely before the ventricles begin their contraction. From the AV node, the atrioventricular bundle descends through the membranous portion of the interventricular septum, then divides into two bundle branches or crura that descend within the endocardium on either side of the muscular interventricular septum. The left crus forms multiple branches that reach the ventricular wall along several trabeculae carneae. The right crus reaches the right ventricular wall by traveling along the septomarginal band, a trabeculum that traverses from the intermuscular septum to the right ventricular wall at the apex of the right ventricle. The two crura end as Purkinje fibers that stimulate contraction of the ventricular myocytes. If the SA node becomes erratic, an artificial pacemaker can be inserted into the heart to regulate the heart rate.

Although the inherent heart rate is set by the SA node, it may be modified by motor innervation from the autonomic nervous system.

Conducting System of the Heart

Conducting System Animation

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VII. Innervation of the Heart

A. Motor Innervation

The sinoatrial (SA) and atrioventricular (AV) nodes are heavily innervated by both parasympathetic and sympathetic fibers that can modify the frequency of action potentials generated by the nodes. Vagal efferent fibers to the heart synapse in ganglia in the cardiac plexus or in the walls of the atria. They decrease the heart rate and constrict the coronary arteries. Sympathetic efferent fibers are nearly all postganglionic fibers from cervical and upper thoracic ganglia of the sympathetic trunk. They increase the heart rate and the strength of each beat.

Because cardiac myocytes do not depend on motor neurons to initiate contraction, innervation of cardiac muscle is not as elaborate as that of skeletal muscle. Axons do not form neuromuscular junctions with individual myocytes. Instead, the axons lie in the endomysium among the myocytes. The axons have periodic dilations containing neurotransmitter, and these endings serve many myocytes. Unlike skeletal muscle, the heart will continue beating when all extrinsic innervation is removed.

Innervation of the Heart

B. Sensory Innervation

Afferent information from the heart, while not part of the autonomic nervous system, travels with the sympathetic and parasympathetic nerves to reach the central nervous system. Vagal afferents carry information related to reflex actions, blood pressure, and oxygen and carbon dioxide content of the blood. They probably do not carry pain sensation from the heart.

Afferent fibers traveling with the sympathetic nerves play a role in reflexes and relay pain information from the heart. Pain from the heart is often perceived in the skin of the shoulder and arm, a phenomenon called referred pain. This occurs because afferent information from the heart and the skin converges on the same dorsal horn cells in the spinal cord. The diagram illustrates a sensory neuron receiving information from both the intestine and a portion of the skin. Therefore, although the pain actually originates in a visceral structure, it is perceived as if it is located in the skin.

Rationale for referred pain

Pain from the skin (somatic sensation) is more common than pain from the viscera, and therefore, more of our nervous system is dedicated to receiving pain from the skin. Having little central representation for visceral pain, the visceral afferents return to the spinal cord, converge with the somatic sensory fibers and synapse on the same cells. Because the afferent fibers from the heart return to the lower cervical and upper thoracic spinal cord levels, the same levels that supply the dermatomes of the shoulder and arm, cardiac pain generally refers to those dermatomes, as shown above. The principle of referred pain can be applied to many organs and is a useful clinical tool for diagnosing disease.

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VIII. Coronary Circulation

In addition to the pulmonary and systemic circulations, there is a small, yet critical, third circulation that supplies the heart.  The coronary arteries are so important you should have a detailed understanding of their distribution. There are two major coronary arteries that branch from the aorta, just above the aortic valve. The left coronary artery arises from the left side of the aorta and passes between the pulmonary trunk and the left auricle to reach the coronary sulcus. It bifurcates into the anterior interventricular and circumflex arteries. The anterior interventricular artery is also called the LAD, or left anterior descending artery. It supplies the right and left ventricles and the anterior region of the muscular interventricular septum. The circumflex artery continues to the posterior side of the heart within the coronary sulcus and supplies the left atrium and left ventricle.

The right coronaryartery arises from the right side of the aorta and lies in the coronary sulcus. Its branches include the sinoatrial nodal artery that supplies the SA node, the right marginal artery that supplies the right ventricle, and the posterior interventricular artery that supplies the right and left ventricles and the posterior portion of the muscular interventricular septum.

Usually, there is a balanced distribution between the two coronary arteries. The left coronary supplies the anterior portion of the interventricular septum, the left ventricle, and the left atrium; the right coronary artery supplies the right ventricle, right atrium, and the posterior portion of the interventricular septum. When variations do occur, there may be a right or left dominant coronary artery. Such variations always occur on the back of the heart. When the right coronary artery is dominant, it extends onto the back of the left ventricle supplying an area ordinarily supplied by the circumflex branch of the left coronary. When the left coronary artery is dominant, the posterior interventricular artery is a branch of the circumflex artery of the left coronary.

Major Branches of the Coronary Circulation

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After the coronary arteries have delivered the blood to nourish the heart, the cardiac veins must return it to the circulation. A cardiac vein accompanies each of the major coronary arteries. The great cardiac vein is the largest; it accompanies both branches of the left coronary artery, the anterior interventricular artery and the circumflex artery. The middle cardiac vein travels with the posterior interventricular artery and the small cardiac vein travels with the right marginal artery. All of these major cardiac veins drain into the coronary sinus, which delivers the deoxygenated blood to the right atrium.

Not all venous blood from the myocardium returns via the coronary sinus. There are anterior cardiac veins that empty directly into the right atrium and smallest cardiac veins that empty directly via microscopic openings into the heart chambers.

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IX. Myocardial Infarction

Although it has been controversial, there is no longer any doubt that there is collateral circulation in the human heart. Connections are present between the anterior and posterior interventricular arteries as well as between the circumflex artery and the distal end of the right coronary artery. If an artery is occluded gradually due to formation of plaque or a clot, the anastomotic vessels enlarge gradually to form extensive collateral circulation that can supply the myocardium adequately. However, if a coronary artery is obstructed rapidly, the collateral circulation is usually inadequate and a heart attack or myocardial infarct occurs. The myocytes die from the lack of oxygen (ischemia). Depending on which side of the heart becomes functionally impaired after myocardial infarction, the patient may show signs of left heart failure, right heart failure, or some combination of these.

Atherosclerosis Leading to Myocardial Infarct

In left heart failure, obvious consequences of severe damage to the muscular walls of the left ventricle are 1) dilatation of the ventricle and 2) incomplete emptying of the ventricle during systole. The part of the myocardium that survives generally shows signs of compensatory hypertrophy. Incomplete emptying of the left ventricle also raises the left ventricular diastolic pressure, which elevates the left atrial and pulmonary venous pressures, with resulting pulmonary congestion.

In right heart failure, equivalent changes of the right ventricular wall are seen. Right heart failure leads to systemic congestion, increased venous pressure, and edema seen chiefly in the ankles or in the presacral region of bedridden patients.

X.  Treatments for Coronary Artery Obstruction

Treatments for coronary artery disease depend on the degree of obstruction and health of the patient.  Options include treatment with medications, minimally invasive angioplasty, and highly invasive coronary artery bypass surgery.

Angioplasty

This procedure involves threading a catheter into the vasculature to reach the site/s of obstruction.  A balloon is inflated to compress the plaque and to increase blood flow.  The provider may insert a stent at the obstruction to help hold the vessel open.

http://www.orchardheart.com/angioplasty/

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Coronary Artery Bypass Grafting (CABG)

This surgical procedure involves suturing a new vessel onto the heart to form a detour or bypass around the site of obstruction.  It involves open surgery to gain access to the heart.  If multiple grafts are placed, the procedure is called a double, triple, quadruple bypass, etc.  Donor vessel/s is/are harvested from the patient and sutured to the aorta and distal to the site of obstruction to restore sufficient blood flow.

CABG Animation from heartpoint.com

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Lungs and Base of Neck

Sarah Traynor

Key Concepts

Students will examine the relationships of the lungs and pleural sacs in the thoracic cavity, and the position of the vagus and recurrent laryngeal nerves; learn the lobes and fissures of the lungs and the organization and branching of the airways and blood vessels within the organs; and follow structures from the superior mediastinum into the neck.

Learning Objectives

- Locate these pleural regions:

- visceral vs. parietal pleura

- costal pleura

- mediastinal pleura

- diaphragmatic pleura

- cervical pleura

- costodiaphragmatic recess/costophrenic angle

- costomediastinal recess

- Explain the purpose, technique and anatomical relationships that should be considered when performing thoracentesis.

- Explain the purpose and technique for inserting a chest tube.

- Identify the lobes and fissures of the lungs.

- superior, middle and inferior lobes

- oblique/major and horizontal/minor fissures

- Describe the course of the trachea in the superior mediastinum. Identify the carina and contrast the anatomical differences between right and left primary bronchi.

- Identify all structures that enter and leave the hilum of the lungs, their relative positions and  functions.

- pulmonary artery and vein

- bronchial artery and vein

- primary/main bronchus

- pulmonary plexus (nerves)

- lymphatics and lymph nodes

- Define bronchopulmonary segments and explain their clinical significance.

- Explain how contraction of the thoracoabdominal diaphragm contributes to respiration, and describe the nerve that innervates its contraction.

- Follow structures from the superior mediastinum into the root/base of the neck.

- phrenic nerves

- left recurrent laryngeal nerve

- vagus nerves

- internal jugular veins

- subclavian veins

- brachiocephalic veins

- great vessels (brachiocephalic trunk, right common carotid, right subclavian, left common carotid, left subclavian)

- Identify the first two branches of the subclavian artery

- vertebral artery

- internal thoracic artery

I. Pleural Sacs and Recesses

Fluid-filled pleural sacs surround the lungs. These sacs have an inner visceral layer that covers the surface of the lung, an outer parietal layer that lines the thoracic wall, and a viscous pleural fluid between the two layers. The parietal pleura can also be named based on its location. The diaphragmatic pleura is the portion of parietal pleura that lines the diaphragm. The costal pleura lines the inner surface of the rib cage and the mediastinal pleura lines the midline organs of the thorax. The cervical pleura lines the apical region of the lungs located in the root of the neck (the lungs extend ~4 cm above the 1st rib ). The visceral and parietal layers are smooth and glide against each other with relatively little friction as the lungs expand and recoil during inhalation and exhalation.

Image adapted from Textbook of Anatomy 4th Ed by Dr. Traynor

Pleural Sacs and Recesses

Because the lungs do not completely fill the pleural sacs, in select locations the parietal pleura reflects on itself to form pleural recesses and is not in contact with the visceral pleura/lung. Inferiorly, the costal and diaphragmatic pleurae form the costodiaphragmatic recess (costophrenic angle) and anteriorly the costal and mediastinal pleurae form the costomediastinal recess (cardiophrenic angle). The recesses of the pleural cavity are mostly occupied by the lungs during inspiration, but not during expiration, as shown in the figure. When exercising, the recesses will diminish with increased and deeper breaths.

Netter Plate 193

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II. Thoracentesis

Typically, the pleural recesses contain a thin film of fluid, but infection or injury can cause accumulation of air (pneumothorax) or significant amounts of blood or other fluid (pleural effusion, hemothorax) in the pleural sac. If the accumulation is substantial, it may cause respiratory insufficiency because the lung will not be able to expand effectively. This accumulation can be heard by a change in the breath sounds, and seen in the chest x-ray.  Depending on the position of the patient, fluid will accumulate in a particular recess.  For example, in the upright position, the fluid will accumulate in the costodiaphragmatic recess, and produce a fluid line on an x-ray.

Pneumothorax and Hemothorax

Thoracentesis is a surgical puncture of the chest wall used to aspirate air or fluid from the chest cavity to restore lung function.  When performing thoracentesis, the needle insertion site depends upon whether there is air or fluid in the pleural sac. For air removal, the needle is typically inserted in the second or third intercostal space, in the midclavicular line. For fluid removal, the needle is inserted one or two intercostal spaces below the fluid level, as determined by x-ray, and 5 to 10 cm lateral to the spine.  The selected site must be above the diaphragm. Using a large syringe and long needle, the physician/assistant pushes the needle through the intercostal muscle onto the rib, then pushes the needle up over the top of the rib to avoid hitting the major neurovascular bundle that runs in the groove under each rib. This neurovascular bundle includes the intercostal nerve, artery and vein.  As the needle passes through the parietal pleura it will pop. By applying back pressure to the syringe, it is possible to draw fluid or air from the pleural space.

Anatomical lines

For more detailed information about thoracentesis procedure, watch this short video:

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III. Chest Tubes

Chest tubes are used in almost every patient having surgery involving the thorax. They perform a function similar to thoracentesis, but are left in the patient for a period of time to continually drain the accumulating fluid, blood, or air. Placement of a chest tube depends on the circumstances, but a common site is in the fifth or sixth intercostal space at the midaxillary line.

Chest Tube Placement

This video describes the steps of inserting a chest tube:

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IV. Lobes and Fissures of the Lungs

The left lung is divided by the oblique fissure into superior and inferior lobes.  You can also refer to these as the upper and lower lobes. Along its anterior margin, the heart forms an indentation called the cardiac notch. The small, tongue-like projection below the notch is the lingula. The right lung has three lobes- superior, middle, and inferior lobes- that are divided by the oblique and horizontal fissures into upper, middle and lower lobes. The oblique and horizontal fissures are also referred to as the major and minor fissures, respectively. You can use either term. Sometimes, the fissures are incomplete and there is less division between the lobes than what we would consider to be typical. Like anything in anatomy, the length and depth of the fissures is variable.

Lobes of the lung

V. Trachea and Bronchi

The trachea begins in the neck at the inferior margin of the larynx, and lies immediately anterior to the esophagus for its entire length. The cartilaginous rings of the trachea are incomplete; the poster portion of the ring is composed on smooth muscle and is adjacent to the smooth muscle of the esophagus. The malleable smooth muscle of the posterior trachea permits a bolus of food to travel through the esophagus into the stomach without incident.  The arch of the aorta crosses the trachea anteriorly. It ends by bifurcating into right and left primary/main bronchi at the level of the T5 vertebral body. At the bifurcation, or carina, it is covered anteriorly by fibers of the cardiopulmonary plexus of nerves (which contains sympathetic, parasympathetic, and sensory fibers). The right main bronchus is shorter, wider, and oriented more vertically than the left. As a consequence of these features, aspirated foreign objects tend to become lodged in the right main bronchus or one of its more inferior branches.

Netter Plate 199 demonstrating primary, secondary, and tertiary bronchi

Trachea and Primary Bronchi with relation to the aortic arch and esophagus

Netter Plate 206; vast network of cardiopulmonary plexus overlying the carina

VI. Hilum of the Lung

Many organs have a region called the hilum (pl. hila), where structures enter or leave the organ. A complete list of structures at the hilum of the lung includes:

- primary bronchus

- pulmonary arteries

- pulmonary veins

- bronchial arteries and veins

- hilar lymph nodes

- branches of the pulmonary plexus of autonomic nerves

Make sure you can identify these structures in both the left and right lungs and explain their function. The primary bronchus is unique in that it has a thick, cartilaginous wall. Because the airway must be open constantly, bronchi are held open by hyaline cartilage. The veins have the thinnest walls, and are usually the most anterior and inferior vessels. The locations of the arteries differ slightly in the right and left lungs. The position of the pulmonary artery relative to the primary/main bronchus in each lung is summarized by the acronym “RALS”. In the right lung the pulmonary artery is anterior to the bronchus (RA= right, anterior), and in the left lung the pulmonary artery is superior to the bronchus (LS= left, superior). In terms of the function of these structures, the pulmonary arteries carry deoxygenated blood from the heart to the lungs; the pulmonary veins carry oxygenated blood from the lungs to the heart. The lung parenchyma (tissue) and bronchial tree require blood as well for nutrients, this oxygenated blood enters the hilum via the bronchial arteries. Bronchial arteries are some of the first direct branches of the descending aorta. Bronchial veins drain to the azygos system. The lymph nodes surrounding the hilum of the lung are prominent and collect lymph from the interstitial spaces within the lung tissue.

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VII. Bronchopulmonary Segments

The air passages (trachea, primary/main, secondary, and tertiary bronchi, and bronchioles) subdivide progressively and distribute throughout the lung, terminating in alveolar sacs. The region of the lung aerated by a tertiary bronchus is called a bronchopulmonary segment.

Tracheobronchial tree

The diagrams below illustrate the boundaries of the ten bronchopulmonary segments in each lung. They are not labeled intentionally because you do not need to know the details. The apex of the segment is directed toward the hilum, so the outlines on the lung surface represent the base of each segment. Look at the lower figure for an appreciation of the tertiary bronchi that aerate each of the bronchopulmonary segments. Note that each segmental/tertiary bronchus branches from the secondary bronchus at a specific angle. This means that in order to place a specific bronchopulmonary segment in an appropriate dependent position for drainage, the patient should be placed in a specific head-up or head-down position, in combination with a specific semi-prone or semi-supine position.

Bronchopulmonary Segments

Tertiary/segmental bronchi that create the bronchopulmonary segments.

Pulmonary arteries branch with and travel alongside the branches of the bronchi, to each bronchopulmonary segment. Pulmonary veins run separately and are found at the interfaces of the bronchopulmonary segments, where they receive tributaries from both segments. Thus, the pulmonary arterial branches are intrasegmental , while the pulmonary veins are intersegmental . This anatomical relationship is clinically important. Due to each segment having an independent supply of both arterial blood and air, a particular diseased segment can be resected to limit the spread of disease to other lung segments. Understanding the anatomy of bronchopulmonary segments is essential for accurate interpretation of radiographs and for surgical resection of diseased areas of the lung. You are not expected to learn them for this course, but be aware of them.

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VIII. Thoracoabdominal Diaphragm

The thoracoabdominal diaphragm is a dome-shaped sheet of skeletal muscle that separates the thorax and abdomen. The muscle fibers radiate away from a central tendon to attach to the ribs, sternum, and vertebral column.

Superior View of the Diaphragm

When the diaphragm contracts, the muscle fibers shorten and pull the central tendon inferiorly, flattening the diaphragm. When the diaphragm flattens, the thoracic volume increases and thoracic pressure decreases. This reduced thoracic pressure draws air into the lungs (inspiration). When the diaphragm relaxes, the muscle fibers elongate and the diaphragm becomes more dome-shaped. When the diaphragm becomes dome-shaped, the thoracic volume decreases and thoracic pressure increases. Due to the increased pressure, air is expelled from the lungs (expiration). Quiet respiration involves mainly the diaphragm expanding the thoracic cavity vertically, but forced inspiration engages other respiratory muscles (intercostals, scalenes, posterior serratus) that elevate the ribs and sternum and expand the thoracic cavity horizontally.

Animation by John Pierce, via Wikimedia Commons

All somatomotor innervation of the diaphragm (a skeletal muscle) is by the left and right phrenic nerves that arise from cervical spinal levels C3-5 (“C3, 4, 5, keep the diaphragm alive!”). The phrenic nerves enter the superior mediastinum between the subclavian artery and the forming brachiocephalic veins. The right phrenic nerve is found on the right side of the right brachiocephalic vein, the superior vena cava, and the right atrium. At the level of the right atrium, it lies between mediastinal pleura and the fibrous pericardium. The left phrenic nerve crosses the arch of the aorta, then travels inferiorly along the pericardium over the surface of the left atrium and left ventricle. It is also located between the mediastinal pleura and the fibrous pericardium. Both phrenic nerves pass anterior to the hilum of their respective lung. Since the phrenic nerves are derived from spinal nerves, they contain somatomotor fibers, somatosensory fibers, and post-ganglionic sympathetic fibers (just like all spinal nerve derivatives). The many sensory fibers of the phrenic nerve transmit information from the mediastinal and diaphragmatic pleura, pericardium and diaphragm.

Thoracoabdominal Diaphragm and Phrenic Nerves

In addition to somatomotor fibers to the diaphragm, the phrenic nerve has sympathetic and many sensory fibers that transmit information from the mediastinal pleura, pericardium and diaphragm. Sensory (afferent) information from these areas is conveyed to the same region of the spinal cord that receives sensory input from the base of the neck and the upper limbs. As a result, irritation of the diaphragm may be perceived as pain in the base of the neck and the shoulder region (dermatomes C3-5). This phenomenon is called referred pain. In the same vein, if the liver is diseased or the gall bladder is inflamed, this visceral inflammation may be also perceived as pain in the base of the neck and shoulder region because these organs are adjacent to the diaphragm.

Image from University of Minnesota-Duluth; schematic of the mechanism underlying referred pain of visceral tissue to skin

VIIIi. Root of the neck

Many nerves and vessels that we first studied in the thorax extend into the neck. We are going to track these vessels into the root of the neck, or the junction between the thorax and the neck.

Structures in the root of neck (all surrounding muscles have been removed)

In order to access these  structures in the neck, you have to remove several layers of muscle. First, the most  superficial muscle covering the anterior neck is the platysma, which is a muscle of facial expression. Underlying the platysma is the sternocleidomastoid muscle (will return to all of these muscles in Unit  3).  Once you reflect sternocleidomastoid from its sternal and clavicular attachments, you can view the infrahyoid muscles in their entirety.

Muscles that have to be reflected in order to access veins, arteries and nerves that are deeper in the neck

Deep to the infrahyoid muscles is the thyroid gland that surrounds the anterior and lateral portions of the first few tracheal rings and sits inferior to the thyroid cartilage. At this point, you can also view the structures in the root of the neck that we took note of in the superior mediastinum like the phrenic nerves, whose pathway has already been described above. Next is the brachiocephalic veins which form at the junction of the internal jugular and subclavian veins. The left and right brachiocephalic veins form the superior vena cava. Deep to the veins are the great vessels branching from the aortic arch: the brachiocephalic trunk(innominate artery) bifurcating into the right common carotid and subclavian arteries, and the left common carotid and  left subclavian arteries. In the neck, the internal jugular vein, common carotid artery and vagus nerve travel together within a sheath called the carotid sheath. Lastly, the recurrent laryngeal nerves ‘recur’ around separate structures on the left and right sides of the body. The right recurrent laryngeal nerve recurs around the right subclavian artery; the left recurrent laryngeal nerve recurs around the ligamentum arteriosum in the thorax. Both recurrent laryngeal nerves travel alongside the trachea in the neck to reach the larynx. Imagine how long the recurrent laryngeal nerve is in a giraffe!!

Following the subclavian artery  laterally, we can finally view the origin of the internal thoracic artery that we have discussed in great detail at this point as it supplies the intercostal muscles  (via anterior intercostal arteries) and provides collateral circulation to the thoracic cage along with the aorta (via posterior intercostal arteries). We can also view the very first branch of the subclavian artery, the vertebral artery. This artery will travel through the transverse foramen of the cervical vertebrae and enter the cranial cavity to supply the brain. The left and right internal carotid and vertebral arteries provide collateral circulation to the brain (we will discuss this further in Unit 3). The internal thoracic artery is the second branch of the subclavian artery. The first couple of branches of the subclavian are important because they supply the brain and the accessory respiratory muscles.

First two branches of the subclavian artery on the right side

Autonomic Nervous System

Karen Krabbenhoft

Key Concepts

The autonomic nervous system, the involuntary motor component of the nervous system, controls activity of smooth muscle, cardiac muscle and glands.

Learning Objectives

- Describe the general organization and function of the autonomic nervous system.

- Summarize the functional and anatomical differences between the sympathetic and parasympathetic nervous systems.

- Identify the location and function of the following sympathetic structures.

- sympathetic trunk and ganglia

- collateral or prevertebral ganglia

- sympathetic communicating rami

- sympathetic thoracic splanchnic nerves

- autonomic plexus

- cardiac, pulmonary, esophageal

- Identify the location and function of the following parasympathetic structures

- vagus nerves

- esophageal plexus

- vagal trunks

- parasympathetic splanchnic nerves

- terminal/collateral ganglia

- Trace sympathetic autonomic pathways from presynaptic nerve cell bodies to ultimate innervation of target tissues in the body.

- Trace parasympathetic autonomic pathways from presynaptic nerve cell bodies to ultimate innervation of target tissues in the body.

- List the general functions of the parasympathetic and sympathetic divisions of the autonomic nervous system.

- Explain the how viscerosensory information for pain and reflexes returns to the central nervous system.

I. Overview of the Autonomic Nervous System

The autonomic nervous system (ANS) controls functions that occur automatically, without conscious control.  It is also termed the visceromotor system, which emphasizes the fact that it tends to innervate organs (viscera) and is a purely motor system that controls contraction of certain muscles and secretion by glands.  In contrast to the somatomotor system, which controls skeletal muscle exclusively, the autonomic nervous system innervates smooth muscle, cardiac muscle, and glands.

Anatomically, the most important distinguishing feature of the autonomic system is that two neurons (as opposed to one for the somatomotor system) are necessary to transmit an impulse from the CNS to the target. The first neuron’s cell body is in the CNS (in a motor nucleus) and is called the preganglionic or presynapticneuron. Its axon leaves the CNS and synapses with a second neuron whose cell body lies in an autonomicganglion. This second postganglionic or postsynapticneuron sends its axon to the target tissue.

Comparison of somatomotor and visceromotor pathways

II. Divisions of the Autonomic Nervous System

The autonomic nervous system has two subdivisions, the sympathetic and parasympathetic systems.  These two divisions work cooperatively to ensure that the body functions appropriately and efficiently in a variety of circumstances.  The sympathetic system is the “fight or flight” system that stimulates its targets during stressful events, while the parasympathetic system is the “rest and digest” system that stimulates its targets during relaxation.

The two divisions of the ANS differ not only functionally, but also anatomically.  The sympathetic division is also called the thoracolumbar division, because its preganglionic nerve cell bodies are all located in the lateral horn of  spinal cord levels T1 to L2.  The parasympathetic division is also known as the craniosacral division because its preganglionic nerve cell bodies are located in the brainstem in association with cranial nerves (III, VII, IX and X) and in the lateral horn of the spinal cord levels S2 – S4.

Parasympathetic and Sympathetic Overview

Both divisions use two neuron pathways, so there are postganglionic neurons in ganglia of the peripheral nervous system.  As with the preganglionic cell bodies, the postganglionic neurons of the divisions are in separate structures.  Sympathetic ganglia are generally close to the spinal cord, located either lateral or anterior to the vertebral column in paravertebral/chain or prevertebral ganglia, respectively.  By contrast, parasympathetic ganglia are generally close to the target organ, in fact they are usually embedded within the wall of the target organ, in terminal ganglia.

Features of the sympathetic and parasympathetic divisions are summarized below.

Comparison of Divisions of the ANS

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III. Components of the Sympathetic Nervous System

Components of the ANS are described in greater detail below.

Sympathetic Components

Sympathetic preganglionic cell bodies:

All preganglionic sympathetic nerve cell bodies are located in the lateral horn at spinal cord levels T1-L2. Although these neurons are located in a relatively small area, they will be disseminated to provide sympathetic innervation to the entire body. The first step in that dissemination is to enter the sympathetic trunks.

Sympathetic trunks:

The sympathetic trunks are chains of ganglia that contain postganglionic sympathetic cell bodies. There is one sympathetic trunk on each side of the vertebral column from the first cervical to the lowest sacral levels. The presynaptic axons from spinal cord levels T1-L2 must synapse in ganglia from cervical to sacral levels. To accomplish this, some preganglionic fibers enter the sympathetic trunk and synapse in a ganglion at that level. Other fibers ascend or descend within the trunk before synapsing in ganglia above T1 and below L2. For example, chain ganglia in the cervical region are supplied by presynaptic fibers from upper thoracic levels, which enter and then ascend in the sympathetic trunks. Likewise, sympathetic chain ganglia below the level of L2 are supplied by preganglionic fibers from lower thoracic and upper lumbar levels, which enter the sympathetic trunk and travel down to lower lumbar and sacral levels. These ascending and descending axons connect the ganglia into a continuous chain. Although not represented in the diagram, it is also possible for postganglionic axons to ascend or descend in the chain on the course to their targets.

Communicating rami are structures that bridge the ventral ramus of a spinal nerve and the sympathetic chain. They may be either white (preganglionic) or gray (postganglionic), depending on whether or not the fibers are myelinated. The color difference between the rami is not evident in the embalmed cadaver.

White or preganglionic communicating rami contain axons from cell bodies in the lateral horn of the spinal cord from T1 to L2. The preganglionic axons leave the spinal cord via the ventral root, continue into the spinal nerve and its ventral ramus, then leave the ventral ramus to enter the sympathetic trunk. White rami connect ventral rami of spinal nerves and sympathetic ganglia only from T1-L2 because these are the levels of the spinal cord where all preganglionic sympathetic nerve cell bodies are located.

Gray or postganglionic communicating rami contain postganglionic axons from cell bodies in the sympathetic chain ganglia that are going to join with ventral and dorsal rami of spinal nerves. Every spinal nerve has a gray ramus and distributes the sympathetic fibers along with its somatomotor and sensory fibers to the periphery. These sympathetic fibers primarily innervate targets inbody wall and extremities.

Sympathetic splanchnic nerves are branches from the sympathetic trunk to viscera. Splanchnic nerves going to the heart, lungs, and other viscera in the thorax contain mostly postganglionic fibers (and synapse in a chain ganglion). The splanchnic nerves going to abdominal and pelvic viscera contain mostly preganglionic fibers. They synapse in collateral ganglia. Sympathetic splanchnic nerves are named according to the level of the sympathetic trunk from which they originate. There are cervical, thoracic, lumbar, and sacral splanchnic nerves.

Collateral or prevertebral ganglia are sympathetic ganglia located immediately anterior to the abdominal aorta (and hence anterior to the vertebral column). The ganglia are located at the sites where major arteries branch from the aorta to supply internal organs. The postganglionic cell bodies in collateral ganglia send their axons to abdominal and pelvic viscera along the major branches of the abdominal aorta; they do not innervate the body wall or extremities.

Autonomic plexuses.  An autonomic plexus is a mixture of sympathetic, parasympathetic, and sensory fibers. Nerve fibers may pass through a plexus without synapsing in it, although some plexuses may contain small ganglia. A nerve plexus is often named for a neighboring organ. For example, near the heart and lungs, the cardiac plexus and pulmonary plexus (often called the cardiopulmonary plexus) receive sympathetic fibers from the sympathetic trunk and parasympathetic fibers from the vagus nerve. The fibers in a plexus are frequently a mixture of pre- and postganglionic fibers, but there is no set pattern. In the cardiopulmonary plexus, the sympathetic fibers are postganglionic (cells bodies in the cervical and thoracic chain ganglia), but the vagal parasympathetic fibers are preganglionic axons.   The latter synapse in microscopic ganglia located within the cardiopulmonary plexus or within the wall of the heart.

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IV. Components of the Parasympathetic Nervous System

This list will focus on parasympathetic structures that innervate targets in the trunk.

Cranial Nerves:  Nerves that arise from the brainstem rather than the spinal cord are called cranial nerves. One cranial nerve, the vagus nerve, is involved in parasympathetic innervation of the trunk. The vagus nerves do have targets in the head and neck, before they deliver parasympathetic fibers to all of the viscera in the thorax and to a portion of the viscera in the abdomen.

The esophageal plexus is formed as the vagus nerves traverse the thorax. The right and left vagus nerves are located on the right and left sides of the esophagus in the upper portion of the thorax, but separate into many strands that intermingle on the surface of the esophagus. This intermingling of right and left vagus nerves forms the esophageal plexus, which contains primarily preganglionic parasympathetic fibers, but also branchiomotor (~somatomotor) and viscerosensory fibers. The mixing ensures that no organ is innervated by just one vagus nerve or the other. It is necessary to have both branchiomotor and visceromotor fibers in the esophageal plexus, because its wall contains both skeletal and smooth muscle. The upper third is strictly skeletal muscle, the middle third is mixed, and the lower third is strictly smooth muscle.

Parasympathetic splanchnic nerves are formed by the sacral portion of the parasympathetic nervous system. The preganglionic parasympathetic axons arise from sacral spinal cord levels S2, S3, and S4 and exit with these spinal nerves. In the pelvis, the axons branch off the spinal nerves to reach their visceral targets in the pelvis and abdomen. Because the fibers emerge in the pelvis they are called pelvic splanchnic nerves.

Parasympathetic ganglia.  As part of the autonomic nervous system, parasympathetic axons must synapse in a ganglion before innervating their target. Parasympathetic ganglia are different from sympathetic ganglia in several respects. They are located very close to or even within the walls of the organs that they innervate. They are widely scattered (unlike the regular chains of the sympathetic system); and they are usually small (even microscopic) in size, in contrast to the large sympathetic ganglia.

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V. Sympathetic Pathways

A. Output to the Sympathetic Trunk

As mentioned previously, all preganglionic sympathetic nerve cell bodies are in the lateral horn of spinal cord levels T1 to L2. The axons of these neurons leave the spinal cord via the ventral root, traverse the spinal nerve and ventral ramus, then enter the sympathetic trunk at that level via a white communicating ramus. All preganglionic axons follow this pathway regardless of their eventual targets. The remainder of the pathway, including the site of the postsynaptic neuron, depend upon the ultimate target.  The possibilities are detailed below.

B. Sympathetic innervation to the skin of the neck, body wall and limbs

Sympathetic fibers to the trunk and extremities all synapse in sympathetic chain ganglia, either at the level of entry or after first ascending or descending within the sympathetic trunk. After synapse, postganglionic fibers join the spinal nerve at that level using a gray communicating ramus. Postganglionic fibers distribute on the ventral and dorsal rami of that spinal nerve to reach its dermatome. These fibers innervate cutaneous targets involved in temperature regulation, including smooth muscle in blood vessels to control the degree of vasoconstriction/dilation, arrector pili muscles that can generate goose bumps, and sweat glands.

C. Sympathetic innervation to the head

There are no sympathetic ganglia in the head. As a result, most preganglionic sympathetic fibers that innervate targets in the head ascend within the sympathetic chain, then synapse in the highest cervical chain ganglion, called the superior cervical ganglion. This ganglion is actually a fusion of the upper four cervical ganglia. This ganglion is adjacent to the internal carotid artery so that postganglionic fibers reach their targets in the head by traveling along branches of this artery.  The specific targets will be discussed during the head and neck unit of this course.

D. Sympathetic innervation of thoracic viscera

The thoracic viscera receive preganglionic sympathetic innervation from the upper thoracic spinal cord levels. These fibers enter the sympathetic chain and ascend to synapse in cervical chain ganglia. Postganglionic fibers leave the cervical sympathetic chain via splanchnic nerves called the superior, middle and inferior cardiac nerves, which converge to form the cardiopulmonary plexus near the arch of the aorta and at the carina/bifurcation of the trachea. Fibers pass from the plexus to the coronary arteries, SA node of the heart, and to bronchi and arteries of the lung.

Sympathetics to Thoracic Viscera

E. Sympathetic innervation of abdominal and pelvic viscera

Innervation of the abdominal and pelvic viscera is the most complicated pathway of the sympathetic system. Preganglionic sympathetic fibers pass through the sympathetic trunk and form splanchnic nerves. In the thoracic region, there are three major splanchnic nerves called the greater (T5-9), lesser (T9-10), and least (T12) thoracic splanchnic nerves. Each of these splanchnic nerves passes inferiorly through the diaphragm and enters the abdominal cavity. They synapse in collateral/prevertebral ganglia located at the branch points of the celiac, renal, and superior mesenteric arteries. Postganglionic fibers from the ganglia travel along all of the branches of the respective artery to reach the target organs.

There are additional prevertebral ganglia in the abdomen, including the inferior mesenteric and hypogastric ganglia, that do not receive preganglionic fibers from thoracic splanchnic nerves. These ganglia receive preganglionic fibers from lumbar or sacral splanchnic nerves, that branch from the sympathetic trunk of lumbar and sacral regions, respectively.

Sympathetics to Internal Organs

Splanchnic Nerves

Preaortic Plexus

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VI. Parasympathetic Pathways

A. The Parasympathetic Nervous System

The parasympathetic system is also called the craniosacral nervous system because its preganglionic cell bodies are located in either the brainstem for cranial nerves III, VII, IX and X, or in the spinal cord for sacral nerves 2, 3 and 4. Thus, the preganglionic axons leave the CNS as a component of a cranial nerve or as part of the ventral root of a sacral spinal nerve. The parasympathetic system not only has a more restricted origin from the CNS, but also has more limited targets, in contrast to the sympathetic system, which sends fibers to all parts of the body. A major distinction is that the body wall and extremities do not receive parasympathetic innervation.

Parasympathetic Nervous System

B. Parasympathetic innervation of the head

Salivary glands, lacrimal glands, and specific smooth muscles in the head are innervated by parasympathetic fibers of cranial nerves. You will study these pathways in detail in the head and neck unit of this course.

C. Parasympathetic innervation of thoracic viscera

A cranial nerve, the vagusnerve, provides parasympathetic innervation to the thoracic viscera. The right and left vagus nerves descend through the neck, where they give off branches to the cardiopulmonary plexus. These preganglionic fibers distribute from the plexus to targets in the heart and lungs. Synapses on postganglionic neurons take place in or near the target organs.

The vagus nerves descend into the thorax, where they break up to form the esophageal plexus. Some fibers innervate the esophagus, and the remaining fibers coalesce to form anterior and posterior vagal trunks that continue into the abdomen.

Parasympathetics to Thoracic Viscera

D. Parasympathetic innervation of abdominal and pelvic viscera

The anterior and posterior vagal trunks pass through the diaphragm with the esophagus into the abdomen. Preganglionic vagal fibers innervate the derivatives of the embryonic foregut and midgut. This includes all organs of the intestinal tract to the junction of the transverse and descending colons. The sacral portion of the parasympathetic system innervates the remainder of the colon and the pelvic viscera. Preganglionic fibers originate in the S2-S4 levels of the spinal cord, exit via the ventral root, and enter the spinal nerves and ventral rami. The preganglionic fibers branch from the S2-S4 ventral rami to reach their visceral targets. These nerves are called pelvic splanchnic nerves. They synapse in microscopic ganglia in or near their target organs.

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VII. Functions of the Sympathetic and Parasympathetic Nervous Systems

The sympathetic nervous system supplies structures in all regions of the body. The parasympathetic system innervates mainly visceral structures, and sends no fibers to the body wall or extremities. Functionally, the sympathetic and parasympathetic systems often have contrasting roles. The sympathetic system is an emergency system that prepares the body for “fight or flight,” whereas the parasympathetic system is a homeostatic system that preserves and restores the orderly functions of the body. Thus, the sympathetic system speeds up the heart rate, dilates the pupils, constricts blood vessels to the skin and gut to increase blood flow to the voluntary muscles, and slows peristalsis of the gut. In contrast, the parasympathetic system decreases the heart rate, constricts the pupil, and increases peristalsis.

In cases where sympathetic and parasympathetic systems innervate the same organ, there is coordination of activity rather than competition. For example, eliminating parasympathetic innervation increases heart rate, but eliminating sympathetic innervation produces no immediate change. This shows that the parasympathetic function is to maintain the activity of the heart at the lowest efficient level. In an emergency, the sympathetic system takes over to increase heart rate and the parasympathetic system ceases to control the heart.

Both sympathetic and parasympathetic nerves innervate the airway from the trachea to the terminal bronchioles in the lungs; the more distal, smaller passages are not under autonomic control. The pulmonary plexus provides parasympathetic fibers from the vagus nerves, and sympathetic fibers from the upper thoracic spinal cord segments. “Fight or flight” situations require increased flow of oxygenated blood to the heart, brain, limb muscles, etc. Sympathetic nerves cause the necessary bronchodilation by relaxing the smooth muscles surrounding the airways. In more relaxed situations, when the demand for blood flow is lower, parasympathetic nerves cause bronchoconstriction by stimulating smooth muscle contraction.

The number of postsynaptic neurons that are stimulated by each presynaptic neuron varies dramatically between the parasympathetic and sympathetic systems. The parasympathetic presynaptic neurons synapse with only one or two postsynaptic neurons, giving the parasympathetic system more precise control over the signals it sends. In contrast, the sympathetic presynaptic neurons synapse on about 20 postsynaptic neurons. This gives the sympathetic system the ability to amplify its message, an essential feature for a system that must react quickly in an emergency situation.

VIII. Visceral Afferents

So far we have considered only efferent/motor fibers to the viscera. However, there are also afferent/sensory fibers from the viscera to the CNS. Although sensory fibers may return to the CNS by following autonomic pathways, these sensory components are not part of the autonomic system, which is strictly a motor system. The visceral afferent fibers carry either pain or reflex sensation. The nerve cell bodies of visceral afferent fibers may be in either dorsal root ganglia of spinal nerves or in sensory ganglia of cranial nerves, particularly the vagus nerve.

The sensory neurons in the dorsal root ganglia that travel with sympathetics carry pain sensation. They follow the pre- and postganglionic sympathetic fibers as they return to the CNS from the walls of the viscera and blood vessels. Although these sensory axons pass through the sympathetic ganglia, they do not synapse in them. Typical stimuli that elicit pain sensation from the skin do not elicit pain from the viscera. Viscera are insensitive to cutting, crushing, or burning, but are sensitive to excessive distention or contraction of smooth muscle, as during excessive intestinal contractions. Visceral pain is poorly localized and tends to be dull or heavy.

Viscerosensory Fibers Travel with Autonomic Pathways. Image from https://clinicalgate.com/viscerosensory-pathways/

In general, the sensory and sympathetic fibers that innervate a particular organ are associated with the same spinal cord levels. Afferent fibers enter the same spinal cord segments that the preganglionic sympathetic fibers exit. For example, sympathetic pathways to the heart originate in the upper thoracic spinal cord segments and the pain fibers from the heart enter the spinal cord through the 1st to 5th thoracic spinal nerves (mostly on the left side). These pain fibers are believed to travel exclusively with the sympathetic components of nerves to the heart, and carry pain that is sharp and localized.

The vagus nerve contains both parasympathetic and visceral afferent fibers from thoracic and abdominal organs. These sensory fibers carry information for reflexes and sensations such as hunger and nausea. The cell bodies for these sensory neurons are in the sensory ganglia of the vagus, and their peripheral processes have a wide distribution. Their terminals are in the heart, walls of the great vessels, esophagus, lungs, stomach, intestines, and kidney.

The pelvic viscera send afferent impulses to the CNS through the pelvic splanchnic nerves (which you will study in the next unit). The cells bodies of these sensory fibers are located in the dorsal root ganglia of sacral nerves 2, 3, and 4.  These neurons form stretch receptors in the bladder, rectum, and cervix and conduct information related to bladder and rectal distention and sexual sensation.

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Thoracic Duct

Karen Krabbenhoft

Key Concept

Lymph from ¾ of the body drains into the venous system via the thoracic duct.

Learning Objective

- Describe the origin and termination of the thoracic duct, and trace its course in the thorax.

I. Thoracic Duct

There are two major lymphatic ducts that return lymphatic fluid to the venous system.  Recall that that the drainage pattern is not symmetrical.  The right lymphatic duct drains lymph from the right upper quadrant of the body, the region superior to the thoracoabdominal diaphragm and on the right side of the body’s midline.  The thoracic duct is the largest lymphatic duct in the body.  It drains lymph from approximately three fourths of the body, including everything inferior to the thoracoabdominal diaphragm and the region superior to the diaphragm on the left side of the body’s midline.

Overview of lymphatic drainage.  The area shaded pink is drained by the right lymphatic duct and the area shaded gray is drained by the thoracic duct.

The thoracic duct originates in the upper abdomen at the cisterna chyli, a lymphatic vessel that receives all lymph from below the diaphragm. The thoracic duct enters the thorax through the aortic hiatus in the diaphragm. It ascends on the right side of the midline in the lower half of the thorax and then shifts to the left side of the midline in the upper half of the thorax. It exits the thorax by passing behind the left subclavian artery, and ends by joining the left internal jugular or left subclavian vein where the two veins join. In its course through the thorax, the duct is quite variable; at some levels, it may be a single large duct, while at other levels it can form several smaller ducts, particularly in the lower thorax. After meals, the lymph in the thoracic duct is often white, because the fat absorbed in the intestines (chyle) drains through this pathway.

Course of the Thoracic Duct

The thoracic duct must be identified during surgery in the posterior mediastinum. A damaged thoracic duct can leak about 60-200 ml of lymph per hour and could cause serious post-surgical complications. The thoracic duct can also rupture as a result of trauma to the thorax. Such an injury may cause a concurrent tear in the parietal pleural. This can result in a chylothorax, the accumulation of lymph in the pleural cavity. Because of the course of the thoracic duct (described above), a right chylothorax suggests damage in the inferior ½ of the thorax, while a left chylothorax suggests damage in the superior ½ of the thorax.

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Venous Drainage of the Thorax

Karen Krabbenhoft

Key Concepts

Both arteries and veins can anastomose to provide alternative supplies or drainage routes. The superior and inferior venae cavae are connected via the azygos system.

Learning Objectives

- Describe the venous drainage of the thorax, including the collateral collections.

- brachiocephalic veins

- superior vena cava

- azygos system

- posterior intercostal veins

I.  The Azygos System

The azygos system consists of an asymmetrical set of veins flanking the vertebral column. The term azygos means “unpaired.” Although the azygos system is highly variable, it usually includes one vein on the right and two veins on the left:

Azygos vein – the main channel of the system connects the inferior and superior venae cavae. The azygos vein ascends on the right side of the vertebral column and receives blood from the abdominal and thoracic walls, mainly through the posterior intercostal veins. It also receives esophageal and bronchial veins. The azygos vein forms the azygos arch just superior to the hilus of the lung where it turns from its vertical course to pass anteriorly and drain into the superior vena cava.

Hemiazygos vein – ascends on the left side of the vertebral column, posterior to the aorta as far as vertebral body T9. At the T9 level, it crosses the midline posterior to the aorta and esophagus to join the azygos vein. It receives a variable umber of posterior intercostal and esophageal veins.

Accessory hemiazygos vein – begins at the 4th or 5th intercostal vein and descends to cross the midline at T7-T8 to join the azygos vein. The accessory hemiazygos receives a variable number of posterior intercostal veins.

Azygos System

Components and Relationships of the Azygos System

Because the azygos system connects the superior and inferior venae cavae, it provides an alternative drainage pathway for venous drainage. If the inferior vena cava were obstructed, venous blood from the lower half of the body could be shunted to the superior vena cava. Alternatively, if the superior vena cava were occluded, blood from the head and upper extremities could be shunted to the inferior vena cava for return to the heart.

Azygos System Anastomoses

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Branches of the Thoracic Aorta

Karen Krabbenhoft

Key Concepts

In addition to its branches to the head and upper limb, the thoracic aorta has both parietal and visceral branches. Posterior intercostal arteries supply the body wall, while bronchial and esophageal arteries supply thoracic organs.

Learning Objective

- Identify these branches of the thoracic aorta and elucidate their targets:

- arch of the aorta

- brachiocephalic artery

- left common carotid artery

- left subclavian artery

- posterior intercostal arteries

- bronchial arteries

- esophageal arteries

I. Branches of the Thoracic Aorta

There are three major regions of the thoracic aorta:

- The ascending aorta exits the left ventricle and gives rise to the right and left coronary arteries.

- The arch of the aorta branches asymmetrically to form the brachiocephalic trunk and the left common carotid and left subclavian arteries.

- The descending aorta has both parietal and visceral branches to supply the body wall and the internal organs, respectively. Its parietal branches include the posterior intercostal arteries, which supply the thoracic and abdominal walls.  These segmental arteries travel between the ribs with the intercostal veins and arteries. The descending aorta’s visceral branches include the bronchial arteries, which deliver oxygenated blood to the lungs, and the esophageal arteries.

Branches of the Thoracic Aorta

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