Lab 18: The Pectoral Girdle
By the end of this lab, students will be able to:
- Recognize the different categories of muscles with respect to movement.
- Describe the different types of muscle contractions
- Identify, describe, and palpate the joints of the pectoral girdle.
- Identify the rotator cuff muscles and describe their action, origin, insertion, and innervation.
- Identify and describe the posterior muscles acting on the shoulder girdle.
- Identify and describe the anterior muscles acting on the pectoral girdle.
Terms to Know
Joints of the Pectoral Girdle
Muscles Acting on the Pectoral Girdle
Today in lab you will explore the joints and muscles of the pectoral girdle. Using the iPads Atlas, you will examine the joints that make up the pectoral girdle and shoulder as well as the muscles acting on these joints. Make good use of the animations to help you understand movement of the scapulothoracic joint and the movements caused by each muscle listed. While exploring in the iPads you will also examine radiology of the pectoral girdle and shoulder. You should compare what you are seeing on the iPads to what you see in the radiological images to help you understand what you are seeing using radiology. The Navigator tool will be available to view both the joints and muscles, and some of the muscles are visible on the 3D printed model. You should also explore the cross-sectional images using this tool.
You will also have the opportunity to explore these muscles on the human upper extremity tissue. Please treat this tissue with great care and respect. It is ok if you feel an emotional response when seeing this tissue for the first time. Some students may feel that the hands are more personal than the organs you have examined earlier this semester, and it is ok if you need to take a minute to feel comfortable examining this tissue. Know that these individuals donated their tissue because they want you to learn everything you can from them. Your TA can help you work through these emotions so that you can get the most out of this incredible opportunity.
Keep in mind that all structures are not visible using all tools, and that is OK. By the end of the lab today you should have identified all of the structures on the list using multiple sources. As a study tip, you may find it helpful to write a description of each muscle/structure as you identify them. When you write it in your own words, it may spark your memory as you look for that structure the next time you study.
Activity 1: Visible Body app and Radiology-Joints and Muscles of the Pectoral Girdle and Shoulder
First, explore the muscles and joints of the pectoral girdle using the iPads.
- Click on the Regions tab and scroll to 8. Axilla. Under the systems tab on the left side of the screen, click on the muscular system so the whole arm is highlighted. Also, while under the systems click on the arterial, venous, nervous and lymphatic systems twice to remove them, since we will not be looking at these systems today.
Explore the muscles of the pectoral girdle and arm listed below. Some of these muscles will be deep to others, thus you will have to remove the larger more superficial muscles (pectoralis major, deltoid, trapezius and platysma) in order to see the deep muscles.
You will be asked to identify the origins, insertions, actions, and innervations of these muscles (this will be listed in the app after you have highlighted the structure and click the book). However, you may not need to know certain specifics for each muscle. Follow what is on the posted muscle charts for the specifics of what you need to know for each muscle. If you would like to see the muscle(s) in action, there are two means to see this, the first is to click on the red pin when you have highlighted the muscle, this will drop down motion videos. The second way to see muscle actions is to go back to the home screen of the app and select the Muscle Actions tab. From here, you will be able to see motions of the shoulder and within those motion sequences, you can individually select muscles to see their action.
- Levator scapulae
- Rhomboid major
- Rhomboid minor
- Pectoralis minor
- Serratus anterior
- Pectoralis major
- Latissimus dorsi
- Teres major
- Rotator cuff muscles
- Teres minor
- Biceps brachii
- Triceps brachii
- Click on the Systems tab under the Skeletal System Views, select 14. Shoulder Girdle. The “shoulder joint” is actually made up of three joints and has many bursae. The bursae in the app will appear purple, you can select these structures and hide them as we will not hold you responsible to know them. The app will also show a few more ligaments then we have listed for you. You will only be responsible for knowing those ligaments listed below. Examine the three joints and the structures listed with them. Highlight the structures and observe the functions listed under the book icon. You may also find it interesting to look at the injury and clinical relevance (stethoscope icon). Sometimes it can be helpful to attach clinical meaning to these structures when trying to remember them.
- Acromioclavicular Joint
- Acromioclavicular ligament
- Coracoclavicular ligament
- **shown as 2 ligaments; conoid portion and trapezoid portion
- Coracoacromial ligament
- Glenohumeral Joint
- Articular capsule
- Glenoid labrum: This will be deep, remove the capsule and other ligaments of the shoulder to see this structure
- Scapulothoracic joint (you cannot really highlight this joint, but you can see how the scapula articulates with the ribs)
- Click on the Muscle Animations tab (on the home page) and scroll to scapular motions to view the muscles that act on the scapula. Explore the animations of elevation/depression, protraction(called abduction in app)/retraction (called adduction in the app), and upward and downward rotation of the scapula.
- While under the Muscle Animations, you can also click on shoulder motions and observe the animations of abduction/adduction, horizontal abduction/horizontal adduction, external(lateral)/internal(medial) rotation, and flexion/extension of the shoulder.
- Now open the Shoulder radiology PowerPoint. This will be in an open documents window on the screen. Explore the muscles and joints listed above in the radiological images. Use the iPad renderings to help you understand what you are seeing in the radiological images.
Activity 2: Navigator and Plastinate
You will use the navigator to examine the anatomy of the pectoral girdle. Some of the muscles and joint structures are visible on the 3D printed model, while others can be seen on the computer model. You should also examine the anatomy of the shoulder complex in the cross-sections. You can also examine the plastinated tissue in this station and compare the plastinated pectoral girdle to what you see in the Navigator computer model and cross sections.
**NOTE: DO NOT save any presets as you are examining these muscles.
- On the computer model, click on “View,” and use the presets to explore the muscles of the pectoral girdle in the list of terms on page 1. You can also click on “Advanced” in the View window to isolate specific muscles or structures. Uncheck pectoralis major so that you can view the pectoralis minor and the subclavius. You should be able to identify most of the muscles in the list of terms, although some are cut because the model only shows the superior portion of the arm.
- One preset shows the computer model with the muscles removed. Examine the ligaments stabilizing the joints of the pectoral girdle.
- Make sure that the highlighter feature is active. If you do not see green highlighted structures as you move the stylus around the shoulder of the 3D printed model, click on this icon, furthest to the right on the toolbar. Also, ensure that you can see two views at the top of the screen, one in the transverse plane and one that is perpendicular to the stylus. Hold the stylus in the sagittal plane and move it around the shoulder region of the 3D printed model. Observe the head of the humerus, scapula, and rotator cuff muscles in the cross sections. As you move the stylus, you will see two intersecting lines on the cross sectional image. As those intersecting lines move over certain structures, those structures will be highlighted. Move the stylus towards the posterior aspect of the model and observe the posterior muscles acting on the pectoral girdle: the trapezius and rhomboids (superiorly) and the latissimus dorsi (inferiorly). Remember, you can freeze the images at any point by pressing the black button on the stylus. Press the black button again to unfreeze the image.
- Spatial relationships are key! Understanding the relationships between structures (e.g. posterior, anterior, medial, lateral, superior, inferior, superficial, deep) will help you understand what you are viewing at each level. For example, with the posterior muscles acting on the pectoral girdle, if you know that the trapezius is superficial (posterior) to the rhomboids, and you position the stylus at the superior thoracic level of the 3D printed model, you should be able to identify that the trapezius is the muscle closest to the skin in the middle back, while the rhomboid muscles would be just deep to the trapezius
- Examine the plastinated upper extremity tissue. Now that you have examined the muscles using the navigator, you can identify several of these muscles on the plastinated arm. Not all muscles are visible in this dissection. The vessels and nerves on this specimen are very fragile. Please DO NOT touch this tissue. You should ask the TAs to go through these muscles with you and move the tissue. You can compare what you see on the plastinated arm to what you viewed on the computer model and cross-section images on the Navigator.
Activity 3: Upper Extremity Tissue
There are four arms that have been dissected for you to examine in the lab. You are able to view all of the muscles on the list of terms either in part or on whole on this tissue. You should wear gloves while handling this tissue. We also have plastic aprons that you can wear while examining this tissue, as it will be wet and may drip. You should look at all arms during each lab. Two arms have been dissected more superficially, while the others have been dissected so that you can identify some deeper structures. Each set of arms comes from the same person, there is anatomical variation between them, including an interesting anomaly that you will examine in this and another lab. You could be asked to identify most of these structures on either arm. It can be helpful for you to have the muscle charts with you while you are examining these structures. Looking at the orientation of muscle fibers and the origin/insertions can help you understand the actions of these muscles.
Handle this tissue with care and respect! Also, be gentle when moving the specimen and feeling the muscles. We need to keep these specimens for many semesters, so please do not pull to hard or damage any of the structures.
- Start by looking at the posterior aspect of the arm.
- Feel for the spine of the scapula. Extending superiorly from the scapular spine, examine part of the trapezius. This muscle has been cut, but you can see its fibers inserting on the scapular spine and, on the superficial dissection, wrapping around anteriorly and inserting on the lateral aspect of the clavicle.
- Along the medial border of the scapula, you can identify three muscles that have been cut. From inferior to superior, these muscles are the rhomboid major (larger), rhomboid minor, (thinner), and levator scapulae (also extending from the superior angle of the scapula).
- Move the trapezius back and observe the supraspinatus muscle sitting just deep to it. It has been partially retracted in the deep dissection, so you can also pull this muscle back to better appreciate how it sits in the supraspinous fossa. Be gentle when retracting the supraspinatus on the deep tissue, as there are nerves and vessels attached to the deep side of this muscle.
- Inferior to the scapular spine, observe the infraspinatus. This muscle can also be retracted in the deep dissection to observe its position in the infraspinous fossa. Again, be careful when retracting this muscle, as there are nerves and vessels attached to its deep aspect.
- Just inferior to the infraspinatus, observe the teres minor. This will be best viewed on the superficial dissection. The teres minor, infraspinatus, and supraspinatus are all part of the rotator cuff group and are the three rotator cuff muscles located on the posterior aspect of the scapula.
- Inferior to teres minor, observe the larger teres major. Also observe the latissimus dorsi just inferior to teres major, which is cut in this dissection. Observe these muscles on the posterior aspect of this tissue, and then turn the tissue so you can observe it from an anterior view. Now notice how the latissumus dorsi and teres major both run to insert on the anterior aspect of the humerus. This is important to appreciate so that you can understand how these muscles contribute to internal rotation of the glenohumeral joint.
- Observe the deltoid on the lateral aspect of the shoulder. This muscle wraps around the shoulder superiorly and comes together at a common insertion point on the humerus (the deltoid tuberosity). Notice how the orientation of the fibers changes as it wraps around the shoulder from anterior to posterior. This can help you to understand why the functions of the posterior, middle, and anterior fibers of the deltoid differ from each other.
- Observe the muscle on the anterior aspect of the scapula that has been cut but has an attachment point on the medial border of the scapula. This is serratus anterior. This muscle extends from the anterior aspect of the medial border of the scapula and wraps anteriorly around the thoracic cage to insert on the ribs. Think about how its structure contributes to its function of stabilizing the scapula.
- On the anterior aspect of the scapula, observe the subscapularis muscle. This is the fourth rotator cuff muscle, and it sits in the subscapular fossa.
- Observe pectoralis major and pectoralis minor. Both have been cut. Pectoralis major is larger and superficial to pectoralis minor. Pectorals major has an insertion on the humerus, while pectoralis minor inserts onto the coracoid process of the scapula. As a result, pectoralis major acts on the glenohumeral and scapulothoracic joints, while pectoralis minor only acts on the scapulothoracic joint.
- Notice a small amount of muscle that is located on the inferior aspect of the clavicle. This muscle has been cut, but what you can see is the superior most portion of the subclavius.
- Now you will start examining the muscles of the arm that cross the glenohumeral joint. First observe the coracobrachialis. This muscle originates on the corocoid process of the scapula and inserts on the humerus. This muscle is normally pierced by a nerve, the musculocutaneous nerve, in the middle portion of the muscle belly. However, notice how it differs on in the deep dissection. In this case, an earlier portion of the brachial plexus (which you will learn about next week) also pierces this muscle, giving it the appearance of having two muscle bellies. Anatomical variations are very common! This is just the first of several variations you will see in this course.
- Now observe the biceps brachii. This muscle has two heads. The long head originates on the supraglenoid tubercle of the scapula. Follow it as high as you can on the deep dissection, towards the head of the humerus. The short head originates on the coracoid process of the scapula.
- Just lateral to the biceps brachii observe the brachialis. This muscle is the primary flexor of the elbow.
- On the posterior aspect of the arm, observe the triceps brachii. The long head is superficial and in the middle, and this is the only portion of the tricpes brachii that crosses the glenohumeral joint. Also observe the medial and lateral heads.
- Observe the acromioclavicular joint on these dissections, you can palpate where the clavicle meets the acromion. On the superficial dissection you may be able to see the acromioclavicular and coracoclavicular ligaments.
- On the deep dissection you can also see the coracoacromial ligament running from the acromion process to the coracoid process of the scapula. Observe how the supraspinatus runs underneath this ligament as it moves towards the greater tubercle.
- Finally, look at the posterior aspect of the deep dissection. The infraspinatous has been cut and the articular capsule has been opened so that you can observe the internal aspect of the glenohumeral joint. Notice how smooth the articular cartilage on the head of the humerus is. Also note how thick the articular capsule is. Lastly, observe the ring of tissue just surrounding the glenoid fossa. This is the glenoid labrum.