Introduction

In this lab, we continue discussing the brain by examining the brainstem, cerebellum, and cranial nerves. We will also explore the bones, landmarks, fossa, and sutures of the skull. We combine the cranial nerves and skull into one lab because the cranial nerves will travel through the foramina of the skull that you will learn about. This material lends itself well to methodical and regular studying, and be sure to study the foramina and cranial nerves that travel through them together.

Lab Activities

Activity 1: Brainstem and Cerebellum

Brainstem:

The three major regions of the brainstem, from superior to inferior, are the midbrain, pons, and medulla oblongata. The brainstem contains important nuclei (collection of cell bodies) that communicate with the cerebrum, cerebellum, and spinal cord, as well as nuclei with cell bodies of the neurons that form the cranial nerves. It also contains sensory and motor pathways connecting the cerebrum and the spinal cord.

Observe the following major brainstem structures.

• Locate the pons on the ventral surface of the brainstem. The pons can be easily identified by its anterior bulge. 
• Rostral to the pons, observe the midbrain. This structure is somewhat hidden by the temporal lobes. It is located just inferior to the thalamus, which is best viewed in the midsagittal section. Observe the following structures of the midbrain:
  • The midbrain surrounds the cerebral aqueduct, which is the pathway that CSF travels between the third and fourth ventricles. This is best seen on a midsagittal view.
  • The cerebral peduncles are located on the ventral side of the midbrain. These white matter pathways carry fibers of the corticospinal tract, originating in the primary motor cortex and descending fibers from other cortical regions. The space between the cerebral peduncles is called the interpeduncular fossa.
• Caudal to the pons, examine the medulla oblongata. The medulla is continuous with the spinal cord. Identify the following on the medulla oblongata:
  • Locate the pyramids, the eminences of the corticospinal (pyramidal) tract that appear as tube-like bulges on the ventral aspect of the medulla oblongata. The pyramids are visible from their emergence at the pons to the pyramidal decussation at the junction between the medulla oblongata and the spinal cord. Identify the pyramidal decussation where the sulcus between them disappears briefly. This is the decussation of the fibers of the corticospinal tract.

Cerebellum:

The cerebellum coordinates voluntary movements and helps to maintain posture and equilibrium. It receives information from the primary motor cortex of the frontal lobe about the intended movement, compares this intended movement with the actual positioning and movement of the body, and sends signals back to the frontal lobe to resolve any differences between the intended movement and current position. The cerebellum is also involved in coordinating higher cognitive functions and emotions.

Locate the cerebellum and observe the two hemispheres. Also, observe the numerous fine grooves, or sulci, on the surface of the cerebellum running transversely. These are called the folia, and they are similar to the gyri of the cerebrum.

Activity 2: Cranial Nerves

Cranial Nerves:

The cranial nerves are peripheral nerves that carry a variety of sensory and/or motor information from/to the head and neck. Ten of the twelve cranial nerves are associated with the brainstem, while cranial nerves I (olfactory) and II (optic) are associated with the cerebrum and thalamus, respectively. The functions of each of these muscles are described here. As part of these descriptions, we will mention structures that you haven’t learned yet. However, we will discuss all of these structures by the end of this unit.

• Olfactory Bulbs and Tracts – CN I: The olfactory nerves arise in the olfactory epithelium of the nasal cavity and course dorsally to the olfactory bulb inferior to the frontal lobe of the cerebral hemisphere. The actual nerves were lost during the removal of the brain. Observe the olfactory bulb, where these nerves synapse on the inferior aspect of the frontal lobe. The olfactory tract originates in the olfactory bulb and runs caudally on the ventral aspect of the frontal lobe.
• Optic Nerves – CN II: The optic nerves begin in the retina of each eye. These nerves course posteriorly and are united in the optic chiasm. The fibers split again immediately posterior to the optic chiasm and extend posteriorly as the optic tracts.
• Oculomotor Nerves – CN III: These motor nerves emerge in the interpeduncular fossa and innervate 4 of the 6 extraocular muscles: the medial rectus, superior rectus, inferior rectus, and inferior oblique.
• Trochlear Nerves – CN IV: These are the only nerves to emerge on the brainstem’s dorsal aspect. They travel around the sides of the midbrain and pons to innervate the superior oblique. You can find these tiny nerves between the lateral rostral pons and the cerebrum.
• Trigeminal Nerves – CN V: These large nerves are the only cranial nerves to emerge from the lateral aspect of the pons. This nerve innervates the muscles of mastication (chewing) and conveys somatic sensation from the entire face.
• Abducens Nerves – CN VI: These nerves emerge near the midline at the border of the pons and the medulla oblongata. They innervate the lateral rectus muscle of the eye.
• Facial Nerves – CN VII: These mixed motor and sensory nerves emerge from the junction of the pons and medulla oblongata lateral to the emergence of the abducens nerves. They carry sensory information from the taste buds of the anterior two-thirds of the tongue and innervate the muscles of facial expression, the lacrimal glands of the eye, and most salivary glands.
• Vestibulocochlear Nerves – CN VIII: These nerves, which carry both auditory and vestibular information, enter the brainstem lateral to (right next to) the facial nerves.
• Glossopharyngeal Nerves – CN IX: These nerves are composed of the most rostral of a series of rootlets that emerge posterior to the olive on the lateral aspect of the medulla oblongata. They receive sensory information from the tonsils, pharynx, middle ear, and posterior tongue and innervates the stylopharyngeus muscle and parotid gland.
• Vagus Nerves – CN X: These nerves emerge caudal to the glossopharyngeal nerves in the same series of rootlets. The vagus nerve has several functions, including providing parasympathetic innervation to organs of the thorax and part of the abdomen and the innervation of muscles of the larynx and pharynx.
• Spinal Accessory Nerves – CN XI: These nerves, which innervate the trapezius and sternocleidomastoid muscles, emerge caudal to the vagus nerve.
• Hypoglossal Nerves – CN XII: These motor nerves innervate the musculature of the tongue. They emerge from the ventral aspect of the caudal medulla as a series of rootlets in the groove just lateral to the pyramids.

Tip: The cranial nerves that have only motor functions (oculomotor, trochlear, abducens, and hypoglossal) emerge medially compared to the other cranial nerves. You cannot see this with the trochlear nerve because it emerges on the brainstem’s dorsal aspect, but you can notice this with CN III, CN IV, CN VI, and XII.

Review the summary of cranial nerves chart. The chart describes the cranial nerves, their passageways through the skull, and their functions.

Activity 2: Skull – Bones and Passageways

Identify the bones of the skull, major features of the skull, and the passageways through the skull.

• First, observe the bones that make up the majority of the external aspect of the cranium: the parietal bones, occipital bone, temporal bones, and frontal bone.
  • On the temporal bone, identify the mastoid process and styloid process. These are important sites of muscle attachment.
  • Look at the inferior aspect of the skull. At the inferior portion of the occipital bone, which makes up most of the inferior portion of the cranium, observe the occipital condyles. These condyles articulate with the atlas of the vertebral column.
  • Observe the places where these bones meet. These are called sutures.
    • The coronal suture is where the frontal bone meets the two parietal bones.
    • The sagittal suture is where the right and left parietal bones meet.
    • The lambdoid suture is between the parietal bones and occipital bone.
    • The squamous suture is between the parietal bone and temporal bone on each side.
• Then observe the bones that make up the face.
  • The frontal bone is the bone of the forehead.
  • The zygomatic bone makes up the anterior cheek. The prominent part of the zygomatic bone meets part of the temporal bone laterally to form the zygomatic arch, which makes up our cheekbone.
  • The nasal bone forms the superior portion of the nose.
  • The maxilla forms the upper portion of the jaw and medial, a portion of the cheek.
  • The mandible is the lower jaw bone. This is the part of the jaw that moves when we chew or speak. The anterior tip of the mandible that forms the chin is called the mental protuberance. Also, observe the articulation between the mandible and the temporal bone. This is the temporomandibular joint. The posterior structure that articulates with the temporal bone is the condylar process. The flat portion protruding up just anterior to this is the coronoid process of the mandible.
• There are a few bones that are best viewed from the internal aspect of the cranium.
  • First, observe that there are three bowl-like spaces formed by bony landmarks within the skull. These spaces are called the anterior cranial fossa, middle cranial fossa, and posterior cranial fossa.
  • The ethmoid bone is a small anterior, medial bone located within the anterior cranial fossa. You can identify it by the several small holes in it.
  • Just posterior to the frontal and ethmoid bones, observe the sphenoid bone. This bone has a ridge that separates the anterior cranial fossa from the middle cranial fossa. The temporal lobes of the brain sit in the middle cranial fossa.
  • The rest of the middle cranial fossa is made up of the temporal bone. The posterior aspect of the temporal bone has a ridge that separates the middle cranial fossa from the posterior cranial fossa. The posterior cranial fossa is made up of the occipital bone, and this space holds the cerebellum.

There are additional bones of the skull and additional features of these bones that we will discuss in this unit, such as the orbit, nasal cavity, and oral cavity. However, we will discuss these bones and features with other labs.

Many nerves and vessels enter and leave the skull. Therefore, there are many passageways within the skull. You will be asked to identify several of these passageways. Additionally, you will be asked to know which nerves or vessels travel through these passageways for many of them.

The stylomastoid foramen cannot be observed on the internal aspect of the skull. This foramen is visible between the mastoid process and the styloid process of the temporal bone.

**Note: The trigeminal nerve gets its name because it divides into three branches (the name means “three twins”). The different branches pass through different foramina of the skull. You do not need to know or identify the branches of the nerve (in parentheses in the chart) in this lab. You will just be asked to know that a branch of the trigeminal travels through specific foramina above.

The clinical connection for this lab reviews assessing the cranial nerves and provides a great tool to remember the cranial nerves. Knowing the function of the cranial nerves helps clinicians assess the dysfunction of the cranial nerves. If dysfunction is found in any cranial nerve, further testing should be performed. The second video provides you an excellent way to draw and remember the location of the cranial nerves. Understanding where the cranial nerves affect the head and neck will help you remember their functions.