Lab 3: Brain Stem 1

Lab Summary

The next two labs will provide you with an understanding of the structural and functional organization of the brain stem. Although the brainstem includes the diencephalon, for the purposes of this lab we will focus on the organization and functions of the most posterior divisions of the brainstem: the medulla, pons, and midbrain.   

You will examine the external and internal morphology of the medulla, pons and midbrain and will review the segmental and inter-segmental functions of the structures and pathways that compose them. We will be inspecting the whole and sagittal brain specimens, and brains in which portions of the cortex and cerebellum have been removed to reveal the entire superficial structure of the brainstem. In addition, you will determine the orientation and serial location of brainstem sections in your slide collection and in the Serial Brainstem Series: 

SECTION 1: Whole Brain Study 

SECTION 2: Brainstem Preparation Study 

SECTION 3: Orientation of Brainstem Sections 

SECTION 4: Ascending and Descending Spinal Tracts  

SECTION 5: Review 

Whole Brain Study

Use the whole brain and sagittal half brain specimens in conjunction with the sagittal, dorsal and ventral brainstem diagrams (Figure 3.1A-E) to review the borders and major surface features of the medulla, pons and midbrain that you learned in the first lab. 

Use your sagittal and whole brains to identify each of the cranial nerves (Figure 3.1D-E), particularly noting their position with respect to each brain division (e.g., which nerves are found at the junction of the medulla and pons?) and also their relation to the surface contours of the brainstem, such as the areas overlying the pyramids, or the inferior olive. Which cranial nerve arises from the dorsal surface of the brainstem?

Review your notes from the brain stem lectures and determine in which region(s) of the brainstem, i.e. medulla, pons, midbrain, ventral, dorsal, etc. you will find nuclei that contain the sensory or motor neurons related to the different functional components of each cranial nerve. Do not look for these nuclei internally now as we will do that in lab 4. In this lab you should try to gain an understanding of how each cranial nerve relates to the 3 divisions of the brainstem, the functions associated with each nerve and what loss of function will occur from damage to the nerve in the periphery? For review, label the cranial nerves on the dorsal, ventral and sagittal views in Review 3.1.

Blood Supply

Use Figure 3.2A-B and your whole brain specimen to locate each of the major arteries and branches on the base of the brain. These include: 1) the vertebrals; 2) branches (identify major branches) from the vertebrals and basilar artery to the cerebellum, pons and cortex; and 3) branches off the Circle of Willis and the arteries to the cortex arising from the Circle of Willis. Blood vessels supplying the medulla and pons include the anterior and posterior spinal arteries, branches of the vertebral artery, the paramedian, long circumferential, and short circumferential branches of the basilar artery, and the posterior inferior cerebellar artery. Branches of the basilar artery, including the posterior cerebral and superior cerebellar arteries, the posterior communicating arteries, and the posterior choroidal arteries, supply the midbrain. Branches from these form the paramedian arteries on both sides of the midline and the long and short circumferential arteries to the dorsal and lateral regions. Examine Figure 3.2C to aid you in determining which general regions of the brainstem each supplies. In the next lab, you will learn which internal structures each artery supplies.

Test yourself on the arteries of the brainstem using the unlabeled figures in Review 3.1. You may want rewatch the youtube video reviewing the arteries of the circle of willis and their distribution – Intracranial Arterial Anatomy – Circle of Willis. 

Brainstem Preparation Study

Ask your instructors to go over the brainstem specimens that have been specifically dissected to allow you to see areas of brainstem not otherwise visible in the whole and sagittal brains. Strategic cuts through the cortex and cerebellum have been made to reveal the areas of the brainstem medial and ventral to the hemispheres. The meninges have been stripped from that half to further reveal surface landmarks. 

Using these dissections, the plastic brainstem model and Figure 3.3, look at the dorsal surface and identify: the cuneate and gracile fasciculi and the tubercles formed by underlying nuclei, the inferior (ICP) and middle cerebellar (MCP) peduncles and the stria medullares of the medulla. The stria medullares mark the approximate border between the medulla and pons on the dorsal surface. On the floor of the fourth ventricle find the trigones formed by the hypoglossal, vagal, facial and vestibular nuclei. Find the superior cerebellar peduncles (SCP) and the anterior (superior) medullary velum that stretches between them to form the roof of the anterior portion of the fourth ventricle. In the midbrain identify the superior (SC) and inferior colliculi (IC) and their brachii that are formed by the fibers entering the superior colliculus from the retina and fibers of the inferior colliculus passing to the medial geniculate nucleus of the thalamus, respectively. Note the positions of the pineal gland which is important in the generation of circadian rhythms and the habenula which is a limbic system structure that is thought to play a role in limbic mediated aspects of reproductive behavior, sleep-waking cycles and learning. Trace the continuity between fourth and the third ventricles via the cerebral aqueduct and note the presence of choroid plexus (Figure 3.4A).

Using the plastic brainstem model, the whole brain and brainstem specimens Figure 3.4B, look at the ventral surface and identify: the interruption of the anterior (ventral) median fissure by the pyramidal decussation (very often crossing fibers are visible); the pyramids and olives; the massive transversely oriented fibers that form the base of the pons and coalesce laterally to form the middle cerebellar peduncles; and the interpeduncular fossa. 

Orientation of Brainstem Sections

In Figure 3.6 are fourteen myelin stained (Loyez method) sections. When used in conjunction with the brainstem preparation and the sagittal half brains, you should be able to begin to develop a three-dimensional view of the brainstem structures. Using the sections in the Figure 3.6 (Serial Brainstem Series) locate the plane of each section through the brain. This is best done by concentrating on the external features of the brainstem for clues and thus it is possible (and necessary!) for you to orient these sections before progressing to their internal organization in the next lab. This skill is useful because sections are often cut at odd angles through the brainstem, e.g., the dorsal part may pass through the pons but the ventral part through the medulla.

An image of a sagittal half-brain can be found in Interactive 3.1. Use this figure to make a record of the orientation of each slide for reference in next week’s lab. Draw a line through the brain representing the plane of each section and label it with the slide number. Try to be as accurate as possible; in many cases the angle will be different for each slide and the slides may not all be in the proper rostral-caudal order. The answer key is found at the end of Figure 3.6.

You may want to gain additional experience using external features and landmarks to align sections within the brainstem. Ask your instructor to provide you with a slide box containing a series of sections similar to that used in Figure 3.6. Using a down projecting microscope that will be provided to you, align these sections to the sagittal drawing of the brain stem. This will provide you additional resources for locating the internal structures of the brainstem in lab 4. Since the sections in each slide box may differ in orientation and order it is important that you use the same slide box for both this week and next week’s labs. Record the number of your slide box. Remember that these sections are stained for myelinated fibers, thus, myelinated axons are darkly stained and the gray matter will be pale. 

LANDMARKS 

Use the following brain stem landmarks and the information in the table to orient your slides: 

pyramidal decussation – a slanting cleft on the ventromedial surface 

pyramids – paired bulges on the ventromedial surface 

olives – bulges on the ventrolateral surface 

gracile tubercles – paired bulges on the dorsomedial surface 

cuneate tubercles – paired bulges on the dorsal surface, not on the midline 

fourth ventricle – changes shape throughout the brain stem; from the obex it gradually widens to lateral recess, then closes down rostrally to form the cerebral aqueduct 

choroid plexus – only formed over the medulla where the roof plate was not covered by cerebellum 

hypoglossal trigone – small mounds in midline formed by hypoglossal nuclei on floor of fourth ventricle 

vagal trigone – small mounds formed by vagal nuclei lateral to hypoglossal trigone 

vestibular area – mounds on the floor and sides of the fourth ventricle overlying vestibular nuclei 

pons – large basal bulge that is widest at mid-pons level, and thickest (dorso-ventrally) in the rostral pons 

inferior cerebellar peduncles – form large “shoulders” on the dorsolateral surface 

middle cerebellar peduncles – massive structures on lateral surface with cerebellum attached dorsally 

superior cerebellar peduncles – form roof over fourth ventricle, separated by thin superior medullary velum 

inferior colliculi – more egg shaped, rounder than superior colliculi 

superior colliculi – wide, slightly flattened bulges on dorsal surface over aqueduct 

Table 3.1 Landmarks useful for identifying levels of the brainstem

Orientation of Brainstem MRI & CT Sections

Examine the MRI Images and CT Images in Figure 3.7 as you did with the myelin stained sections, and locate the plane of each section through the brainstem. Concentrate on the external features and shape of the brainstem for clues as you orient these images. Draw theses sections on the sagittal half-brain image in Interactive 3.2. Discuss your results with your lab instructor. What landmarks were most useful?

When you are comfortable identifying levels of the brainstem in the MRI images, test your 3 dimensional understanding of the brainstem using Review 3.2 to try to match sections across the axial, sagittal and coronal planes of section. You can gain additional practice with this skill by using the materials available at the Harvard Whole Brain Atlas website using the Normal Anatomy in 3-D with MRI/PET viewer (http://www.med.harvard.edu/AANLIB/cases/caseNA/pb9.htm). 

Ascending & Descending Spinal Cord and Brainstem Tracts

Review the tracts of the spinal cord using the Ascending Spinal Cord Tracts and Descending Spinal Cord Tracts tables at the end of this section and in Figure 3.8.

Follow these tracts in the Serial Brainstem sections in the Interactive 3.3 brain stem sections that you put in sequence earlier in this lab. 

Review

Short Answer Questions