MODULE 2

DIVISIONS of the BRAINSTEM

- - You are now ready to put your knowledge of surface anatomy to use in identifying the level of cross sections through the brainstem. The lines in Figure 2-19 serve to define the borders between divisions of the brainstem. In Figure 2-21 the 4 lines drawn on the mid-sagittal view show the plane of cut for the sections in the figure. In each section, a few of the landmarks that help to identify the level of the section are given. For example, the presence of the colliculi, the cerebral aqueduct and the cerebral peduncle in the section shown in the lower left part of the figure tell you that this section HAS TO BE through the midbrain. However, because the slides in the Cornell slide series cut through most of the brainstem at an oblique angle, we will rarely sections that look exactly like these "typical" ones.

- - Before you look at the slides we also want to introduce you to three terms that we will use starting with Module 3. They are shown in our last mid-sagittal view of the brainstem - Figure 2-20.

----- The TEGMENTUM refers to the central core of the brainstem that extends from the medulla through the midbrain. Most (but not all) of the ascending sensory and descending motor pathways of the brain pass through this region. Cranial nerve nuclei are found here and a diffuse collection of neurons, called the reticular formation, occupies much of this region.

----- At the pontine level, a large mass of neural tissue, called the BASILAR PONS, lies ventral to the tegmentum. We will discuss it when we do the cerebellum.

----- At the midbrain level, the TECTUM lies dorsal to the tegmentum. It consists of the superior and inferior colliculi.

- - To give you a way to start in identifying some of the structures we have shown you on brainstem sections, we give you a list of 25 structures or spaces below, followed by three figures in which all of the items on the list have been identified by number. Work back and forth between the list and the views and see how many structures you can eventually identify on your own. If you want to be reminded of where in the module you saw a structure, click on it to jog where you saw it. If you want to see other views of the structure, go to "I want to" and click on the module 2 glossary.

STRUCTURES TO BE IDENTIFIED

• 1) PYRAMID - part of the major descending motor pathway
• 2) CENTRAL CANAL of SPINAL CORD - will open into the fourth ventricle shortly
• 3) FASCICULUS CUNEATUS - part of a major ascending sensory pathway from the upper body
• 4) NUCLEUS GRACILIS - part of a major ascending sensory pathway from the lower body
• 5) CEREBELLUM - essential for carrying out movements smoothly
• 6) INFERIOR OLIVE - neurons here send axons to the cerebellum through the inferior cerebellar peduncle
• 7) FOURTH VENTRICLE - the most caudal of the ventricles; CSF enters it by descending through the cerebral aqueduct
• 8) CHOROID PLEXUS - forms a roof for the third and fourth ventricles; secretes some CSF
• 9) TEGMENTUM OF THE MEDULLA - ascending pathways, descending pathways, the reticular formation and cranial nerve structures are found here
• 10) INFERIOR CEREBELLAR PEDUNCLE - carries input to the cerebellum from the spinal cord and inferior olive
• 11) MIDDLE CEREBELLAR PEDUNCLE - carries input to the cerebellum from the cerebral cortex
• 12) SUPERIOR CEREBELLAR PEDUNCLE - the output pathway for the cerebellum
• 13) ANTERIOR MEDULLARY VELUM - passes between the two superior cerebellar peduncles and forms a roof over the anterior part of the fourth ventricle
• 14) TEGMENTUM OF THE PONS - see 9)
• 15) BASILAR PONS - lies "underneath" the tegmentum of the pons - contains descending motor fibers and nuclei that send axons to the cerebellum by way of the middle cerebellar peduncle
• 16) INFERIOR COLLICULUS - part of the ascending auditory pathway
• 17) SUPERIOR COLLICULUS - part of the visual system
• 18) CEREBRAL AQUEDUCT - connects the third and fourth ventricles
• 19) TECTUM - name for the region containing the superior and inferior colliculi
• 20) BRACHIUM OF THE INFERIOR COLLICULUS - part of the auditory pathway
• 21) LATERAL GENICULATE NUCLEUS - part of the visual pathway - most optic tract fibers terminate here
• 22) MEDIAL GENICULATE NUCLEUS - part of the auditory pathway
• 23) CEREBRAL PEDUNCLE - contains descending motor fibers, and a pathway from the cerebral cortex to the cerebellum
• 24) INTERPEDUNCULAR FOSSA - a space
• 25) THALAMUS - part of the diencephalon
• 26) THIRD VENTRICLE - CSF enters from the lateral ventricles and exits by way of the cerebral aqueduct

- - To make the jump from surface anatomy, first look at Figure 2-22. This shows, in some detail. the plane of section of the slides we will be working with for the rest of the course.

- - Next, call up the following three composite images. Most of the structures labeled on these slides are ones you have identified in earlier views of this module. Try to reconcile the appearance of the structure here with that in views of the brain's surface. All the sections, except the 3 spinal cord ones, are clickable. Clicking on one leads you to enlarged view of the section, with much more detail.

• Figure 2-23
• Figure 2-24
• Figure 2-25

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