MODULE 2

DORSAL VIEWS of the BRAINSTEM

- - In any attempt to look at the dorsal and lateral surfaces of the brainstem, the first problem is that this region is hidden below the cerebral hemispheres and cerebellum. You will recall that the expanding cerebral hemisphere extends caudally to completely cover the diencephalon and midbrain Figure 2-3. How can we expose the brainstem to view? The obvious answer is to dissect away the tissue of the cerebral hemisphere (and of the cerebellar hemisphere, as well) which gets in the way. The most "pure" approach , at least in conceptual terms, would be to cut the relevant fiber bundles and then just lift off the hemispheres (cerebral and cerebellar). A careful look at Figure 2-2 shows where to make the cuts. After making cut #1 (and this is pretty schematic) you would be able to lift off the cerebral hemisphere, leaving behind the basal ganglia, the diencephalon and the brainstem. After making cut #2 you would be able to lift off the cerebellum. The result would be to give you a good exposure of the dorsal surface of the brainstem (plus the basal ganglia and diencephalon). Figure 2-4 gives you this view, and as the course progresses we will draw in many pathways on it. Figure 2-5 is a copy, in which the spinal cord has been cut off, and a great many labels added. Blumenfeld's Figure 12.2 B is similar, but does not include the cerebral hemisphere

- - You are about to be confronted with the names for a great many structures, without being told much about their function. For the moment we have two goals: 1) we want you to begin to relate a name to a structure, so that when we do take up functional issues you will at least have an idea of where the structures are in the brainstem and 2) we want you to begin making the transition from surface view of the brainstem to cross sections through the same region.

- - Now, let's look at some real brains in which the dorsal surface of the brainstem has been exposed. The frame in Figure 2-6 shows the region included in the following views.

• Figure 2-7 shows how the first dissection was carried out (i.e., where the cuts were made).
• Figure 2-8 labels parts of the ventricular system (the body of the lateral ventricle has been left out, so that we can see the structures that form some of its borders) and the subdivisions of the caudate nucleus. Compare it with DiganatA_5.6 and DiganatA_5.7.
• Figure 2-9 shows the course of the choroid plexus and identifies other structures.

- - Switch back and forth between these copies and be certain you can identify the following:

• 1) the septum pellucidum. This thin membrane runs between the corpus callosum and the fornix and separates the anterior horns of the two ventricles from each other.
• 2) the caudate nucleus.
• 3) the stria medullaris (of the thalamus). This is a longitudinal bundle of axons which also marks the point of attachment of the choroid plexus that normally roofs the third ventricle (but is removed in this dissection).
• 4) the habenular nucleus.
• 5) the pineal.
• 6) the anterior medullary velum. This membrane forms the roof of the anterior part of the fourth ventricle.
• 7) the superior and inferior colliculi.

- - A second dissection was photographed at a slightly different angle, thus revealing more of the brainstem.

• Figure 2-10 shows how the cuts were made. In this instance part of the fornix and related structures (the hippocampus and hippocampal commissure - don't worry about them, for now) were left in place on the left side, partially obscuring the dorsal surface of the diencephalon.
• Figure 2-11 identifies a variety of structures.

- - Keep it on the screen and relate it to the views called up in the list below.

• 1) the choroid plexus. You should realize that our dissection has exposed much of the lateral ventricle, as well as the dorsal surface of the brainstem . The choroid plexus remained behind, when the hemisphere was removed, and can be seen passing around into the inferior horn of the ventricle, just as shown in Figure 2-9.
• 2) the diencephalon. The more caudal part of the diencephalon tends to "overhang" the midbrain and may obscure it from view . In Figure 2-5 this part of the diencephalon has been shown on the right but "removed" on the left to reveal underlying structures.
• 3) the brachium of the inferior colliculus. One of the many bundles of fibers which happen to run on the surface of the brainstem and therefore create an identifiable surface landmark. Brachium is one of several names used for structures of this sort. You may want to look at Figure 2-12 for more examples of terms we will begin to use.
• 4) the superior, middle and inferior cerebellar peduncles. Here we see the cut surface of the peduncles. They are, in a sense, embedded in surrounding tissue, and thus their borders are not easily defined in the photograph; they are shown more precisely in Figure 2-5.
• 5) the fourth ventricle. It is exposed to view because the cerebellum and choroid plexus have been removed.
• 6) the nucleus cuneatus, fasciculus cuneatus and tubercle of the nucleus gracilis. These structures lie at the most caudal level of the brainstem, where the medulla becomes continuous with the spinal cord. The tubercle is simply the surface bump, created by the underlying nucleus

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