MODULE 1

THE CEREBRAL HEMISPHERE

INTERNAL ANATOMY

- - One of the most difficult topics for students of neuroanatomy to master has to do with the internal anatomy of the cerebral hemispheres. There are actually only a few structures (and names) involved, but the manner in which the brain has developed has caused the relationships between them to be complex. Added to this is the fact that the most convenient way to study the hemisphere involves the use of serial sections - and that no matter what plane of section one picks, the cuts will pass through some structures at odd angles. The end result for the student is confusion which usually does not begin to lift until the last weeks of the course. We hope that by focusing on this subject at the start we will this. But, be prepared for some tough going!

- - Early in the module we felt obliged to say a little about brain development because we wanted to explain how the ventricular system came to be. So, when we last looked at the developing hemisphere, in Figure 1-8, it was just a blister, extending laterally from the diencephalon. In fact, things are more complicated, as shown by a schematic frontal section through the hemisphere and diencephalon, Figure 1-46. As we know, a sheet of cells, the cerebral cortex, lies just below the cortical surface. Underneath the cortex lies the "white matter" - actually a jumble of axon bundles running every which-way. They are colored yellow in Figure 46, but the myelinated axons give a white appearance to the region in fresh brain specimens. Because mylein stains black, the white matter appears black in histological sections through the brain, such as Figure 1-35. As Figure 1-46 shows, buried within the white matter are several nuclei - each of them a region of densely packed neuron cell bodies. As the axons of the white matter try to pass to and from the cortex, and from one cortical region or hemisphere to another, they must pass between and around these so-called deep nuclei. The nuclei are, in a sense, the troublemakers; as a group, they are known as the basal ganglia and they include the caudate nucleus, the putamen, the globus pallidus, the amygdala and the claustrum. The major white matter pathways threading between the nuclei include:

FURTHER DEVELOPMENT

- - If the brain remained as depicted in Figure 1-8, then the internal anatomy of the hemisphere would not be difficult to visualize. But the fact is that as the hemisphere enlarges, it extends back over the diencephalon and midbrain, with part of it then pushing forward again to form the temporal lobe Figure 1-44, Figure 1-45. The results of this revolting (from our point of view) development include the following:

- - In summary, deep within the cerebral hemisphere lie a group of nuclei (nuclei = groups of neuronal cell bodies packed together) called the basal ganglia. These nuclei include the caudate, the putamen, the globus pallidus, the amygdala and the claustrum. The migration of neural tissue, as the hemisphere has enlarged, has caused these nuclei (particularly the caudate and the associated lateral ventricle) to assume odd shapes. Threading between these nuclei are the pathways that connect the cerebral cortex with other regions of the brain. It is useful to be able to visualize the complex anatomy that results in order to interpret radiographic sections through the hemisphere.

- - Of course, you will never see these structures (in real life) as they have been depicted in the above views. Rather, you will see them in radiographic views which are sections through the brain in a variety of planes. To get an idea of what lies ahead call up DiganatA_4.0 ; keep it on your screen and click on the boxes to see a remarkable comparison of gross brain slices and MRI views.

A LOOK AT WHOLE BRAIN SLICES

- - Time to try your hand at identifying some of these structures.

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