MODULE 2
THE BRAINSTEM
- - Probably we should start by defining the brainstem. Figure 1-23 shows that it consists of all the brain caudal to the forebrain (excluding the cerebellum) . Practically, however, the thalamus (part of the Diencephalon) is so closely related to the brainstem that we are going to include it in our discussion. In most of our drawings in this module we will follow the color code of Figure 1-23
- - The brain is, from an embryological point of view, the rostral end of the neural tube - continuous caudally with the spinal cord in the region of the foramen magnum. Unlike the spinal cord, in which each segment looks more or less like the next one, regional differences in development give each division of the brainstem its own peculiar appearance. The various names applied to this part of the brain are given in Figure 2-2.
- - How can we expose the brainstem to view, so that we can describe it? You are already familiar with the way in which the cerebral hemisphere develops by evaginating outward from the side wall of the diencephalon. The cerebellum, in contrast, develops as a proliferation of the cells in the edge of the neural fold (the so-called rhombic lip, because it forms a margin of the rhomboid - shaped fourth ventricle) at the pontine level. The cell masses on either side join in the midline to become the vermis; lateral to this are the cerebellar hemispheres. Both the cerebral hemisphere and the cerebellum are "anchored" to the brainstem by massive bundles of fibers (axons) which are, in fact, the input and output pathways of these structures. In the case of the cerebral hemisphere, this fiber bundle is the internal capsule, which you have already studied. The cerebellum has 3 separate fiber bundles of this sort, known as the cerebellar peduncles.