Sheep Brain Dissection : Lab 5

SHEEP BRAIN DISSECTION : LAB 5 Make your final coronal cut immediately anterior to the splenium of the corpus callosum. Just as the fornix and the caudate nucleus are "C-shaped structures, the hippocampus is also. Thus, it can sometimes be seen in two places, depending on the plane of dissection . Both the dorsal and ventral aspects of the hippocampus are readily apparent at this level. Notice a band of fibers connecting the two regions of the hippocampus; these axons are called the fimbria. The long narrow space just dorsal and lateral to the fimbria is the lateral ventricle. The caudal region of the thalamus, the metathalamus is seen in this plane of dissection. Two important nuclei found in this region of thalamus are the lateral geniculate nucleus and the medial geniculate nucleus. The location of these two nuclei is ill-defined. The lateral geniculate nucleus is located dorso-laterally, slightly below the fimbria. Immediately ventral to the lateral geniculate nucleus is the medial geniculate nucleus. Make note of the general location of these two nuclei and take care to remember the functions they serve. The lateral geniculate is a major relay nucleus for visual information that is on its way to occipital cortex. Thus, axons carrying visual information from the retinas synapse in this nucleus. Cell bodies located in the lateral geniculate nucleus project their axons to visual cortex to complete one of the primary visual pathways. The medial geniculate nucleus is a site where axons carrying auditory information synapse along the pathway carrying information to the temporal lobe, the auditory receiving area of cortex. The cerebral peduncles are seen as a large fiber mass on the ventrolateral edge of the brain stem. Stop now and look at the ventral surface of your intact hemisphere to observe the peduncles from another perspective. Note that when you are caudal to the mammillary bodies you are no longer in the region of the hypothalamus and that the cerebral peduncles are the most prominent structure on the ventral surface in this area of the brain. Return to your coronal section and observe that the cerebral peduncles form the lateral border of a large nuclear mass, the substantia nigra. The cells of the substantia nigra produce the neural transmitter, dopamine. Low dopamine levels have been related to the movement disorder, Parkinson's disease, while excessive amounts of dopamine have been linked to the mental disorder, schizophrenia. Find the cingulum, which is much larger now than as seen in coronal Section D. As in earlier sections, it is surrounded by the cingulate gyrus. Recall that the cingulum is a fiber bundle connecting areas of the frontal and parietal lobes with the hippocampus. This completes the last of the dissection. Review the earlier sections and also this last one. As in previous sections, you should be able to define, identify, or locate each boldfaced term, as appropriate. In preparation for the practicum, you will find it helpful to study your dissected specimen in conjunction with the figures. The tissue, though, presents a qualitatively different experience from the two-dimensional images on the website and it should be your primary focus. Individual differences in brains mean that none of the brains in the practicum will look exactly like the drawings, so it is important to be able identify structures on actual tissue, and not just on the figures. As you study, try to visualize how the major structures are interrelated and how they course rostrally and caudally, or dorsally and ventrally. Study in groups. Use the lists, or index cards of boldfaced terms you constructed at the end of each dissection to quiz one another. Point to the structures and require that the structure be named. Conversely, provide a name and require that the structure be located. In all instances, where relevant, indicate the structure’s function. Do not destroy your dissected tissue. Your “ticket” to the practicum is the container and the dissection kit that was checked out to you at the beginning of the semester. Please return it and all your sheep brain tissue on the day of the practicum. Your grade for the course cannot be assigned until your dissection kit is returned.