Sheep Brain Dissection: Lab 4

SHEEP BRAIN DISSECTION: LAB 4 Coronal Sections. Many structures of the brain are visible neither on its external surface or medial face, but rather they lie deep within the hemispheres. To reveal these internal structures, it is necessary to make serial sections parallel to the coronal plane. Section B. Look at the medial face of one of your mid-saggital sections before cutting your specimen. Find the anterior portion of the corpus callosum. See the bend? It is called the genu of the corpus callosum. In Latin, genu means knee. Follow the corpus callosum ventrally and caudally from the genu to the point at which the fibers end in a point. This pointed tip of the corpus callosum is the rostrum. Now locate the optic chiasm. Your first coronal cut should be made just caudal to the rostrum of the corpus callosum and just anterior to the optic chiasm. Use the large brain knife, not a small scalpel to make your cut. Create this first coronal section by making one smooth, continuous movement. Set the frontal pole aside. Look at the coronal face of the hemisphere; it should look like Section “B” shown to the left of the screen. Your sheep brain specimen is unstained tissue. As a result, it will not show the great contrast between tissue consisting mostly of cell bodies or axons. Nonetheless, there are readily visible differences in the unstained brain which we can examine. In unstained brain tissue, areas rich in axons appear white or are lighter in color because of the myelin sheath surrounding the axons. In contrast, cell bodies, which are not coated in myelin sheath, appear slightly darker and grey. This contrast between cell bodies and axons is the basis for the terms gray matter (cells) and white matter (axons). Look at the cortex, the outer layer of grey matter surrounding the inner core of white matter. It should be apparent that the folds of the gyri have greatly increased the surface area of the cortex. Ask your instructor or the lab assistant to show you the preserved mouse, rat and cat brains, which vary considerably in terms of the amount of cortex present. Join the frontal pole you removed with the remaining brain. Look at the lateral aspect of the hemisphere and find the rhinal fissure. Follow the fissure around to the coronal surface. You will see a layer of cortex above the fissure and a layer below the fissure. The tissue inferior to the fissure is paleocortex and is thinner than the layer of neocortex above the fissure. This difference in thickness is a consequence of neocortex (“new-cortex”) consisting of six layers of various types of cortical cells, while the rhinencephalic paleocortex, or archicortex, consists of only three layers of cortical cells. It is the increase in amount of neocortex that characterizes the development of the brain phylogenetically. Fibers (axonal white matter) fill much of the interior of the hemispheres, but there are also many groupings of cell bodies present. A large cluster of cell bodies located within the brain, is called a nucleus, while a cluster of cell bodies located outside the CNS is called a ganglion. Bundles of axons located within the CNS are called tracts, but bundles of axons found outside the CNS are referred to as nerves. A noteworthy example of this, which you have already encountered, is cranial nerve I, the optic. Recall that the bundle of optic fibers that are anterior to the optic chiasm are called the optic nerve, while those same fibers that are located caudal to the chiasm that are internal to the brain are called the optic tract. There are many fiber groups, or bundles of axons, present inside and outside the brain. Some of these bundles of axons are sensory, some are motor, and some are called “association” fibers, fibers that connect one region of cortex with another. The largest band of association fibers is the corpus callosum, which you have already seen in sagittal section. Recall that it is a band of transversely oriented fibers that connects similar areas of cortex in the two hemispheres. Dorsal to the corpus callosum is the cingulate gyrus; embedded within it is the cingulum. The cingulum is a band of “association” fibers, that is, fibers that are neither sensory nor motor, but that connect two cortical areas. The cingulum connects regions of the frontal and parietal lobes; thus, these fibers run anterior to posterior (front to back), while the corpus callosum runs side-toside. Be certain to note that the cingulum is a fiber structure consisting of axons, while the cingulate gyrus is cortical matter that consists primarily of cell bodies. This means that the cingulum is white matter, while the cingulated gyrus is grey matter. The cingulate gyrus has recently been determined to be involved in the perception and regulation of pain. Locate the large space just below the corpus callosum, the lateral ventricle. Note that the corpus callosum forms the roof of the lateral ventricles. The large cell mass forming the inferior border of the ventricle is the head of the caudate nucleus. Just lateral to the head of the caudate nucleus you’ll see white speckles that are bundles of axons beginning to converge into a band of fibers called the internal capsule. The internal capsule consists of axons descending from cortex. Many, but not all, of these fibers are from the pre-central gyrus or motor cortex. The fibers of the internal capsule will converge into a more compact bundle and will migrate toward the ventral surface of the brain, where they are known as the cerebral peduncles. These same fibers appear along the ventral surface of the medulla where they are known as the pyramidal tract. Looking at your coronal section , note that there is a nucleus lateral to the internal capsule, the putamen. It is not clearly defined, just know the general region within which it appears. The arrangement of nuclei and fibers (caudate nucleus, putamen, and internal capsule), gives the area a striated appearance, which prompts this area to be called the corpus striatum. The corpus striatum is important for movement; dysfunction of the structures within the corpus striatum and/or in projections to or from it, are associated with movement disorders such as Parkinson’s Disease and Huntington’s Chorea. Section D. This cut should be made just anterior to the mammillary body. If done correctly, your section should look like the image to the left. The complexity of the relationship between the ventricles becomes apparent in this section with the appearance of the 3rd ventricle. It can be located by finding the thalamus which serves as the ventral border of the III ventricle and the body of the fornix, that provides its dorsal, or upper, limit. Immediately superior to the fornix is the lateral ventricle and superior to it is the corpus callosum. The internal capsule continues to be a prominent fiber structure. It has become a more compact bundle and is migrating toward the ventral surface of the brain. Now, locate the corpus callosum and fornix. You should see a small cellular mass lateral to those structures and the ventricles; it is the tail of the caudate nucleus. The caudate nucleus is a large structure; the head of the caudate nucleus is seen anteriorly in Section B. Three nuclei are seen in this plane of dissection: the caudate nucleus, globus pallidus, and putamen. Collectively, they are known as the basal ganglia. The basal ganglia receive input from and send output to frontal and prefrontal cortex and the parietal lobes of cortex. One of the functions of this loop appears to be the selection and initiation of willed movements. The basal ganglia are involved in habit learning and may be related to obsessive-compulsive disorder. In a recent research program PET scans were taken in two groups of patient volunteers with obsessive-compulsive disorder. One group were treated with medication and one with behavior therapy alone. After ten weeks, patients in both groups who responded to treatment showed a significant change in the glucose metabolism in the right caudate nucleus, which is thought to be a regulatory center that serves to filter out unwanted thoughts and behaviors. Patients who did not respond to therapy failed to show these changes. Coronal section, "D," lies caudal to the optic chiasm; therefore, those fibers carrying visual information from the retinas are now called the optic tract. It is the thick band of fibers on the ventral rim of the section. A new structure can be seen lateral to the optic tract, the amygdala, a large nucleus in the ventromedial rhinencephalon. It is wedge-shaped or somewhat triangular. Take a moment to locate the amygdala on the external surface of your intact hemipshere, also. It is the little mound at the anterior end of the rhinencephalon lateral to the optic chiasm. Can you recall the kinds of behaviors in which the amygdala is involved? Note that the thalamus has become very prominent. This large cellular structure can be subdivided into a number of ill-defined regions: the anterior, lateral, medial , and ventral thalamus. The metathalamus, another subdivision of the thalamus is located in the posterior, or caudal regions of the thalamus and will be considered in Lab 5. We will concern ourselves only with the ventral thalamus. The ventral thalamus contains the largest somatic relay nuclei. One of these nuclei, the ventro-postero-lateral nucleus, is the projection site of somatosensory information coming from cells in the nucleus cuneatus and nucleus gracilis. (Refer to SHEEP BRAIN DISSECTION: LAB 2, to review the information on these two somatosensory nuclei and the ascending somatosensory tracts in the spinal cord.) Two fiber bundles, one immediately superior to the other, appear at the ventral midline. The inferior bundle is the anterior column of the fornix projecting to the ipsilateral mammillary body and other structures. The superior bundle is the mammillo-thalamic tract carrying information from the mammillary bodies to the anterior thalamus. Before you proceed to the next section, find the corpus callosum and note that it forms the dorsal roof of the lateral ventricle. You should note that both the III and lateral ventricles can be seen in this coronal section with the lateral ventricle lying superior to the III ventricle. To complete this section of the guide, locate the cingulum and the cortical tissue that surrounds it, the cingulate gyrus.