Sheep Brain Dissection: Lab 3

SHEEP BRAIN DISSECTION: LAB 3 Medial Face of the Hemisphere: Be certain that all the meningeal covering has been ruptured along the course of the longitudinal fissure before attempting to bisect the brain. In order to eliminate cutting artifacts, the cut should be made with a knife sufficiently long to affect the separation in a single stroke. If the cutting is not done with one smooth stroke, "sawmarks" will obscure some of the fine details and structures of the medial surface that you need to identify. With the brain resting on its ventral surface (on several paper towels) and with the frontal poles toward you, place the tip of the knife at the anterodorsal limit of the cerebrum. Sight along the blade and position it so that it falls exactly over the midline for the whole length of the specimen. Push the knife down and away from you, as you carefully maintain the midline position until you have cut through the cerebellum, brainstem, and spinal cord. As the knife cuts through the fibers connecting the two hemispheres they will separate and the medial faces of the two hemispheres will be revealed. If you made a true mid-sagittal section, the medial faces will be mirror-images of one another. The right hemisphere will be used for identifying the prominent structures visible on the medial face, while the left hemisphere will be used to make coronal sections. You will find this figure, helpful in identifying structures during this portion of the dissection. You can print drawings of the right and left medial face. These drawings can be used to test your mastery of the material and for making notes. While looking at the medial face of the right hemi-section locate the central canal at the most caudal end. The canal courses anteriorly to the point where it enlarges under the cerebellum; this triangular-shaped space within the cerebellum is the 4th ventricle. At the anterior end of the cerebellum, you should see a thin membrane, the anterior medullary velum. Continue tracing the ventricular system rostrally. Immediately anterior to the anterior medullary velum, the ventricular space becomes the Aqueduct of Sylvius. The Aqueduct of Sylvius opens into the 3rd ventricle, a thin, flat ventricle located between the two lobes of the thalamus. The III ventricle is no longer visible as a hollow chamber in midsagittal section, because in making your midsagittal cut you have cut the hollow space in two. You will have to imagine what the structure looked like, when the brain was intact. The spatial relationships of the 3rd and lateral ventricles with the surrounding brain tissue is rather complex. Have the instructor or the lab assistant show you the three-dimensional model of the ventricles. It should help to clarify how the ventricles are interconnected and the shapes they assume in the interior of the brain. This diagram provides a view of the ventricular system from both lateral and dorsal perspectives. Realize that these are views of the ventricles with all of the surrounding brain tissue removed (i.e., imagine filling the ventricles with plaster, then peeling away the brain until only the plaster remains). Compare the two-dimensional drawings with the life-size model of the ventricles available in the lab. The model should clarify how the ventricles change their shape in different regions of the brain. It should become apparent why the shape and location of the ventricles changes in the coronal sections you will produce later. The tissue dorsal to the Aqueduct of Sylvius is the tectum (roof) while the tissue ventral to the Aqueduct is the tegmentum (floor). Recall that the tectum is made up of the corpora quadrigemina. By what more common names are the corpora quadrigemina known? Notice the thick band of white fibers between the colliculi and the Aqueduct of Sylvius; this band of fibers is known as the lamina quadrigemina. Under close observation this structure appears to be layered, like a piece of laminated plywood. Hence its name. The superior colliculus is a component in one of the pathways conveying visual information to the brain. This structure is involved in the detection and processing of quickly moving stimuli in our peripheral vision. While it is not part of humans' primary visual system, it is more important in lower-order organisms. The inferior colliculus is involved in the processing of auditory information, in particular, the location of auditory signals. There are a number of commissures to be identified on the medial surface of the brain. A commissure is a band of fibers that connects corresponding structures on each side of the brain with one another. The first of the commissures to be considered is the corpus callosum, which is a large fiber structure that makes connections between homotopic regions of the cerebral hemispheres. The corpus callosum extends for a considerable distance in the anterior-posterior direction. At its anterior limit it makes a bend ventrally and caudally. This "bend" is called the genu. This part of the corpus callosum may remind you of a bent knee, so, it may not surprise you to know that the Latin word, 'genu' means knee. At the most caudal end of the corpus callosum, near the cerebellum, you will see another bend; this part of the callosum is the splenium. The portion of the corpus callosum between the genu and the splenium is called the body of the corpus callosum. Two other commissures, the anterior commissure and the posterior commissure, will be discussed later. Immediately ventral to the caudal region of the body of the corpus callosum and the splenium, you will see a large round structure, the thalamus. Like the hypothalamus below it, the thalamus consists of a number of nuclei. All sensory systems, with the exception of olfaction, synapse on cells located in sensory nuclei in the thalamus. Cells in the sensory nuclei of the thalamus send the sensory information they have received forward to the appropriate projection site in the cerebral cortex. For example, visual information being transmitted by cranial nerve II, the optic nerve, will be relayed by the lateral geniculate nucleus (LGN) of the thalamus to occipital cortex. The medial geniculate nucleus (MGN) transmits auditory information from cranial nerve VIII, the vestibular -cochlear nerve, to auditiory cortex. The ventro-postero-lateral nucleus (VPL) of thalamus receives somatosensory information. Don't proceed until you can state to which region of cortex to which the cells in VPL send their signals. These nuclei are deep in the brain and cannot be seen until coronal sections are made. Not all nuclei in the thalamus have a sensory relay role; other functional sites are also present. For example, it has been established, recently, that regions in the thalamus are involved in the development of emotional learning, such as, fear conditioning. Dorsal and caudal to the thalamus locate the small piece of tissue that appears to be rather tenuously attached. This is the pineal body, which Descartes thought was the "seat of the soul." In birds, the pineal gland plays a role in establishing and maintaining circadian rhythms. There are large species differences, however. For example, pinealectomies do not disrupt circadian rhythms in mammals. The area just inferior to the thalamus, extending down to the ventral surface of the brain, is the hypothalamus (hypo means below or under, thus the hypothalamus is located below the thalamus). The thalamus forms the dorsal border of the hypothalamus, the mammillary body is the caudal limit, and the optic chiasm the rostral limit. The hypothalamus is a complex structure composed of many nuclei; it is in control of the autonomic and endocrine systems and organizes behaviors that are important to the survival of a species. Hunger and thirst, temperature regulation, and sexual and reproductive behaviors are but a few of the concerns of the hypothalamus. Find the pituitary gland that you removed when studying the external features of the brain. Bisect the pituitary by making a mid-sagittal cut. You should be able to locate a distinct line separating the pituitary into two sections. The posterior pituitary (also called the neurohypophysis) receives direct neural connections from the hypothalamus above it. Cells in the hypothalamus project axons to the pituitary where they release oxytocin, a hormone that produces uterine contractions during childbirth and that stimulates the mammary glands to eject milk during suckling. The hypothalamus also projects axons to the pituitary that stimulate release of vasopressin (antidiuretic hormone), which regulates the reabsorption of water by the kidneys. The anterior pituitary or adenohypophysis is non-neural tissue. This glandular tissue is connected to the hypothalamus by a vascular route. The hypothalamus injects releasing factors into the portal blood system; the factors travel to the anterior pituitary to stimulate the release of hormones. Among the hormones released are luteinizing hormone (LH), follicle stimulating hormone (FSH), and prolactin, all of which are involved in reproductive behavior. These hormones travel to their target organs via the blood supply. Malfunctions in the pituitary may be reflected in disorders of organs located some distance from the pituitary, depending on the specific hormone involved. The pituitary also releases growth hormone (GH), which is necessary for the normal growth of all tissues. Just inferior to the genu of the corpus callosum is an area of archicortex called the septal area. This area has been implicated in behaviors, such as, aggression. At the caudal edge of the septal area, locate the anterior commisure. This commissure looks like a small white dot. This fiber tract connects the olfactory bulbs, amygdalae, and hippocampal areas, among others. Recently, it was discovered that the anterior commissure is larger in women and gay men than it is in heterosexual males. The functional significance of this finding, however, is not clear. Look at the ventral surface of your specimen, at the anterior limit of the rhinencephalon notice the small mound, the amygdala. Considering that the sense of smell is important for survival, especially in lower order animals, it is not surprising that the olfactory bulbs would have close connections with the amygdala via the anterior commissure. The amygdala is another small structure with far reaching effects. For example, emotional responses have three components: behavioral, autonomic and hormonal. These components are controlled by separate systems, but are integrated by activities of the amygdala. The amygdala plays a special role in responding to activities and stimuli that have biological importance, such as those that signal pain, or the presence of food, water, or danger, and has recently been implicated in the development and maintenance of drug dependence. Immediately dorsal to the body of the corpus callosum (i.e., that part of the callosum between the splenium and genu) locate the callosal sulcus. The cortical tissue just dorsal to the callosal sulcus is the cingulate gyrus, which is limited dorsally by the cingulate sulcus. Follow the cingulate gyrus caudally until it begins to course ventrally and then laterally where it disappears under the occipital pole of the cerebrum and where it becomes continuous with the hippocampal gyrus. These gyri, the cingulate gyrus and hippocampal gyrus, are archicortex. When combined with the septal area, amygdala, hypothalamus, and other structures they constitute the limbic system, sometimes referred to as the visceral brain. Among other things, the anterior cingulate cortex is involved with the perception of pain. The cingulate gyrus also has integrative functions; it provides an interface between the frontal cortex (invovled in decision making), the emotional processes of the amygdala, and the brain mechanisms involving movement. The cingulate gyrus is, therefore, intimately involved in emotions and motivated responses. If the brain was bisected exactly along the midline, there will be a membrane, the septum pellucidum, extending ventralwards from the body of the corpus callosum and immediately caudal to the genu. This membrane separates the two lateral ventricles from each other.