READ BEFORE CLASS
Using the following list of equipment you will SLOWLY dissect a Sheep Brain. Remember: The slower you work, the more likely your dissection will come out with the structures intact. You will complete this lab over the next 2 weeks. The equipment required is: 1) Scalpel 2) Dissection tray 3) Surgical Gloves (the tighter the better).
Caution: The scalpel blades are very very VERY sharp. When making cuts, always slice in a direction that is away from the body, keeping your fingers retracted from areas in front of the blade. The idea here is to investigate the structures of the brain, not the fingers.
In the following pages you will receive instruction in the dissection of the sheep brain. Please do not be shy about using the atlases that are in the room, making direct comparisons to the human brains that you handeled last week, or asking questions of the instructors. After-all, the idea here is to learn more about the human brain through comparison. At the end of this text you will find a few Xeroxes of both the sheep brain and human brain atlases.
HAVE FUN THE NEXT TWO WEEKS, KEEP THE STRESS LEVEL LOW, BUT ALSO BE SERIOUS ABOUT WHAT YOU ARE DOING! KEEP IN MIND that you cannot see all the structures mentioned. As is usually the case, many of the descriptions provided are meant to give you additional information and to provide you with an orientation to the brain and the many structures of the brain.
1. External Features of the Brain
Examine the surface anatomy of the sheep's brain. In order to study the surface structure of the brain, it is necessary to remove the meninges, which has already been done for you. The meninges are three separate sheaths of membrane which enclose the central nervous system and include: 1) the dura mater, the outer-most coating, composed of a sturdy fibrous sheet of white collagen fibers--as can be seen on the mason jar sample, 2) the arachnoid mater, a poorly vascularised membrane of loose connective tissue, and 3) the pia mater, the inner-most membrane, intimately adherent to the surface of the brain. Much of the brain you will be dissecting still has this pia mater attached. Astrocytic (a type of CNS cell) end-feet are apposed to the inner surface of this. Between the pia and the arachnoid is the sub-arachnoid space, through which cerebro-spinal fluid (CSF) passes, as do many blood vessels. Because the arachnoid follows the contour of the dura, and because the pia adheres to the brain's surface, a number of sub-arachnoid cisterns reside in regions where the arachnoid bridges over sulci, forming pools of CSF.
Keep in mind that each of major division of the brain is identifiable on the basis of its developmental origin as a distinct vesicle. In the adult, each division is associated with one ventricular cavity, and each division has a distinct pattern of organization. We shall consider these divisions and their compartments next, but keep in mind that you will be able to see only a few surface features.
2. Forebrain
The forebrain or prosencephalon forms the largest part of the brain. It is divided into the diencephalon, which forms around the third ventricle, and the telencephalon, which encloses the lateral ventricles. The diencephalon can be divided into a smaller ventral portion, the hypothalamus, which is concerned with autonomic and endocrine mechanisms, and a larger dorsal part, the thalamus, which contains a number of subnuclei, most of which are associated with distinct cortical regions. The telencephalon can be divided into the larger, surface, portion, the cerebral cortex, and a smaller, though still considerable, deeper part, the basal ganglia. Only the outer mantle, the cerebral cortex, can be seen from the surface. Notice the median longitudinal fissure dividing the two cerebral hemispheres. In the human brain, we classically divide the neocortex into four lobes on the basis of the sulcal pattern. This is not so readily apparent in the sheep brain. Nevertheless, we can refer to the caudal-most end as the occipital cortex (visual), the rostral-most end as the frontal cortex (motoric, cognitive, intellectual). The region between these two ends is divided into two parts and we refer to the dorso-medial portion as the parietal cortex (somatosensory; spatial) and to the lateral portion as the temporal cortex (auditory). Just rostro-medial to the temporal cortex runs the rhinal sulcus along the ventro-lateral surface of the brain, defining the pyriform cortex (olfactory) medially, which is paleocortex. Along this ventral surface runs the lateral olfactory tract, carrying fibers to the pyriform cortex from the olfactory bulb--seen in some of the preparations at the far rostral tip of the brain. At the midline, notice the optic chiasm, where the "partial decussation" of the optic pathway takes place, giving rise to the two optic tracts. Caudal to the optic chiasm are the mammillary bodies, a subdivision of the hypothalamus. Just rostral to this, immediately caudal to the optic chiasm, is the location at which the pituitary stalk attaches, which has been removed in most of these samples.
3. Midbrain
The midbrain or mesencephalon forms caudal to the diencephalon around the ventricular space called the cerebral aqueduct. The part of the midbrain lying dorsal to the aqueduct is called the tectum (roof), which is made up of the corpora quadrigemina, four bulges composed of two pairs of colliculi: a rostral pair called the superior colliculi (optic tectum), and a caudal pair called the inferior colliculi (auditory tectum). You should be able to see the four colliculi from the dorsal surface by gently easing the cerebellum down from the occipital pole.
Ventral to the aqueduct resides the tegmentum (floor), and directly beneath this, the crus cerebri. The term cerebral peduncle is loosely used to refer to the fibers of the crus cerebri, but in fact should mean both. The crus cerebri-cerebral peduncle-contains the major output fibers of the cerebral hemisphere. This is the large fiber pathway at the base of the brain just medial to the temporal lobe, extending from the rostral edge of the pons to the caudal edge of the optic tract. The cavity behind the mammillary bodies and between the two cerebral peduncles is called the interpeduncular fossa. The midbrain tegmentum contains two large nuclei, the red nucleus and the substantia nigra, both of which subserve motor functions, and neither of which are apparent from the brain's surface.
4. Hindbrain
The hindbrain-rhombencephalon-is the most caudal of the embryonic vesicles, forming around the fourth ventricle that has a rhomboid shape in the adult. The structures grouped around it are the medulla oblongata, furthest caudally, and the pons, rostrally, both of which are ventral to the ventricle, and the cerebellum, which overlies the fourth ventricle dorsally. The rhombencephalon is often subdivided into a myelencephalon-medulla oblongata-and metencephalon-pons and cerebellum.
Notice along the base of the hindbrain two large fiber tracts on each side of the ventral median fissure, called the pyramidal tracts. Dorsal to the pyramidal tract is the inferior olivary nucleus, one of the major sources of innervation to the cerebellum. The hindbrain also houses a number of cranial nerve nuclei, giving rise to some of the cranial nerves. Most of the cranial nerves have not remained attached to the brain, but you should be able to find the oculomotor nerve (III), arising from the interpeduncular fossa, and possibly the trigeminal nerve (V), arising from the lateral surface of the pons. You have already seen the optic nerve (II) and possibly the olfactory nerve (I). You should familiarize yourself thoroughly with the cranial nerves and the respective functions (from your text).
On Old Olympus' Tufted Top A Fat Armed German Viewed Some Hopps
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Olfactory Optic Oculomotor Troculear Trigeminal Abducens Facial Auditory Glossopharyngeal Vagus Spinal Accessory Hypoglossal
At the caudal end of the hindbrain, where the two pyramidal tracts cross the midline at the pyramidal decussation (not obvious), is the rostral end of the spinal cord, only a portion of which is attached in the samples provided.
The cerebellum is attached to the brainstem by three peduncles, called superior, middle and inferior, the latter two of which are not readily distinguished. Using a scalpel, gently sever these peduncles, cutting underneath all of the cerebellar folia. The cerebellum should come off easily, revealing the full extent of the fourth ventricle and its rhomboid shape. Notice the dorsal median sulcus, which ends at the obex. Just lateral to this point on each side reside the two main secondary sensory nuclei for somatosensis of the spinal cord, the gracile nucleus medially, and the cuneate nucleus laterally. These are fed through fibers traveling up the spinal cord on its dorsal surface (the dorsal columns). The floor of the fourth ventricle, rostral to these nuclei, contains a series of cranial nerve nuclei, including the hypoglossal (XII) and abducens nuclei (VI).
Using the scalpel again, sever the brainstem at the junction between the hindbrain and the midbrain, just rostral to the pons at the ventral surface and just caudal to the inferior colliculus at the dorsal surface. Examine the coronal surfaces of the two sections. Are there obvious patterns evident?
5. Internal Features of the Brain
The instructions that follow will guide you as you sever the brain in the sagittal axis, which when finished should result in the brain being separated into two equal halves. Carefully pry open the two cerebral hemispheres. Notice as you do so how deep you must open them before you reveal the corpus callosum, the large fiber pathway connecting the two halves of the neocortex. The rostral end of this is called the genu, the middle region called the body, and the caudal end called the splenium. Also notice the cingulate gyrus overlying the callosum. Carefully sever the callosum. You should continue to sever the brain using smooth unhurried movements of the scalpel. As you do, notice the differing structures that you are transecting. Continue ventrally to sever the septum pellucidum directly beneath the callosum rostrally, and then the fornix (which connects the hippocampus with the septum), anterior commissure (through which the two temporal lobes are interconnected, rather than passing fibers through the callosum), the thalamic midline (the large round midline attachment, massa intermedia), and caudal to this, the large pineal gland. Continue ventrally to sever the mammillary bodies and optic chiasm rostrally and the brainstem caudally.
Once you have separated the two halves, go back and identify each of the above structures. Also identify the superior colliculus, the tegmentum, the cerebral aqueduct, and the third ventricle.
Now take one of the two sagittal sections and using the scalpel make a coronal cut from the dorsal surface downward to the level of the corpus callosum--on the mid-sagittal surface. Extend this cut laterally around to the ventral surface of the brain and gently lift the caudal portion away exposing the surfaces lying below the telencephalon (neocortex). Examine these exposed areas identifying the superior and inferior colliculi, optic tract, and thalamic nuclei. Can you see any other structures.
CLEAN-UP
That is it for this week. Now please clean up your lab area disposing of the brain parts in the bag (bucket) provided, along with gloves, Kim-wipes, etc. Place the remaining intact protions of your sheep brain in the container provided. Mark the container and than please wash out your dissecting pans and rinse off your instruments. Remove the scapel blade carefully and dispose of it in the Sharps Container. Dry the pan (not the instruments) and return all the clean items to the instructor. We will continue the dissection next week!
6. Internal Capsule
Remove the hemisphere on which you have already dissected the caudal half of the cortex. Starting at a mid-lateral location rostral to the level of the optic chiasm in the rhinal sulcus, use the scalpel to scrape away the overlying cortical mantle exposing the underlying white matter. You may find yourself able to discriminate the claustrum, lentiform nucleus and associated extreme and external capsules, but this is very difficult in the sheep. Eventually, you will find yourself within the internal capsule, and should note the very prominent radiate appearance of its fibers, coursing to and from the overlying cortex. Gradually scrape away the overlying cortex to reveal the full extent of the internal capsule rostrally, dorsally and caudally. Notice how easily the grey matter of the neocortex comes away, often leaving the underlying white matter intact. Follow that underlying white matter into the internal capsule. Examine the brain from the rear view, where you have made a coronal cut during the previous dissection. Follow the cortical white matter down into the revealed internal capsule just rostral to the optic tract. Carefully scrape away the tissue to reveal the point where the internal capsular fibers disappear beneath the optic tract and then re-emerge caudal to it as the cerebral peduncles.
Notice the striated appearance of the internal capsule as it courses through the striatum, for which it is named. In contrast, notice the less striated appearance of the white matter just above the internal capsule. Note that the internal capsule separates the caudate and putamen (the striatum), but that the latter are really linked by a number of cellular bridges in the capsule. The striated appearance you see now is due to this intermingling of grey and white matter within this region of internal capsule/striatum.
Turn the tissue over to reveal the medial side of the brain. Notice the anterior commissure. Insert the blunt end of the scalpel into the crevice between the corpus callosum and the fornix at the caudal end, where you have previously made a coronal cut, and gently lift up the callosum to reveal a portion of the choroid plexus. (This may have been accidentally removed previously). Peer in beneath and continue to open this rostrally to see the body of the caudate nucleus. This appears as a prominent bulge along the lateral wall of the lateral ventricle, running in the rostro-caudal axis. As you continue rostrally, you may need to remove a portion of the septum beneath the genu of the corpus callosum in order to see it.
7. Cerebellum
The cerebellum is made up of a cortex, the outer layer of grey matter, and an inner core of white matter (the medullary center) surrounding centrally placed aggregations of nerve cells called the deep cerebellar nuclei. The entire surface of the cerebellum is covered in closely set fissures running in the transverse plane, which delineate individual ridges called folia. All the connections between the cerebellum and the remainder of the CNS pass through one of three main peduncles, called the superior, middle and inferior cerebellar peduncles. Identify them on the ventral surface.
The inferior cerebellar peduncle consists mainly of fibers entering the cerebellum, the majority coming from the inferior olivary nucleus. This peduncle also consists of fibers leaving the cerebellum to terminate in the vestibular nuclei. The middle cerebellar peduncle consists of fibers entering the cerebellum from the pons. The superior cerebellar peduncle consists of primarily efferent fibers from the deep cerebellar nuclei, and some afferent fibers from the spinal cord.
The cortex of the cerebellum contains a median region called the vermis. It is most pronounced along the ventral and caudal surface. Lateral to it are the cerebellar hemispheres, divided into an anterior and posterior lobe by the primary fissure on the dorsal surface. On the ventral surface, the part of the cerebellum directly between the peduncles is called the nodule. That portion of the cerebellum lateral to the peduncles is called the flocculus. Together they constitute the flocculo-nodular lobe, primarily concerned with vestibular functions. Note that many fibers of the vestibular nerve enter the brain and bypass the vestibular nucleus, projecting directly to the flocculo-nodular lobe of the cerebellum.
Remove the remaining pial membrane from the surface of the cerebellum and examine the pattern of folia with the scalpel, peering in between adjacent folia. Examine deep between the longitudinal fissures defining the above regions. Using the sharp end of the scalpel, make a mid-sagittal cut into the anterior lobe, about 1 cm deep. Now, from one side, cut away one half of the anterior lobe so that you can view the mid-sagittal (cut) surface. Notice the characteristic appearance of the cerebellum with its white matter branching out to the many folia. The parallel fibers course through the folia in the plane of their long axis, while the Purkinje cell dendrites extend into the molecular layer across this axis, ensuring maximal opportunity for the Purkinje cells to be contacted by the parallel fibers. Repeat this exercise in the posterior lobe in one of the cerebellar hemispheres at a lateral position, and in the vermis, all on the same half of the cerebellum as before. Notice the identical anatomical organization. All of the cerebellar cortex is identical in both gross anatomical features and in micro-circuitry.
Use your scalpel and make smooth, even strokes cutting the other half of the cerebellum from the lateral edge in the parasagittal plane into roughly 5 mm thick sections, until you have reached the sagittal plane (the midline). Lay out each sequential piece and examine the details of the folia. Now, try to detect the deep cerebellar nuclei hidden found just lateral to the midline within the white matter. The four deep cerebellar nuclei are the fastigial nucleus, the dentate nucleus, the globose and the emboliform nuclei (the latter two comprising the nucleus interpositus). They will appear as a slightly grayer region deep in the white matter, though nothing as grey as the overlying cerebellar cortex. You will probably be unable to discriminate them in this tissue. Use the remaining half (or whatever is left of the remaining half) of the cerebellum, and gradually dissect away the cortex and white matter. Notice how the grey matter of the cortex comes apart from the underlying white matter. Examine the details of the folia while dissecting through the cerebellar cortex. See if you can identify the deep cerebellar nuclei.
8. Limbic System
The term "Limbic Lobe" was coined by Broca to refer to the hippocampus, parahippocampal gyrus and cingulate gyrus. The term "limbic system" is rather less precise, and includes the above plus the amygdaloid body, septal nuclei, mammillary body, and anterior thalamic nuclei. Interconnecting bundles of fibers are also a part of the system: the fornix/fimbria; mammillo-thalamic tract; stria medullaris; stria terminalis; and cingulate bundle. The limbic system is involved in memory, in emotions, and in the control of visceral and motor responses associated with defense and reproduction.
Use the intact hemisphere and turn it to view the medial surface. Identify the cingulate gyrus, directly overlying the corpus callosum. The cingulate sulcus delimits its dorsal extent. Notice how the cingulate gyrus continues rostrally, curving ventrally and then caudally over the genu of the corpus callosum, extending towards the septal area as the subcallosal gyrus. Caudally, the cingulate gyrus curves ventrally over the splenium of the corpus callosum, turning laterally to become the parahippocampal gyrus, hidden from view. Using the scalpel, gently scrape away the cortex of the cingulate gyrus using longitudinal strokes to reveal the underlying white matter. Notice that many of the fiber bundles run for considerable distances in the direction of the cingulate gyrus. This is the cingulate bundle, one of many association fiber pathways, in this case interconnecting many parts of cortex on the medial surface (subcallosal, cingulate, parahippocampal gyri).
At the back end of the medial surface of the cerebral cortex, gently raise the cortex off the superior colliculus, and detect the continuation of the cingulate gyrus as it curves ventrally and laterally, and then rostrally down into the temporal lobe. Turn the brain over and carefully scrape away the parahippocampal gyrus (the ventral surface of the temporal lobe) passing through neocortex, then white matter, and then a separate fiber pathway, quite distinct from the cortical white matter, which overlies the hippocampus. Medial to this lies the parahippocampal gyrus, the target of much of the cingulate bundle. One portion of the parahippocampal gyrus, called the entorhinal cortex, in turn, provides the major input to the hippocampus.
The hippocampus is a complex infolding of archicortex situated in the lateral ventricle. It is made up of several distinct regions which form as an infolding of the cortex continuous with the parahippocampal gyrus. The hippocampus sends a large fiber projection rostrally. The fibers originate from the ventricular side of the hippocampus, called the alveus. As the pathway increases in size, it is called the fimbria, a flattened longitudinal bundle of white matter which curves around the lateral ventricle to become the fornix.
Note, that the fornix carries fibers in both directions between the septal area and the hippocampus. It also carries fibers from the hippocampus to the mammillary bodies via the post-commissural fornical columns (passing behind the anterior commissure). Try to identify the anterior commissure on the medial surface of the intact hemisphere. Notice its relationship with the fornix underneath the corpus callosum. Now, with the narrow tip of the scalpel, carefully scrape along the route of the fornix rostro-ventrally to follow the post-commissural fornical columns down into the mammillary body.
The mammillary body sends a projection to the anterior thalamus via the mammillo-thalamic tract. This passes caudal to the fornical columns. Carefully scrape away the mammillary body and overlying hypothalamus and medial thalamus to reveal the course of the mammillo-thalamic tract. The anterior thalamic nuclei, in turn, project to the cingulate gyrus.
The structures you have identified include the cingulate bundle passing deep to the cingulate gyrus and parahippocampal gyrus. The entorhinal area of the latter projects to the hippocampus. The hippocampus in turn projects to the mammillary body via the fornical columns, and the mammillary body sends a projection to the anterior thalamic nuclei via the mammillo-thalamic tract. The anterior thalamus projects via the internal capsule back to the cingulate gyrus, thus completing a circle of connections, called the Circuit of Papez, along the medial edge, or "limbus", of the hemisphere.
CLEAN-UP
Now please clean up your lab area disposing of the brain parts in the bag (bucket) provided, along with gloves, Kim-wipes, etc. Please wash out your dissecting pans and rinse off your instruments. Dry the pan (not the instruments) and return all the clean items to the instructor. Before moving on please make sure that your lab area is clean and dry.
See ya--have a great Day.