Human Brain and Senses October 13, 2008 Page 1 Examination of the Human Brain With only a few hours today we can only begin to scratch the surface of a complex subject like neuroanatomy. The purpose of this exercise is not to get a detailed knowledge of neuroanatomy but rather to try to become familiar with some of the structures that we will meet during the course and to learn where they are in a general sense. During this lab we will examine the gross surface features of the human brain and then look at sections of the spinal cord and brainstem. Try to relate the surface features with the internal structure of the brain. Work in pairs. Each pair will be provided with one whole and one half brain. I ve bolded structures that you might hear about in your studies of the brain. Each group should take turns looking at the sections of brains under the microscopes and the sections in plastic. Try to associate functions with the structures. A useful exercise is to ask yourself for each of the structures: what system does it belong in (e.g. sensory or motor, and what modality?), what is the nature of its afferents (inputs) and where do they come from, and where does it project? I know, we haven t covered any of this! Treat the brains gently so they can be used in future years.. Instructions: 1. No dissection will be done. Do not let the brains dry out; keep them moist with wet paper towels. 2. Study the features that can be seen on the lateral, medial and basal surfaces of the brain using the lists below as a guide. Use the midsagitally-sectioned half brain to examine the medial surface. Find and identify the 12 cranial nerves on your brain. 3. Look at each of the 6 trays with partially dissected brains using these notes as a guide. 4. After lunch we will go to the microscopes and look at the sections of the spinal cord and brainstem. What kind of stain was used? Are the stained substances cell bodies, dendrites, or axons? See how cells and axons are aggregated together to form nuclei and fiber tracts Lateral Surface Cerebral hemisphere Cerebellum Brain stem Frontal pole of cerebral hemisphere Occipital pole of cerebral hemisphere Lateral fissure (or Sylvian fissure) Central sulcus Precentral gyrus Frontal lobe Parietal lobe Occipital lobe Temporal lobe Transverse temporal gyri (buried in the dorsal aspect of the Sylvian fissure) Medial surface Cerebral hemisphere Cerebellum (vermis and hemisphere) Brain stem Frontal pole of cerebral hemisphere Occipital pole of cerebral hemisphere Central sulcus on medial wall of hemisphere Corpus callosum Thalamus Hypothalamus Midbrain (tectum, aqueduct & tegmentum) Superior colliculus & inferior colliculus Pons Medulla Fourth ventricle Basal Surface Olfactory (I) nerve or bulb & tract Optic (II) nerve, optic chiasm & optic tract Mammillary bodies (of the hypothalamus) Pons Medulla Inferior olive Pyramids & pyramidal decussation Cervical spinal cord
Human Brain and Senses October 13, 2008 Page 2
Human Brain and Senses October 13, 2008 Page 3 I. SURFACE FEATURES OF THE BRAIN A. Lateral Surface The lateral surface of the cerebral hemisphere may be divided into four lobes: frontal, parietal, occipital and temporal. The frontal lobe is demarcated from the parietal lobe by the central sulcus, and from the temporal lobe by the lateral fissure of Sylvius.. Each lobe may be divided into a number of gyri (convolutions) separated by sulci (grooves). The central sulcus is functionally important because it separates the primary motor cortex from the primary somatosensory cortex. Thus, the precentral gyrus subserves motor function while the postcentral gyrus subserves somatosensory function (touch, position sense, vibration sense). The transverse temporal gyri subserve auditory function. B. Medial Surface In medial sagittal section of the brain, one sees a massive band of commissural fibers, the corpus callosum, which connects the two cerebral hemispheres. Above the corpus callosum lies the cingulate gyrus. The region of cerebral cortex around the banks of the calcarine fissure consists of striate cortex which subserves visual sensation. The diencephalon is divided into two major regions, the thalamus and the hypothalamus. The left and right thalami are joined through the third ventricle by a bridge of tissue known as the massa intermedia. The midbrain is divided into a ventral portion, the tegmentum, and a dorsal portion, the tectum (roof), by the aqueduct. The tectum consists of four protuberances; the rostral pair, known as the superior colliculi, are related to the visual (and other) systems while the caudal pair, known as the inferior colliculi, are related to the auditory system. Rostral to the former is the pineal body, one of the only unpaired neural structures. C. Basal Surface Immediately lateral to the interhemispheric fissure is the gyrus rectus (straight gyrus), lateral to which is the olfactory bulb, which sends fibers into the olfactory tract. Descending fibers course through the massive cerebral peduncles, the most ventral parts of the midbrain. In the interpeduncular fossa (the space between the peduncles), the oculomotor (third nerve) emerges. Note the spatial relationship between the optic chiasm, infundibulum (stalk of the pituitary), and mammillary bodies, and the structures of the hindbrain, including the cranial nerves. II. CRANIAL NERVES There are 12 cranial nerves that are identified by their number and name below. Since some cranial nerves are very small and delicate, they often get ripped off and are missing from the brains. Pool resources with your
Human Brain and Senses October 13, 2008 Page 4 neighbors if you can t find some of the cranial nerves on your specimen. Name Function I Olfactory Sensation of smell from the nose II Optic Vision from the eye III Oculomotor Eye movements, accommodation of the lens, constriction of the pupil IV Trochlear Moves the contralateral eye down and out V Trigeminal Sense of touch and muscle receptors from the face, muscles of mastication VI Abducens Moves the ipsilateral eye laterally VII Facial Control of muscles of facial expression; controls salivation and tearing; sense of taste VIII Vestibular/cochlear Hearing and balance from the inner ear IX Glossopharyngeal Motor output to the pharynx, larynx; taste from the tongue X Vagus Parasympathetic outflow to the heart, lungs and gut; taste from the back of the mouth XI Spinal accessory Control of shoulder and neck muscles XII Glossopharyngeal Moves the tongue to ipsilateral side III. DEEP STRUCTURES IN THE BRAIN 1. Look at the lateral aspect of the brain where the temporal lobe has been removed. The dissection has gone deep into the depths of the temporal lobe to expose the hippocampus, which lies on the medial side of the inferior horn of the lateral ventricle, which permits us to make the dissection without
Human Brain and Senses October 13, 2008 Page 5 touching the hippocampus. Note how shiny the surface of the hippocampus looks, compared to say the surface of the cortex. Note the long fiber bundle interconnecting the occipital lobe with the temporal lobe. What information might these fibers be carrying? Note the capsular, or cave-like, shape of the internal capsule, which is the bundle carrying all descending fibers from the cortex to the brainstem and spinal cord. 2a. On the medial aspect of this brain, find the fornix, the fiber bundle connecting the hippocampus with the mammillary bodies. Identify the columns of the fornix and imagine how the fibers are coursing towards the mammillary bodies. Trace the fornix posteriorly and imagine how it connects up with the hippocampus which you see from the lateral view. Carefully look inside the lateral ventricle by pushing aside the fornix (without ripping it please!). Note the large head of the caudate nucleus rostrally and the small bump on the thalamus known as the anterior tubercle. The anterior nuclei of the thalamus lie under this bump. Also note the fibers of the internal capsule on the lateral aspect of this brain. Find the dark substantia nigra which lies dorsal to the cerebral peduncle, the extension of the internal capsule in the midbrain. 2b. Most of the cerebral cortex has been dissected away in this brain to show various features. You should be able to identify the fornix in the medial view and see how it curves around anteriorly to form the columns of the fornix making its way towards the mammillary bodies. On the ventral view follow the optic nerves to form the chiasm and then follow the optic tract back into the posterior aspect of the thalamus where you can see the bumps that make the lateral geniculate and medial geniculate nuclei. You can also see a nice view of the midbrain with the cerebral peduncles, the dark staining substantia nigra, red nucleus, aqueduct and tectum. On the lateral aspect you can see the fibers going to and from the cerebral cortex radiating out and collecting to form the internal capsule. You can also see the fibers that are going medially to form the corpus callosum. Find the substantia nigra. 3a. On the lateral aspect of this brain the dissection has exposed the fibers of the internal capsule which form a concave structure around the lenticular nuclei, some of which has been removed. Note the difference between the white matter and the grey matter. Note how fibers originate from all areas of the cortex and funnel into a small area. Would you imagine that the effects of a small stroke are more devastating in the internal capsule or in the cortex? Find the hippocampus with its shiny surface that has been exposed by dissecting the temporal cortex from the lateral view and follow the fibers of the fornix. On the medial side the thalamus has been removed. Find the large head of the caudate and the prominent anterior tubercle in this brain. 3b. The other brain in this bucket has not had any cortex removed but the it has been dissected and separated to show the course of the fornix from the hippocampus. On the ventral view trace the optic tract fibers posteriorly until you see a bump on the ventral lateral aspect of the thalamus. This is the lateral geniculate. Note the cerebral peduncles, substantia nigra and red nucleus in the midbrain. 4a. On the lateral views of the brain with the most cortex still present, the long distance fiber bundles have been exposed by blunt dissection. Note how there are fibers that run from the tips of the frontal lobe back to the occipital and temporal lobes. Find the arcuate fasciculus, a large curving bundle of fibers that interconnects the temporal lobe with the frontal lobe that is believed to be important for connectiing Broca s and Wernicke s areas. Note that at one point the most medial dissection reveals
Human Brain and Senses October 13, 2008 Page 6 some grey matter, which looks different from the white matter around it. What nucleus is this grey matter? Find the columns of the fornix. Carefully lift up the fornix as it overlies the thalamus to reveal the bump of the anterior tubercle.. 4b. The other brain has had much of the cortex dissected away. On the medial view find the columns of the fornix. Look inside the lateral ventricle and find the small bump on the thalamus which is the anterior tubercle and the large bulging head of the caudate. Turn the brain so that you can study the dorsal aspect. Note a how the fibers that are dissected here are again running in a different direction. What fibers are these and what structure are they forming? Now inspect the ventral aspect of this brain. Parts of the temporal lobe have been exposed. You should be able to find the following structures: optic chiasm, optic tract, lateral geniculate body, hippocampus, fornix, amygdala which sits just rostral to the hippocampus, cerebral peduncle, the substantia nigra. By carefully exposing the fornix you should be able to make out almost its entire length and C-shape. Trace it from the hippocampus all the way along its C shape and see how it arcs forward and then dives ventrally toward the mammillary body. 5. These are bits of the cerebellum and brainstem. Review the cranial nerves, examine the floor of the IV ventricle, and see how the superior cerebellar peduncle forms the roof of the IV ventricle. In the slice through the cerebellum you should be able to make out the deep nuclei embedded in the white matter, especially the dentate nucleus. 6. This bucket has sections of the brain cut in different planes to show the internal structures. Look at the similar sections embedded in plastic Coronal sections of brainstem and forebrain The brainstem is traditionally divided into three divisions from caudal to rostral: medulla, pons and midbrain. To appreciate the functional organization of the brainstem, it is important to understand its multiple roles: as a transition from the spinal cord caudally to the forebrain and thalamus rostrally, as a conduit for pathways traveling from the forebrain to the spinal cord and vice versa, as the major conduit for fibers to and from the cerebellum, which sits astride the brainstem, and as the site of origin and termination for the 12 cranial nerves. We will discuss the brainstem from caudal to rostral, touching upon the highlights of each division using representative sections. A useful exercise is to determine where these sections were made by relating the internal structures to the external features that can be seen in the midsaggital view of the brainstem above. The sections illustrated here are from human tissue but the sections we have in Nissl and Weigert are from monkeys so there are some differences in the angle of the cut and shape of nuclei but the overall organization is the same. The mid-medulla region is easy to identify because of the unmistakable appearance of the inferior olivary nucleus, part of the system that is connected to the cerebellum. Ventral to the inferior olive run the fibers of the pyramids, which decussates (crosses) just caudal to this level. For fiber tracts it is essential to keep in mind which way the information is going, e.g. if you had a lesion of the pyramids on one side, what kind of deficit would
Human Brain and Senses October 13, 2008 Page 7 you expect and which side of the body would be affected? Just off the midline dorsally is the hypoglossal nucleus. What is its function? In the caudal thalamus parts of the pons and midbrain show up ventrally while more dorsally we have the thalamus. In the midbrain region we see the large fiber bundles on either side that make up the cerebral peduncles, or what s often referred to as the peduncles. These fibers carry all information descending from the cortex to the brainstem and spinal cord as well as many ascending afferent fibers. Above the peduncles is a large cell group known as the substantia nigra which is an essential part of the basal ganglia. In the thalamus we see the characteristic shape of the lateral geniculate nucleus laterally and medial geniculate medially. And now in the temporal lobe we see the hippocampus with the fibers that will make up the fornix on its surface. Be sure to admire the LGN and hippocampus in the Nissl stained section. The fornix can also be seen dorsally against the lateral ventricles.. In the rostral thalamus we can see the large internal capsule, which divides the thalamus medially from the putamen and globus pallidus laterally. The latter is also an important part of the basal ganglia. At the most ventral aspect of this section we can see the infundibulum, or stalk of the pituitary, with the optic tract on either side. The area just above the infundibulum is the hypothalamus. In the thalamus the most medial and dorsal nucleus is the anterior nucleus which pushes up into the lateral ventricles to make a bump called the anterior tubercle. At the midline just medial to the lateral ventricles we see the prominent fiber tract, the fornix which connects the hippocampus with the mamillary bodies. The fornix is in a C-shape curve and we see it again in the hypothalamus as it makes its way towards the mamillary bodies. Dorsolateral to the thalamus is the body of the caudate. In the temporal lobe we can see a large cellular mass which is the amygdala. Microscope Sections We have Nissl and Weigert stained sections of the spinal cord and brainstem of macaque monkeys to compare with the human sections. Obviously there are differences between the human and monkey but in general there is very good correspondence. Furthermore, there are differences in the plane of section so it should not be surprising that the sections don t all look exactly alike. In most cases we have paired Nissl and Weigert stained sections. Some structures or features are more easily seen with one stain than the other. The microscopes can accommodate 5 observers, so pair up with another pair when using the scope. One person who is familiar with the use of microscopes should be the driver or find one of us to help you. The glass slides are fragile so please handle the slides carefully!! They are valuable and difficult to replace. With improper use of the microscopes it is possible to break the glass slides!
Human Brain and Senses October 13, 2008 Page 8 Station 1: Spinal cord (1770/1771) Note the large alpha motoneurons in the ventral horn, fasciculus gracilis and cuneatus, the substantia gelatinosa. Where are the corticospinal fibers? On which side is the cell body, and the muscles they innervate? Caudal medulla (1667/1668) This section represents the transition between the spinal cord and medulla. You can still see the basic butterfly shape of the gray matter of the spinal cord with some differences as some brainstem nuclei begin to make their appearance. Along the ventral aspect the most prominent feature is the massive decussation (crossing) of the pyramids as the corticospinal fibers move from their position on the ventral surface of the medulla to the opposite lateral column of the spinal cord. Look at the cerebellar cortex and find the single row of large Purkinje cells at the outermost aspect of the cell-rich granule cell layer. In the cerebral cortex, we are in the most posterior aspect of the brain where the occipital lobe is found. See if you can find the border between area 17 and area 18 based on the cytoarchitecture of the cortex. Station 2: Mid medulla (1523/1524) At the ventral edge find the pyramids and the beginning of the inferior olive. At the dorsal aspect near the midline, find the large motoneurons of the hypoglossal (XII) nucleus. What muscles does the hypoglossal innervate. Think about where you found the hypoglossal nerve in the whole brain and see if you can see the rootlets of the hypoglossal nerve exiting the brainstem. Find the nucleus of the dorsal motor of the vagus (X). In the white matter of the cerebellum, find the deep cerebellar nuclei: the lateral dentate nucleus and the more medially placed interpositus Rostral medulla (1435/1436) This section is similar to the one in 1523 except the inferior olive is more pronounced. Station 3: Caudal thalamus (1123/1124) In this section the pons is seen ventrally, the midbrain above it and the thalamus above it. This is due to the fact that the brainstem has a flexure to accommodate our upright stance so coronal sections of the forebrain do not correspond to sections perpendicular to the long axis of the brainstem. In the midbrain area, find the cerebral peduncles with the substantia nigra just above it. Look laterally in the thalamus 1123 to find the lateral geniculate with its beautiful laminar structure. Just ventral to it is the hippocampus with a striking cytoarchitecture. Where does the term hippocampus come from? Station 4: Rostral thalamus/basal ganglia (882/883) At this level you can see the thalamus medially and the putamen and globus pallidus laterally, separated by the large fiber bundle known as the internal capsule. Find the caudate nucleus and note how similar it looks to the putamen. Together these two nuclei are known as the striatum and they are well known because the dopaminergic fibers from the substantia nigra end in the striatum. On the midline most ventrally you can see the pituitary, or hypophysis. Just above it is the hypothalamus which controls many of the hormones released by the pituitary. Find the fiber bundle known as the corpus callosum and the smaller fornix. Looking with the Nissl section (which doesn t stain axons) you can nevertheless see that the fibers of the corpus callosum are traveling back and forth between the left and right sides of the brain while the fibers of the fornix are going in and out of the section? Look at the cytoarchitecture of the cerebral cortex at this level and compare it with that seen in the occipital lobe at station 1.