Download sheep brain dissections

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SHEEP BRAIN DISSECTIONS
This lab will be carried out in pairs. Pick a partner. Ask the TAs for help if you can't find one.
Collect your supplies. Each pair of students will need two sheep brains. You will be using one
of the brains for Parts I, II and IV, and the other brain for part III. You will also need a stainless
steel tray or two to lay the brains on, latex gloves, a dissecting knife, a razor blade, and a fine
blunt probe.
The lab will be carried out over two lab periods, so don't rush to do everything in the first
session. Take your time and make sure you have really studied the material at each stage before
proceeding. You might try to pace yourself so you get through Parts I and II of the dissection by
the end of the first lab. The instructor and TAs will be happy to help you and answer any
questions you may have. The exercises listed here are what we want everyone to do. If you have
enough time and interest, feel free once you’ve gotten through them to dissect the brains further
and look for other structures and relationships you are interested in. In addition to the figures
included at the end of this handout, we strongly recommend that you take a look at this online
Sheep Brain Atlas from Dr. John Irwin Johnson at Michigan State.
At the end of the each lab period, store your brains in plastic bags identified with your name as
directed by the TAs. Be sure to clean and dry your dissection trays and tools and return them in
the designated spot.
PART I - SURFACE FEATURES
Dura - The brain should be largely covered with a thick translucent membrane. This is the dura
mater. On the ventral surface of the brain, small areas of skull have been left attached in order to
leave the cranial nerves, olfactory bulbs, and pituitary relatively intact. Begin by removing the
bone, leaving as much of the embedded neural structure intact as possible, by prying the dura and
brain away from the skull and cutting them apart where necessary. Now, note the superior
sagittal sinus, a rostrocaudally running venous space that lies at the midline on the dorsal
surface. This sinus collects much of the venous drainage from the hemisphere, and also
represents the point at which cerebrospinal fluid empties into the vascular system. Using the
razor blade, remove the dura from first one of the hemispheres and then the other by cutting just
on either side of the sinus and along the caudal margin of the occipital lobe. Note that there is a
sheet of dura mater (called the falx cerebri) that extends down from the superior sagittal sinus
into the sagittal fissure (i.e., the space between the hemispheres). Note also that a second flap of
dura inserts itself between the occipital lobe of the hemisphere and the cerebellum. This is the
tentorium cerebelli. You are about to remove the dura from the cerebellum and brainstem, but
before you start, realize that the cranial nerves are anchored in the dura (which is continuous with
the connective tissue sheath that wraps these nerves). Therefore, to leave the nerves as intact as
practical, you would like to gently dissect the dura free with your razor blade rather than simply
yanking the dura off. Finally, remove the dura from the cerebellum.
Cerebral Hemisphere - Note that there are two cerebral hemispheres. Looking down on the
brain from the dorsal surface, note the deep sagittal fissure that separates them. This is the space
in which the falx cerebri lies. Virtually the entire surface of the cerebral hemisphere is covered
with the cerebral cortex. Note that the cortex of the sheep, like our own, is thrown into a large
number of folds. The "hills" are called gyri and the "valleys are called sulci. The cortex is
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actually a single continuous sheet. Using a blunt tool like the back of a scalpel handle, pry apart
a sulcus or two to convince yourself of this. Note that just behind the cerebral hemisphere lies
the cerebellum; you have already removed the tentorium from between the two structures.
Gently pry the cerebellum away from the hemisphere and look down into this space; you are
looking at the dorsal surface of the upper brainstem.
Ventral Surface of the Forebrain - Now turn your attention to the ventral surface of the
hemisphere. Note the two olfactory bulbs and the olfactory tracts that extend back from them.
These convey information about odors to parts of the basal forebrain that are part of the limbic
system. Identify the optic chiasm, the main decussation in the visual pathway. (If you can't find
it, check the skull you removed; it may have been torn away from the brain). You may able to
see the stumps of the two optic nerves entering the chiasm from the anterior side. Behind the
chiasm, the optic fibers split again into two optic tracts, one on each side. These carry visual
signals to various visual nuclei of the brain. Just behind the chiasm lies the hypothalamus, a part
of the diencephalon. The hypothalamus does not appear very big from its ventral aspect. It
continues back only as far as the midline mammillary bodies. (In human and other mammalian
brains these form two distinct humps, but in the sheep brain they tend to fuse). Between the
chiasm and the mammillary bodies, you may find the pituitary (if it wasn't lost when removing
the skull), a bulb hanging off the ventral surface of the diencephalon. If the pituitary was lost in
dissection, you should see instead a small midline protuberance and perhaps a small hole in its
center. The hole represents the most ventral tip of the third ventricle.
The Cerebellum - The cerebellum is highly infolded, like the cerebral cortex, but here the
"hills" are called a folia rather than gyri. You may be able to see a subtle ridge marking the
midline of the cerebellum. This is the vermis. It is probably easiest to see from the posterior
aspect of the cerebellum. Look at the ventrolateral aspect of the cerebellum and you may be able
to see a thick fiber bundle that connects it to the brainstem. This is middle cerebellar peduncle,
one of three such peduncles on each side of the brain. We'll take these up again later.
The Brainstem - Viewing the brainstem from the ventral surface, you should easily be able to
identify the pontine protuberance. This marks the ventral surface of the pons, the middle of the
three major brainstem divisions. If you can see any small bit of brain between the rostral end of
the pontine protuberance and the mammillary bodies, this is the ventral surface of the midbrain,
the most rostral of the three divisions of the brainstem. You may be able to get just a glimpse of
two big fiber bundles on either side of the midline here. These are the cerebral peduncles, which
are made up of fibers originating in the cortex and bound for the brainstem and spinal cord.
Behind the pons lies the medulla, the most caudal of the three brainstem divisions. Right along
the midline, the ventral surface of the medulla is marked by two narrow humps, the pyramids.
At rostral levels of the medulla, just behind the pontine protuberance you may be able to see two
slight bulges just lateral to the pyramids; these are the olives.
The Cranial Nerves - Depending on the quality of your specimens, you may be able to identify
most or all of the cranial nerves. Take advantage of the fact that you are sharing two brains and
each brain has two sides. This gives you four shots at seeing a clear example of each nerve. If
you still have trouble finding one or more of the nerves, ask around and see if any of the other
groups have been able to identify them.
Starting from the rostral end of the brainstem, look for the oculomotor nerve (III) near the
midline of the midbrain, just in front of the pontine protuberance. If you see a very fine nerve
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just lateral to this, you're looking at the trochlear nerve (IV). The trigeminal nerve (V) is easy:
it's thick and is the only one emerging through the pontine protuberance. Look for it well away
from the midline. Just at the junction of the pons and medulla (i.e., at the caudal margin of the
pontine protuberance), there are three nerves, numbered from medial to lateral. The one nearest
the midline on each side is the relatively thin abducens nerve (VI). Much farther lateral, and
possibly even partly obscured by the cerebellum, are the facial nerve (VII) and, just lateral to
this, the auditory or vestibulocochlear nerve (VIII). The last three nerves all emerge from the
medulla. The first of these, the glossopharyngeal (IX) comes out just behind the facial and
auditory nerve and may be tough to distinguish from them. The last three nerves to exit the
medulla typically emerge as fine rootlets that don't coalesce into a unified nerve until a short
distance outside the brain. Just behind the glossopharyngeal nerve you should be able to detect a
series of fine rootlets of the vagus nerve (X). The rootlets of the spinal accessory nerve (XI) lie
just caudal to those of the vagus and are usually tough to distinguish clearly from them. Finally,
the rootlets of the hypoglossal nerve (XII) emerge ventral and medial to those of the vagus and
spinal accessory, nearer the midline and just lateral to the pyramid.
PART II - A CLOSER LOOK AT THE BRAINSTEM
Many of the important features on the surface of the brainstem are obscured in the intact brain by
the cerebral hemisphere and cerebellum. To get a better look at the brainstem, you will now
dissect it free from the surrounding structures in one of your two brains.
Performing the Dissection - Begin by removing the cerebellum. Using your dissecting knife,
make a cut through the cerebellar peduncles beginning at the point at which the cerebellum meets
the pontine protuberance. As you make the cut, the tip of your blade should be pointed dorsally
at about a 45 degree angle so that you don't cut into the brainstem proper. It may make you less
nervous if you start with a shallow cut that doesn't completely sever the peduncles, then follow
this with one or more deeper cuts to separate the cerebellum from the brainstem on that side.
Now repeat the cut on the other side and lift the cerebellum off the brainstem.
The next step is to separate the brainstem from the cerebral hemisphere. To do this you want to
make a transverse cut through the brainstem at a level just in front of the midbrain. Before you
make your cut, get yourself oriented. On the ventral surface of the brainstem, find the
mammillary bodies again. You will be cutting right through these. Now get a good look at the
dorsal surface of the brainstem by viewing the brain from the back and gently prying the back of
the hemisphere and the brainstem apart. You should be able to see two pairs of prominent
bumps: the inferior and superior colliculi. Your cut should pass a few millimeters rostral to the
superior colliculi. When you are oriented, make the cut through the mammillary bodies, trying to
cut through the entire brainstem without doing too much damage to the surrounding cerebral
hemisphere. Again, there is no harm in making the cut in several steps. If you did your job well,
the brainstem should easily come free from the hemisphere. If not, you may need to help it along
with a few extra cuts, but try not to chew up the brainstem too badly.
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Cerebellar Peduncles - Study the relationship between the cerebellum and the brainstem
you've just separated it from. To cut the cerebellum off, you had to cut through three cerebellar
peduncles. These are usually not grossly distinguishable as three different clusters of fibers, but
try nonetheless to get a sense of where they are. The large middle cerebellar peduncle connects
the pons to the cerebellum and will probably appear in your dissection a an oval stump at the
level of the pons. The smaller inferior cerebellar peduncle interconnects the medulla with the
cerebellum and may be hard to distinguish from the middle cerebellar peduncle; its fibers may
simply make up the caudal and medial component of the same stump. If you look closely at the
lateral aspect of the medulla just behind the stump, you may be able to see a bulge that is
continuous with the cut stump; this is the caudal continuation of the inferior cerebellar peduncle
and at this level it is more easily distinguished from the middle. Finally, look at the rostral and
medial aspect of the same stump you've been studying. You should be able to see a small band
of cut fibers that interconnect the cerebellum with the caudal part of the midbrain. This is the
superior cerebellar peduncle.
Fourth Ventricle - Note that medial to the cerebellar peduncles, the dorsal surface of the
brainstem has not been cut. This is the floor of the fourth ventricle. Likewise, the underbelly of
the cerebellum has not been cut medial to the peduncles. This part of the cerebellum forms much
of the roof of the fourth ventricle. On the brainstem, you should be able to see that caudal to the
cerebellar peduncles the fourth ventricle tapers to a point, called the obex. This is the most
caudal end of the ventricular system. The region just behind the obex is still the medulla, but a
part of the medulla that has no overlying fourth ventricle (only a rudimentary central canal at its
core). Rostrally, the fourth ventricle also tapers, giving it overall a diamond or rhomboid shape
(hence the term "rhombencephalon" for the embryonic pons-plus-medulla). At the rostral tip of
the fourth ventricle, you should be able to see that the ventricular system starts to be roofed over
by the dorsal part of the midbrain, that is, the tectum. At this point the fourth ventricle is
confluent with caudal end of the cerebral aqueduct. The fourth ventricle is at its widest just
behind the middle cerebellar peduncles. Lying at the lateral margin of the fourth ventricle at this
point (but not really visible in your material) are the lateral apertures or Foramena of Luschka,
the main exit point for the ventricular system where cerebrospinal fluid passes into the
subarachnoid space.
Dorsal Surface of the Midbrain - Note the four bumps on the tectum (roof) of the midbrain.
The caudal two are the inferior colliculi; the rostral two are the superior colliculi. Just behind
the inferior colliculi, you may be able to see a very fine pair of nerves emerging from the caudal
midbrain. These are the trochlear nerves (cranial nerve IV). Just in front of the superior
colliculi you may see a single unpaired bulb right at the midline. This is the pineal body, a
structure Descartes thought was the seat of the soul. Just in front of this (assuming you have
anything in front of this!) you should see a slot at the midline. This is the top of the third
ventricle. On either side of the third ventricle lies the thalamus.
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PART III - MIDSAGITTAL HEMISECTION: THE MEDIAN ASPECT OF THE BRAIN
Now take the second (intact) brain and with your dissecting knife cut it completely in half along
the midline, in the plane of the superior sagittal fissure. Now lay each half of the brain on its
lateral surface so you can study the cut (median) surface.
The big fiber bundle connecting the two cerebral hemispheres is the corpus callosum. The
enlargement at the caudal end of the callosum is called the splenium, while the curved rostral
end is called the genu (which means "knee"). Identify the medulla, pons and midbrain. Now
study the ventricular system and its relationship to these divisions of the brainstem. Note that the
fourth ventricle is, from this perspective, a tent-shaped space lying between the cerebellum
above and the pons and medulla below. Follow the ventricular system rostrally from the fourth
ventricle. If you made a perfect midline cut, you should be able to see it taper down until it is
nothing more than a thin channel through the midbrain. This is the aqueduct. If your cut is a
little off the midline, you may want use a razor blade to "shave" down the cut face of the
midbrain on the appropriate side of the brain until you reveal the aqueduct. Rostral to the
midbrain the aqueduct opens up to form the third ventricle. This is probably the hardest
ventricular space to recognize in this view. Look for a place just rostral to the midbrain where
your knife cut not through tissue but through a space walled on either side by the uncut
diencephalon. The wall is formed dorsally by the medial surface of the thalamus and ventrally
by the hypothalamus. Just in front of the thalamus, you may be able to find a small aperture
heading laterally, toward the core of the cerebral hemisphere. This is the interventricular
foramen or the foramen of Monro, the point of confluence of the third ventricle with the lateral
ventricle. Obviously, there are two such openings, one for each hemisphere. Between the
thalamus and the genu of the corpus callosum lies a thin sheet of white matter called the septum
pellucidum. If you made a perfect midline cut, this will be intact on both sides of the brain. If
you were slightly off the midline, or if you ripped this sheet, you will see instead into the
ventricular space within the hemisphere on that side. This is the lateral ventricle. Finally, look
at the cerebellum and note the structure of the folia; they have a thin rind of gray matter on the
outside (the cerebellar cortex) and a core of white matter on the inside (the cerebellar white
matter).
PART IV - INTERNAL STRUCTURE OF THE CEREBRAL HEMISPHERE
Take the paired cerebral hemispheres from which you earlier removed the brainstem and
cerebellum and lay it on its ventral surface. Make sure you know which end is the front. Then,
just for marking purposes, make two shallow cuts in the cortex at the levels shown in the figure
at the back of these lab notes. The first should be at a rostrocaudal position that is between twofifths and half the way back from the frontal pole of the brain and lies at about the same position
as the optic chiasm. The second mark should be made about three-fifths or two-thirds of the way
back from the frontal pole. Now make two complete transections of the brain in the transverse
(i.e., the frontal or coronal) plane at the level of your two marking cuts.
Rostral Level - Now lay out the middle slab of the brain flat on your table caudal side down
and examine the structures visible on the more rostral cut face, identified in the figure appended
to these lab notes. First, note the distinction between the cerebral cortex and the underlying
cortical white matter. The large fiber bundle near the center of your slice that interconnects the
white matter on the two sides is the corpus callosum. The callosum forms the roof of two
butterfly-wing-shaped spaces near the middle of your slice; these are the lateral ventricles. They
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are separated from one another at the midline by the thin septum pellucidum and the thicker
fornix, a fiber bundle that interconnects the hippocampus with other limbic structures. Just
lateral to the lateral ventricle you should find the caudate nucleus, part of the basal ganglia.
You may be able to see a series of cell bridges streaming ventrally and laterally from the caudate
toward a second large subcortical cell mass at the ventrolateral margin of your slab, the putamen.
The large fiber bundle that separates the caudate from the putamen is the internal capsule.
Caudal Level - Now flip the slab over and study the caudal face, again referring to the relevant
figure at the back of the lab notes. Several of the features visible at the rostral level should also
be apparent here, including the cortical white and gray matter, corpus callosum and lateral
ventricles. The fornix has widened and forms a flat sheet of fibers lying under much of the
lateral ventricles. The caudate is still present at the lateral margin of the lateral ventricles but has
become much smaller. The putamen will probably no longer be visible at this level, but the
internal capsule is very prominent. Ventrally, the internal capsule can probably be seen to be
continuous with the cerebral peduncles which should be visible at the ventral margin of the
brain, somewhat off the midline. Just lateral to this, the optic tract should be visible as a distinct
compact bundle of fibers.
The most important new feature at this level is the thalamus, which makes up most of the large
mass of gray matter that you see at the core of your slice. The thalami on the two sides are
probably joined at the midline by the interthalamic adhesion. Notice the ventricular space lying
just above this adhesion, below the fornix, and occupying the midline; this is part of the third
ventricle. Another part of the third ventricle can probably be seen as a cavity at the midline just
below the interthalamic adhesion. The gray matter surrounding this hole and extending all the
way to the base of the brain is the hypothalamus. Just lateral to the cerebral peduncles and optic
tract, you should see gray matter that is the sheep's equivalent of the temporal lobe; depending
on the exact level of your section, the grey matter in the most medial part of this gray-matter
region may represent either the amygdala or the hippocampus.
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