Sheep
Brain Lab Manual v1.0
Important Safety
and Handling Notes
Your sheep brain was preserved in formalin, an aqueous
solution of the colorless, odorous formaldehyde gas. Before you begin working with your preserved
sheep brain, please take care to heed the following safety precautions. However, please also keep in mind that there
is no need for alarm: your specimen has been thoroughly washed such that when the
precautions below are followed, your exposure level will be well below that
which is safely permissible for humans.
1) NO FOOD or DRINK
inside the lab classroom.
2) WEAR GLOVES
when handling your preserved specimen, tray, tools, soiled paper towels, etc.
3) REMOVE GLOVES
when you are not working at the lab bench or handling your specimen.
4) CONTAIN YOUR
SPECIMEN inside the dissection tray.
5) USE DESIGNATED GARBAGE
CONTAINERS at the end of each lab bench for disposal of specimen litter,
gloves, and soiled paper towels.
6) IN THE EVENT OF
CONTACT: Don’t panic. Wash skin with
soap and water; flush eyes with water.
There is an eye-wash station at each sink. Also, please inform your
TAs so that they may help you with safety procedures.
Clean-up
procedure
After the lab, please clean-up as follows:
1) Use a paper towel to scrape ALL specimen litter out of
your dissection tray into designated garbage containers.
2) Thoroughly rinse your tray in the sink and stack upside
down with the other clean trays.
3) Dispose of your gloves in designated garbage containers
4) Wash your hands with soap and water
Objectives
The goals of this lab are to:
1) Familiarize yourself with basic anatomical terminology
2) Identify major structures of the mammalian brain and
their function
3) Understand the anatomical relationships, i.e. pathways, among
structures
4) Develop a 3-dimensional perspective of the brain and
apply this to the interpretation of 2-dimensional anatomical images (e.g. various
2-D sections).
Materials
sheep brain
dissection tray
gloves
knife
razor blades
probe
forceps
Procedure
A.
External view of the sheep brain
1. Orient
yourself with the intact sheep brain using the following terminology:
dorsal
ventral
medial
lateral
superior
inferior
anterior/rostral
posterior/caudal
Try to use these terms when pointing out structures to
others.
2. Learn
some basic neuroanatomical terms:
-Brains are often sectioned to reveal the interior
structure. Learn the following planes of
section:
coronal
sagittal
horizontal
-A cluster of cell bodies in the central nervous system
(CNS) is called a nucleus (e.g. lateral geniculate nucleus) whereas a
cluster of cell bodies in the peripheral nervous system (PNS) is called a ganglion (e.g. dorsal root ganglion)
-Tract, fasciculus,
and peduncle are are all terms to describe axon bundles in the CNS
-An axon bundle to or from the PNS is called a nerve.
3. Familiarize yourself with gross external features
(Don’t cut yet!)
-Peel off a small bit of the transparent, delicate pia mater, the innermost layer of the
three meninges which surround and
protect the brain. The tough, outermost dura mater and the middle arachnoid membranes have already been
removed.
-Locate the following major brain divisions:
spinal
cord
brainstem
midbrain
cerebellum
diencephalon
cerebrum
-The cerebrum is shaped like a sheet that is folded into bumps
called gyri (singular:gyrus) and grooves
called sulci (singular:sulcus). The deepest grooves are sometimes called fissures.
-Locate the following fissures/sulci:
medial
longitudinal fissure
rhinal
sulcus
sylvian (=lateral
in humans)
ansate
(=central in humans)
Not so conveniently, many anatomical structures go by
several names and can vary by species.
However, the names often make sense once you learn the basic
terminology. Can you guess the synonym
for midsaggital fissure?
Conveniently, these fissures form boundaries for the various
brain lobes. Where are the following
structures located relative to the above fissures? Don’t worry if you can’t find distinct
boundaries; they are often hard to find in the sheep brain.
frontal
lobe
parietal
lobe
temporal
lobe
occipital
lobe
pyriform
lobe
-Look for gray matter
vs. white matter. White matter gets its color from myelinated
axons whereas gray matter is composed of cell somata. (Hint: White matter is easier to see in the
next two steps where we will look at the corpus callosum).
4. Reveal
a few more occluded structures with some gentle prying
-Grasp the cerebral hemispheres and gently pull them apart. You might need to tear the thin pia but do not tear other structures. The
white matter you see inside is the corpus
callosum, which contains axon fibers interconnecting the two hemispheres.
-Grasp the cerebrum and cerebellum and gently pry them
apart, again you may tear pia, but do not rip off the cerebellum. You should be able to see the tectal nuclei: the superior colliculus and the
inferior colliculus
5. Identify
any intact cranial nerves which are
axon bundles coming from and going to the sensory organs and muscles in the
head. In addition to their descriptive names, they are numbered based on where they
enter the brain along the rostral to caudal axis. Most are visible from the ventral surface of
the brain. Most cranial nerves are
typically torn off during specimen preparation, so refer to the Cooley and
Vanderwolf figure for a complete image of the cranial nerves. Here are some prominent ones that you can
probably find:
optic n.
(II)
oculomotor
n.(III)
trigeminal
n. (V)
abducens
n. (VI)
What kinds of information are carried along these nerves?
6. Identify
additional structures visible from various external views of the brain 
-Above are ventral views of the sheep brain with the pituitary gland still intact. On the
right, the pituitary has been lifted to reveal the infundibulum and mammillary
bodies. The infundibulum is also
known as the pituitary stalk, and it contains axons of hypothalamic
neurosecretory cells which release hormones in the pituitary. The pituitary on your sheep brain was
probably removed with the dura.
-Notice how blood vessels are present nearly everywhere in
the brain. The brain is a metabolically
expensive organ! You might be able to
see some major arteries that converge at the circle of Willis, which is an arterial structure with redundant pathways.
B. Interior
midline view of the sheep brain
1.
Carefully cut along the midline of the brain and identify structures within:
C. Dissections
to reveal sub-cortical structures (use the RIGHT hemisphere)
1. Make a
preliminary T-shaped, shallow incision to facilitate dissection:
- Carefully read the instructions through the figure below before
you begin! Use the right hemisphere and set aside the left for section D. Use a
razor blade to cut through cortex. You
should stop when you hit the corona radiata (white matter), roughly 1cm deep. The
top of the T will be a partial coronal slice, roughly at the level of thalamus
along the rostral-caudal axis. The stem
of the T will be a partial sagittal slice roughly midway through the
medial-lateral extent of the right hemisphere, starting from your coronal
section and ending near the rostral pole of the brain. Try it!
2. Remove rostral-medial
portions of cortex and scrape away corona radiata to reveal basal ganglia structures
-Just use your fingers to pick away chunks of cortex!
3. Reveal
portions of internal capsule
-Continue with your dissection by removing the caudate and
scraping at the white matter to reveal its striated appearance. You should see the internal capsule, a
continuation of fibers to and from the corona radiata. As you will see in later coronal sections,
the internal capsule is so-called because it consists of discrete axon bundles
incapsulated within surrounding grey tissue.
Notice how many of the fibers course towards thalamus.
4. Remove portions
of cortex to reveal hippocampus (continue
with the right hemisphere)
Now remove the chunk of cortex caudal to your partial
coronal slice. Use your hands to pick at
cortex until you find the hippocampus, a smooth, curved structure that wraps
from the medial part of the brain towards the underside of the pyriform lobe.
D. Coronal
sections (use LEFT hemisphere)
-Cut coronal sections at roughly the levels indicated in the
figure below. Refer to the labeled
figures from Cooley and Vanderwolf for help with identifying structures. A good trick is to align facing sections
along the midline so that you can get an approximation of what a whole brain
section would look like.
Discussion
Learn at
least one motor and one sensory pathway, and work your way through these
pathway on the sheep brain. Refer to the provided pathway figures from Cooley
and Vanderwolf.
Here are
some additional topics to stimulate discussion\:
1) How does the sheep
brain compare (e.g. with regard to relative sizes and organization of various
brain structures) to the human brain which you have studied on the Comparative
Brains website, www.brainmuseum.org?
2) Consider the bilateral symmetry of the brain. Are there any unpaired structures? Where are there decussations (crossings) of
axonal tracts, and what does this suggest about left/right processing in the
brain?
3) What are some
neurological pathologies you find interesting (e.g. movement coordination,
anterograde amnesia, vision loss, death) and for each of these symptoms, in which
key brain area(s) might you expect to find a lesion? Can you identify them on the sheep brain?
4) Can you identify
any finer structural details with your naked eye? Using a knife, cut a cross-section of the
cortex and the hippocampus. Can you see
any laminar structure?
5) Reassemble the
left hemisphere of the sheep brain (i.e. the hemisphere you used to make
coronal sections) and see if you can visualize in your mind the internal brain
structures. Does it all make sense? Quiz yourselves by choosing a new plane of
section, predict what you will see, and then go ahead and cut to see if you are
correct. Make some more sections (but
you might be careful to avoid confusion by keeping your sections in order.)
References
An excellent free online guide: http://academic.scranton.edu/department/psych/sheep/
The Sheep
Brain: A Basic Guide by Cooley and
Vanderwolf
The Sheep
Brain: A Photographic Series
by Vanderwolf and Cooley