Lab 4
Back
to
Table
of Contents
Highlighted Words
SHEEP BRAIN DISSECTION: LAB 4
Coronal Sections.
Many structures of the brain are visible
neither on its external surface or medial face, but rather they lie deep
within the hemispheres. Thus, it is necessary to make serial sections
parallel to the coronal plane to see these features.
Section B.
Use Figure 2.4, p. 46, as a guide for cutting your specimen.
Use the large brain knife, not a small scalpel or knife. Remember, you
want to be able to perform the sections with one smooth, continuous movement.
Remove the frontal pole of the hemisphere by making section "B".
This plane of dissection should be made just caudal to the rostrum of the
corpus callosum and just anterior to the optic chiasm. The rostrum
is the caudal tip of the genu. Think of a leg flexed at the knee
and with the toes of the foot pointed. Remember that genu means knee
in Latin. Think of the genu as representing the knee of a flexed
leg. Follow the corpus callosum ventrally and caudally from the genu
to point at which the fibers end. Continuing with our analogy this
endpoint would represent the pointed toes at the end of our flexed leg;
this tip of the corpus callosum is the rostrum. After you have made
your first coronal cut, set the frontal pole aside. Look at the remainder
of the hemisphere, you should see something which approximates Figure 2.5
Section B, p. 47.
Notice that the coronal sections in Figure 2.5 are
ringed by a relatively dark rim of tissueand that the central area
of the section is a mixture of darker and lighter tissues. These
figures were produced by drawing thionine-stained coronal sections.
Thionine stains Nissl bodies which are found only in the soma of
the neuron, and not in the axon. Thionine staining thus provides
a measure of cell body density; the darker the tissue, the greater the
relative amount of cellular (non-axonal) material present.
Highlighted Words
Your sheep brain specimen is unstained tissue. As a result,
it will not show the great contrast between cell body matter and axonal
matter that is seen in the illustrations. Nonetheless, there are
readily visible differences in the unstained brain which we can examine.
In unstained tissue, the fiber areas appear white or light because of the
myelin sheath surrounding the axons. Cell bodies do not have a
myelin covering, so areas in which cell body density is greater appear
slightly darker and greyish. This relationship is the basis
for the terms gray
matter (cells) and white
matter (axons).
Look at the cortex, the outer layer of grey matter surrounding
the inner core of white matter. It should be apparent that the folds
of the gyri have greatly increased the surface area of the cortex, when
compared with a smooth or non-convoluted brain. Ask the instructor
or the lab assistant to show you the preserved mouse, rat and cat brains,
which vary considerably in terms of the amount of cortex present.
Join the frontal pole you removed with the remaining brain.
Look at the lateral aspect of the brain and find the rhinal fissure.
Follow the fissure around to the medial surface. You will see a layer
of cortex above the fissure and a layer below the fissure. The tissue
inferior to the fissure is paleocortex and is thinner than the layer
of neocortex above the fissure. This difference in thickness
is a consequence of neocortex (“new-cortex”) consisting of six layers of
various types of cortical cells, while the rhinencephalic paleocortex,
or archicortex, consists of only three layers of cortical cells.
It is the increase in amount of neocortex that characterizes the development
of the brain phylogenetically. Look at Figure 1.8, p. 36 once again
and notice the marked increase in the amount of neocortex that appears
as one moves up the phylogenetic scale.
Fibers (white matter) fill much of the interior of the hemispheres,
but there are also many groupings of cell bodies present. A large cluster
of cell bodies that resides within the brain, is called a nucleus,
while a cluster of cell bodies located outside the CNS is called a ganglion.
Bundles of axons located within the CNS are called tracts, but bundles
of axons found outside the CNS are referred to as nerves.
A noteworthy example of this, which you have already encountered, is cranial
nerve I, the optic. Recall that the bundle of fibers anterior to
the optic chiasm are called the optic nerve, while those same fibers caudal
to the chiasm that are internal to the CNS are referred to as the optic
tract.
Highlighted Words
There are many fiber groups present,
some of them are sensory, some are motor, and some are called “association”
fibers, fibers that connect one region of cortex with another. The largest
band of association fibers is the corpus callosum, which you have
already seen in sagittal section. Recall that it is a band of transversely
oriented fibers that connects similar areas of cortex in the two hemispheres.
Dorsal to the corpus callosum is the cingulate gyrus;
embedded within it is the cingulum. The cingulum is a band
of “association” fibers, that is, fibers that are neither sensory nor motor,
but that connect two cortical areas. The cingulum connects regions of the
frontal and parietal lobes; thus, these fibers run anterior to posterior
(front to back), while the corpus callosum runs side-to-side. Be
certain to note that the cingulum is a fiber structure consisting
of axons, while the cingulate gyrus is cortical matter that consists
primarily of cell bodies. This means that the former is white matter,
while the latter is grey matter.
Locate the large space just below the corpus callosum,
the lateral ventricle. Note that the corpus callosum
forms the roof of the lateral ventricles. The large cell mass forming
the ventro-lateral wall of the ventricle is the head of the caudate
nucleus. Just lateral to the head of the caudate nucleus, find
the band of fibers called the internal capsule, which consists of
axons descending from cortex. Many, but not all, of these fibers
are from the pre-central gyrus or motor cortex. These
fibers will converge into a more compact bundle and will migrate toward
the ventral surface of the brain, where they are known as the cerebral
peduncles. These same fibers appear along the ventral surface
of the medulla where they are known as the pyramidal tract.
Look at Figure 1.12, p. 42, which shows the internal
capsule as it would look if the cortex were stripped away.
Looking at your coronal section , note the nucleus lateral
to the internal capsule, the putamen. It is not clearly defined,
just know the general region within which it appears. The arrangement
of nuclei and fibers (caudate nucleus, putamen, and internal capsule),
gives the area a striated appearance, which prompts this area to be called
the corpus striatum. This area of the brain is important
for motor behavior; dysfunction of the structures within the corpus
striatum and/or in projections to or from it, are associated with movement
disorders such as Parkinson’s Disease and Huntington’s Chorea.
Section D.
This cut should be made just anterior to the mammillary body
at "D"
as shown in Figure 2.4 , p. 46, Use Figure 2.7, Section D , p. 48,
as a guide to locate the following structures. The lateral ventricles
have increased in their lateral extent but have lost the inferior component
that projected ventrally and parallel to the midline. The complexity
of the relationships between the ventricles becomes apparent in this section
with the appearance of the 3rd ventricle. It can be located
by finding the thalamus which serves as the ventral border
of the III ventricle and the body of the fornix, that provides its
dorsal , or upper, limit. Immediately superior to the fornix
is the lateral ventricle.
Highlighted Words
The internal capsule continues to be a prominent fiber
structure; it is becoming a more compact bundle as it moves toward the
ventral surface of the brain. Immediately lateral to the internal capsule,
a new nucleus has appeard, the globus pallidus. Immediately
adjacent and lateral to the globus pallidus is the putamen, which
was immediately lateral to the internal capsule in coronal Section
B. Now, locate the corpus callosum and fornix. You should
see a small cellular mass lateral to those structures and the ventricles;
it is the tail of the caudate nucleus. The caudate nucleus
is a large structure; you saw the head of the caudate nucleus located more
anteriorly in Section B. These three structures, the caudate nucleus,
globus pallidus, and putamen are known collectively as the basal ganglia.
The basal ganglia receive input from and send output to the frontal,
prefrontal and parietal lobes of cortex. One of the functions of
this loop appears to be the selection and initiation of willed movements.
The basal ganglia region of the brain is a center involved in habit learning
and may be related to obsessive-compulsive disorder. In a recent
research program PET scans were were taken in two groups of patient volunteers
with obsessive-compulsive disorder who were treated with either medication
or behavior therapy alone. After ten weeks, patients in both
groups who responded to treatment showed a significant change in the metabolism
of glucose. These changes appeared in the right caudate nucleus,
which is thought to be a regulatory center that serves to filter out unwanted
thoughts and behaviors. Patients who did not respond to therapy
failed to show these changes.
Coronal section, "D,"
lies caudal to the optic chiasm; therefore, those fibers carrying
visual information from the retinas are now called the optic tract..
A new structure can be seen lateral to the optic tract, the amygdala
(or amygdaloid nucleus), a large nucleus in the ventro-medial rhinencephalon.
It is wedge-shaped or somewhat triangular. Take a moment to locate
the amygdala on the external surface of your intact hemipshere, also.
It is the little mound at the anterior end of the rhinencephalon near the
optic chiasm. Can you recall the kinds of behaviors in which the
amygdala is involved?
Note that the thalamus has become very prominent. This large
celular structure can be subdivided into a number of ill-defined regions:
the anterior, lateral, medial , and ventral thalamus. The metathalamus,
another subdivision of the thalamus is located in the posterior, or caudal,
regions of the thalamus and will be considered in Dissection V.
We will concern ourselves only with the ventral thalamus. The ventral
thalamus contains the largest somatic relay nuclei. One of these
nuclei, the ventro-postero-lateral nucleus, is the projection site of somatosensory
information coming from cells in the nucleus cuneatus and nucleus
gracilis. (Refer to SHEEP BRAIN DISSECTION: LAB 2,
to review the information on these two somatosensory nuclei and the ascending
somatosensory tracts in the spinal cord.)
Highlighted Words
Two fiber bundles, one immediately superior to the other, appear
at the ventral midline. The inferior bundle
is the anterior column of the fornix projecting to the ipsilateral
mammillary
body and other structures. The superior bundle is the mammillo-thalamic
tract carrying information from the mammillary bodies to the anterior
thalamus. Before you proceed to the next section, find the
corpus
callosum and note that it forms the dorsal roof of the lateral
ventricle. You should note that both the III and
lateral
ventricles can be seen in this coronal section with the lateral ventricle
lying superior to the III ventricle. To complete this section of
the guide, locate the cingulum and the cortical tissue surrounding
it, the cingulate gyrus.
This completes LAB 4 . Make a list of all the boldfaced terms
and structures that were presented in this section. Now would be
an excellent time to take advantage of one of the more robust phenomena
in the cognitive literature, the spacing effect, by reviewing the
structures presented in the earlier dissections. Studying with
friends always helps!!!
Back
to
Table
of Contents