#4
A.
MOTOR AND VESTIBULAR SYSTEMS, CEREBELLUM
Motor System
Before tracing the motor pathways study of some of the cells involved is warranted. The
slides described below also include some of the connective tissue elements of the CNS,
i.e., the glia.
B.
1.
Pyramidal Cells. These pyramidal cells of cells of Betz (NH10) are among the
largest neurons in the brain and are found in the cerebral cortex. The apical
dendrite projects toward the surface of the cerebral cortex. Some lateral
dendrites can be observed extending from the cell body to make intracortical
connections. The centrally projecting axon is not stained well in this Golgi
preparation but may be seen as a stubby process arising from the cell body.
These are projection neurons and their axons may proceed all the way down to
lower levels of the spinal cord.
2.
Pyramidal Cell. This slide shows a high power view of the cell body of a
pyramidal cell (NH11) stained with hematoxylin and eosin. Nissl bodies are
evident. Note the fairly large nucleus of this cell plus the prominent, darkly
stained nucleolus. Smaller glial cells of the cerebral cortex can be seen
surrounding this large pyramidal cell.
3.
Motor End Plates. This slide illustrates axons branching in association with
skeletal muscle. Notice the terminal swellings of the axons. These are motor
end plates (NH26).
The corticospinal and corticobulbar system (190-193).
1.
The features of the corticospinal system that can be identified grossly should first
be viewed or reviewed. Identify the precentral gyrus (H2-11) and the premotor
and supplementary motor areas (H2-11). The cerebral peduncle (H2-23)
consists of descending fibers, some of which are corticospinal and some are
corticobulbar (crus cerebri). Moving caudally the next structure containing
corticospinal fibers is the pyramid (H2-20). The decussation of the pyramid
(H2-20) represents the crossing of approximately 80% of the corticospinal fibers.
2.
An overview of the corticospinal system is best obtained by viewing it in a sagittal
section (163, 165). Follow the fibers of the posterior limb of the internal
capsule (S4L) into the cerebral peduncle. Then, follow the corticospinal fibers
(S3L) which can be followed into the pons as the pyramidal tract and onward into
the pyramid in the medulla.
3.
Notice the continuity of fibers extending from the posterior limb of the internal
capsule (T9CL) into the cerebral peduncle (H5-31). Follow the peduncle
downward into the pons (T9AL, T9BL, T8AL, T8BL or H5-30, H5-26, H5-25, H524, H5-23, H5-22, H5-21, H5-20, H5-19 & H5-18). Examine slide T7L. What has
happened to the pyramidal tract at this level? Follow the tract down to the
pyramids in T5L and T3L or H5-12. Notice that these fiber bundles are so strong
that they break up the ventral gray columns at the point of their decussation (H58). Finally, visualize the position of the lateral and ventral corticospinal tracts
(T1DL or H5-4) in the spinal cord.
C.
4.
Motor nuclei of cranial nerves (201, 203) are activated by descending fibers from
the cortex. Many of these nuclei are reidentified in the following description.
Return to slide and Identify the oculomotor nerve nucleus (T9BL or H5-24, H525 & H5-26). Locate the fourth nerve nucleus (T8AL, T8BL or H5-23). The
trochlear nerve (H5-20) can be seen ascending around the cerebral aqueduct
(H5-20) to emerge from the dorsal surface of the brain. The fourth nerve is the
only motor nerve which decussates as it is emerging from the brain. Identify the
motor nucleus of V (T6HighL or H5-18) and the sixth nerve nucleus (T5L or
H5-17). Identify the motor nucleus of VII (T5L or H5-17). Identify the course of
the seventh nerve through the brain stem. What is meant by the internal genu of
the seventh nerve? What is the facial colliculus? If the nucleus ambiguus were
labeled on our slide sets, it would be dorsal to the inferior olivary nucleus
(T4AL or H5-9, H5-10, H5-11 & H5-12). What motor nerves arise from the
nucleus ambiguus? The nucleus of XII (T4AL, T3L or H5-10) can be seen just
ventral to the fourth ventricle. How do the fibers of the twelfth nerve reach the
surface of the brain? Determine the position of the nucleus of the spinal
accessory nerve (T2AL, T2BL or H5-8); it is not labeled.
5.
Corticobulbar fibers are axons of cell bodies in the motor cortex which terminate
on all motor nuclei except those which innervate extraocular eye muscles. The
latter nuclei are activated by fibers which originate in the frontal eye fields.
The Vestibular System (228-229)
1.
Fibers of the vestibular part of the eighth nerve (T5L) terminate primarily in the
four vestibular nuclei. This nerve can be seen entering the brain stem on this
slide.
2.
On slides T4AL, T4BL, and T5L, identify the four vestibular nuclei lateral to the
sulcus limitans in the floor of the fourth ventricle on the dorsal portion of the
medulla (H5-11, H5-12 & H5-13). It will not be necessary to differentiate specific
nuclei. Fibers from vestibular nuclei reach the cerebellum in a tract adjacent to
the inferior cerebellar peduncle (T4BL) called the juxtarestiform body (H513). Some fibers in the juxtarestiform body are direct projections from the
vestibular ganglion to the cerebellar vermis.
3.
Connections between vestibular nuclei and cerebellar structures course in both
directions through the juxtarestiform body. Although not labeled, look for the
fastigial nucleus of the cerebellum (S1L, T4BL or H5-15 & H5-16). It is one of
the roof nuclei located amongst the fibers of the cerebellar peduncles.
4.
One of the important functions of the vestibular system is the maintenance of
posture and the posture of the body in space. This is accomplished through
vestibular reflexes. To serve this function, secondary connections are made from
the vestibular nuclei to motor nuclei of nerves associated with the extraocular
muscles, muscles of the neck, trunk, and limbs in that order of importance.
5.
D.
Identify the medial longitudinal fasciculus (T3L or H5-12). This tract is made
up of a number of fiber types as a quick route to connect various brain stem
nuclei. Fibers projecting from the vestibular nuclei use this tract to project to
motor nuclei of cranial nerves. Coordination of eye movements also makes use
of this tract. Follow this tract forward (T4AL, T4BL, T5L or H5-17, H5-18 & H519) and note its proximity to the nucleus of VI (H5-17). Continue following the
medial longitudinal fasciculus (T6HighL, T7L, T8AL, T8BL) rostrally. Locate
the nucleus of IV (H5-23), close to the medial longitudinal fasciculus. The
nucleus of cranial nerve III (T9BL or H5-24) is located in the same approximate
position as IV, and its proximity to the medial longitudinal fasciculus can be seen.
The medial longitudinal fasciculus (H5-4) also projects caudally where it
moves into the ventral funiculus of the spinal cord. In the ventral funiculus, this
tract is also called the sulcomarginal tract. Fibers from the sulcomarginal tract
terminate indirectly in association with ventral horn cells. This connection allows
muscles of the neck and upper limbs to be brought under reflex control of the
vestibular system. Another pathway serving a similar function projects down the
spinal cord from the lateral vestibular nucleus. This is the lateral
vestibulospinal tract (H5-3) and it descends in the ventral funiculus to caudal
segments of the spinal cord. Fibers also terminate indirectly in association with
ventral horn cells. Projections of the vestibular nuclei also reach various visceral
nuclei, and irritative lesions may result in vomiting, sweating, motion sickness,
etc.
The Cerebellum (204-211)
1.
Examine the hemisected cerebellum (H2-31, H2-32) and, with the aid of the
diagram in the atlas, identify the primary fissure. This separates the anterior
from the posterior lobe of the cerebellum.
2.
Now turn to the posterior (caudal) portion of the cerebellum and identify the
vermal segment called the nodule (H2-32, marked x) Laterally it is continuous
with the flocculus (H2-31) These structures are associated with the vestibular
system and collectively form the flocculonodular lobe of the cerebellum. It is
separated from the rest of the cerebellum by a fissure occurring on the medial
surface of the vermis between the uvula and the nodulus. The name of this
fissure isn=t important. The area between the primary fissure and the
flocculonodular lobe and its continuation laterally constitutes the posterior lobe
of the cerebellum. It is obviously the largest lobe of the cerebellum.
Use the following images to study the histology of the cerebellum.
3.
Cerebellum. This photomicrograph was taken at a very low power to illustrate a
folium (or leaf-like process) of the cerebellum (NH6). Observe the lightly stained
central core of white matter within the folium. Surrounding the white matter is a
layer of purple stained nuclei. This is the granular layer of the cerebellar cortex.
The lightly stained peripheral layer of the folium is the molecular layer of the
cerebellar cortex .
4.
Cerebellar Cortex. This slide illustrates the three layers of the cerebellar cortex
(NH7). At the top of this projected slide can be seen the deeply stained granule
cell nuclei making up the granular layer. The molecular layer, containing
comparatively few cells but many processes can be seen at the bottom of this
slide. A few nuclei of other interneurons are evident in this layer. Some rather
large cell bodies occur between the granular layer and the molecular layer.
These are Purkinje cells of the Purkinje cell layer. Other than Purkinje cells, all
neurons of the cerebellar cortex are interneurons.
E.
F.
5.
Purkinje Cells. Note the two large, flask-shaped cell bodies in this slide. These
are cell bodies of Purkinje cells (NH8) and are classified as projection neurons.
6.
Purkinje Cell. This Purkinje cell (NH9) is stained by means of the Golgi method
which demonstrates both the cell body and its processes. Note the large, black
cell body and highly branched dendrites extending down to the lower left corner of
this slide. Dendrites of Purkinje cells project in a fairly orderly manner into the
molecular layer of the cerebellar cortex. Purkinje cell axons proceed into the
underlying white matter of the cerebellum to synapse with deep cerebellar nuclei.
Cerebellar Afferent Fibers
1.
Afferents to the cerebellum come from all areas of the brain (206, 207). From the
upper body there are cuneocerebellar fibers which originate in the lateral (or
accessory) cuneate nucleus (T3L or H5-9). From the lower body fibers reach
the cerebellum in the spinocerebellar tracts.
2.
Identify the position of the dorsal nucleus of Clarke (H5-3) on the slide of the
thoracic cord (T1CL). Locate the position of the dorsal and ventral
spinocerebellar tracts (T2AL, T2BL or H5-4). Locate their nuclei of origin. Are
fibers in these tracts crossed or uncrossed at this level? The dorsal
spinocerebellar tract reaches the cerebellum via the inferior cerebellar
peduncle (T4AL, T4BL, S4L or H5-10, H5-11 & H5-12). The ventral
spinocerebellar tract travels to the cerebellum via the superior cerebellar
peduncle (T5L, T6HighL, T7L, S3L or H5-17, H5-20, H5-23).
3.
Areas of the cerebral cortex send fibers through the internal capsule and cerebral
peduncle to synapse in the pontine nuclei. Postsynaptic fibers project from the
pons to the cerebellum via the middle cerebellar peduncle. Confirm the course
of this pathway through the middle cerebellar peduncle (T8AL, T8BL, T7L,
T6LowL or H5-19) The termination of the middle cerebellar peduncle in the
cerebellum is shown on slide S5L. (169) The inferior cerebellar peduncle (S4L,
T4AL, T4BL) carries fibers to the cerebellum from the limbs and trunk. What
tracts contribute to this entity?
4.
Study slides T4BL, S3L, S4L or H5-12, H5-15, H5-16, 163-169) On these slides
identify the inferior olivary nucleus, origin of olivocerebellar fibers, the inferior
cerebellar peduncle, the cerebellar cortex, and each of the four Aroof@ nuclei,
fastigial, interposed (globose and emboliform), and dentate nuclei.
Olivocerebellar fibers (T4BL (not labeled) or H5-12 (labeled)). What do we call
these fibers in the cerebellum?
Cerebellar Efferent Fibers
G.
1.
Cerebellar efferent connections (210-211) are made by fibers originating from the
three deep cerebellar nuclei. From medial to lateral they are the fastigial,
interposed and dentate.
2.
The juxtarestiform body (T4BL or H5-13), is the pathway for fastigial efferent
fibers to the brain stem. The superior cerebellar peduncle (T5L or H5-13) first
appears on these slides and is well shown emerging from the cerebellum in slide
S3L (165). Continue tracing the superior peduncle rostrally. Note its
decussation (T9AL or H5-23). This tract terminates in the red nucleus (T9BL,
T9CL, S1L, S2L, S3L or H5-25). Note that some fibers of the superior peduncle
pass through the red nucleus and are joined by fibers originating in this nucleus
to proceed to the dorsal thalamus. Observe these dentatothalamic and
rubrothalamic fibers (T9CL or H5-26 (labeled)) as they proceed to the ventral
lateral (VL) nucleus of the thalamus (T10L or H5-31). Some of these fibers
may be seen on slide S2L and S3L; they are not labeled. Where do fibers travel
from the ventral lateral nucleus? Why are the effects of lesions of the cerebellum
expressed on the same side as the lesion? There are also afferent fibers in the
superior peduncle. What part of the body is represented?
Basal Ganglia (214-217)
1.
The basal ganglia is a collection of subcortical nuclei which include:
a.
b.
c.
d.
e.
The caudate nucleus
The putamen
The globus pallidus
The subthalamic nucleus
The substantia nigra
2.
The caudate nucleus (head or body and tail), putamen, two parts of the
globus pallidus, claustrum, external and extreme capsules and insular
cortex should be viewed on the horizontal sections of the gross brain (H4-12).
The relationship of the lentiform nuclei (putamen and globus pallidus) and
caudate nucleus to the internal capsule should be noted and as well the
relationship of the caudate nucleus to the ventricular system. The claustrum,
internal and external capsules and insular cortex are readily apparent on most
specimens. Many of these same structures as well as the substantia nigra (H532, H5-33 & H5-34) can also be identified on axial sections.
3.
View slides T9AL, T9BL, T9CL and identify the caudate nucleus. Why can it be
seen both at the top and the bottom of section T9CL or H5-31? Follow the
caudate rostrally (T10L, T11L, T12L, T13L and H5-36). Identify the putamen
and globus pallidus (T10L, T11L, T12L or H5-34). You will notice several
bundles of darkly stained fibers medial to the internal capsule projecting along the
lower surface of the thalamus. These bundles represent the major efferent
fibers of the basal ganglia (H5-34). These fibers will reach the ventral anterior
and ventral lateral nuclei of the thalamus. Fibers from the ventral portion of the
globus pallidus form the ansa lenticularis (T10L, T11L or H5-34) which loops
around the internal capsule. Fibers from the dorsal portion of the globus pallidus
pass across the internal capsule and enter the lenticular fasciculus (T10L,
T11L or H5-33) lying adjacent to the subthalamic nucleus. The lenticular
fasciculus forms the ventral border of the zona incerta (T10L, S4L or H5-33), an
extension of the reticular formation into the diencephalon. The thalamic
fasciculus (T10L, S4L or H5-33) represents combined fibers of the ansa and
lenticular fasciculi passing laterally along the ventral border of the thalamus.
4.
Certain thalamic nuclei receive information from the basal ganglia and convey this
information to motor areas of the cerebral cortex. Three important nuclei having
connections with the basal ganglia and the cerebral cortex are the ventral
anterior nucleus (H5-33), the ventral lateral nucleus (H5-32) and the
dorsomedial nucleus (H5-31, H5-32) having connections to the amygdaloid
complex. The ventral anterior nucleus (T12L, S2L, S3L, S4L) receives fibers
from the globus pallidus and substantia nigra. The ventral lateral nucleus
(T10L, S1L, S2L, S3L, S4L) also receives input from these two areas; it also
represents the major thalamic termination (direct or indirect) of axons projecting
forward from the cerebellum.
5.
The subthalamic nucleus (T10L, S4L or H5-31), projects fibers to the globus
pallidus and the substantia nigra. The red nucleus (T9BL, T9CL or H5-26) can
also be seen in the rostral part of the midbrain tegmentum and extends forward
into the caudal part of the diencephalon just dorsal to the subthalamic nucleus.
The red nucleus receives projections from the cortex and the cerebellum. Fibers
proceeding from this nucleus decussate immediately in the ventral tegmental
decussation. These fibers proceed down the brain stem as the rubrospinal tract,
and in the spinal cord they occupy a position just ventral to the lateral
corticospinal tract to eventually terminate on ventral horn cells and spinal
interneurons. Another descending bundle of fibers that may be seen on your
slides is the central tegmental tract (H5-25, H5-24, H5-20, H5-18). It is
identified on slides T6L, T6HighL and T5L and can be traced rostrally from this
point. Fibers in this tract originate from the cerebral cortex, tegmental, reticular
and other subcortical nuclei. The majority of these descending axons terminate
in the inferior olivary nucleus (T5L or H5-12).
6.
The substantia nigra (T9BL (unlabeled), S2L, S3L or H5-24, H5-25). Locate it
on these slides and define its boundaries. This important nucleus projects to the
striatum. Degeneration of dopamine containing neurons in this nucleus occurs in
Parkinson=s disease.
7.
Located ventral to the anterior limb of the internal capsule is the nucleus
accumbens (T13L) where it is continuous with the caudate nucleus and
putamen (H5-36). This area, also known as the ventral striatum, projects to the
basal nucleus of Meynert (H5-35) containing neurons considered to be an
extension of the globus pallidus. The nucleus of Meynert may be seen just lateral
to the substantia innominata and ventral to the anterior commissure (T11L).
Projections from this nucleus eventually affect descending motor systems.