#3 BRAIN STEM, CRANIAL NERVE NUCLEI, AND
SOMATOSENSORY SYSTEM
A.
B.
Brain Stem
1.
Before using sections of the brain stem to follow pathways of ascending and
descending fibers, it will be helpful to get an overview of the appearance of the
various parts of the brain stem in section, identify some characteristic landmarks
in each, and correlate the gross structures with those seen in section. Examine
the brain, half brain, brain stem or plastinated sections of brains. A videotape of
a demonstration of a brain stem is available for study.
2.
On the ventral aspect of the medulla identify the pyramids (H2-20) (control of
skeletal muscle) and the decussation of the pyramid (H2-20). The decussation
is the landmark for separation of medulla and spinal cord. The feature is not
always as prominent as diagrams suggest. On the lateral surface note the
inferior olive (H2-22) and the trigeminal eminence (tubercle) (H2-22). (The
eminence is a manifestation of a tract of the trigeminal nerve. It is labeled
tuberculum cinereum in H2-36, but, since there is another structure with this
same name, it is best to use trigeminal eminence.) The trigeminal eminence is a
not very prominent bulge just dorsal to the olive.
3.
On the dorsal surface of the medulla identify the cuneate fasciculus (H2-34), the
cuneate tubercle (H2-34), the gracile fasciculus (H2-34) and the gracile
tubercle (H2-34). Find the inferior cerebellar peduncle (H2-34, labeled
restiform body in the atlas), afferent to the cerebellum, between the lateral border
of the fourth ventricle and posterolateral sulcus (H2-34) in the rostral portion of
the medulla. Look for a slight transverse elevation formed by the dorsal and
ventral cochlear nuclei (H2-34) (hearing), on the surface of this peduncle.
Follow this elevation laterally to the acoustic nerve (H2-34).
4.
Observe the floor of the fourth ventricle in the diagram (ND3) and re-identify the
superior cerebellar peduncle (H2-34), the median sulcus (H2-34) and the
sulcus limitans (H2-34). What is the significance of the latter? The bulge on
either side of the median sulcus is the median eminence. The part of this
eminence just rostral to transverse ridges called medullary striae (H2-34) is the
facial colliculus (H2-34). Caudal to these ridges identify the vagal trigone (H234) and the hypoglossal trigone (H2-34). Can you guess the reasons for these
names? The vestibular area (H2-34) lateral to the sulcus limitans is for
vestibular nuclei.
5.
Re-identify the inferior and superior colliculus (H2-34) on the dorsum of the
midbrain, the cerebral peduncles (H2-20) and interpeduncular fossa (H2-20)
on the ventral surface of the midbrain and the cerebral aqueduct (H2-30) on the
sagittal section.
Horizontal Section of Hemispheres - With a brain knife make horizontal sections of one
half brain. Be sure one section shows a cut of the genu of the internal capsule (H4-
12).
C.
1.
Study a horizontal section of the hemisphere. Structures to be identified are
shown in the atlas.
2.
First note the lateral ventricles (H4-12) which have been opened. The posterior
horn in the occipital lobe will not be seen but the anterior and inferior horns
(H4-12) are readily seen. Choroid plexus (H4-12), the origin of CSF, will be
seen as a tufted tissue in the lateral ventricle. Next locate the internal capsule
(H4-12), the genu (knee) (H4-12) and anterior and posterior limbs (H4-12).
These are ascending and descending fibers passing to and from the cerebral
cortex. Thalamic nuclei (H4-12) lie medial along with the head of the caudate
nucleus (H4-12), a portion of the basal ganglia. Other portions of basal ganglia
lie laterally. These nuclei are the putamen (H4-12) laterally and globus pallidus
(H4-12) medially. External to these nuclei you should be able to discern the
external capsule (H4-12), claustrum (H4-12) and extreme capsule (H4-12)
and then the gray matter of the insular cortex. If the section is positioned properly
the third ventricle between the thalami should be evident.
Brain Stem in Section (Start with the UNMC slides, but if you would like to view additional
examples click on the Haines slide numbers. Because the UNMC slides and the
sections shown in the Haines atlas are not always at exactly the same level or the same
plane, there will be minor discrepancies. Sometimes looking at the adjacent section will
help). NOTE: For sections where the slide numbers are Bolded, click on the slide
number rather than the terms in Bold. When there is more than 1 slide showing a
particular structure click on the first image and use the left and right arrows on the sides
of the image to move forwards and backwards through the images or you can click on
the individual slide numbers.
1.
S3L & H6-4. This is a sagittal section of the brain stem. Identify the medulla
(inferior to pons and containing inferior olivary nucleus), pons, midbrain (area of
cerebral peduncle, crus cerebri and colliculi) and diencephalon (rostral area with
thalamic nuclei and optic tract).
2.
T2AL, T2BL & H5-8. These are sections of the medulla just above the spinal
cord. External features such as the gracile tubercle, cuneate tubercle,
pyramid and pyramidal decussation are readily identifiable. The trigeminal
eminence (tubercle) created by a tract and nucleus of the trigeminal can be seen
on the lateral side.
3.
T4BL & H5-11, H5-12, H5-15 This section of the medulla features the inferior
olivary nucleus, also simply called the olive because of its appearance. This
Aribbon@ nucleus has a very distinctive appearance and it projects laterally as an
external bulge that creates the anterolateral sulcus between the pyramid and
olive and posterolateral sulcus posterior to the olive. This section of the
medulla shows the relationship of the medulla to the fourth ventricle and
cerebellum. Laterally a large fiber bundle, the inferior cerebellar peduncle
(restiform body), is seen connecting to the cerebellum. This is the inferior of three
cerebellar peduncles.
4.
T6LowL, T6HighL & H5-15 This is a characteristic picture of the pons. Two
cerebellar peduncles are shown. On the lateral sides the very large middle
cerebellar peduncle should be identified. The superior cerebellar peduncle
lies dorsally on either side of the fourth ventricle. The ventral surface of the pons
shows characteristic bundles of transversely oriented fibers, which will end in the
cerebellum and bundles of fibers cut in cross section. The latter are descending
motor fibers (corticospinal tract).
D.
5.
T8AL, T8BL & H5-22 This section is still in the pons ventrally (bottom of slide)
but into the midbrain dorsally. The specific area is the inferior colliculus. The
cerebral aqueduct is an identifying characteristic of the midbrain.
6.
T9AL & H5-24 This section shows the superior colliculus. Lateral to the
superior colliculus the geniculate bodies can be seen. The latter structures are
important stations in the auditory and visual pathways. Geniculate bodies (H525 & H5-26) are also shown in these atlas figures.
7.
T9CL & H5-26, H5-30 Dorsally this is a section through the diencephalon. The
midline cleft is the third ventricle. The interpeduncular fossa and the cerebral
peduncles, prominent features of the midbrain, are on the ventral surface of this
section. The thalamic nuclei are located lateral to the third ventricle. The fibers
of the peduncle can be seen coming from the internal capsule situated lateral to
the thalamus. The red nucleus, another prominent feature of the midbrain, is cut
in this section.
8.
T10L & H5-32 The major features of this section of the diencephalon include the
optic tract ventrally and the basal ganglia lateral and thalamic nuclei medial to
the internal capsule. The third ventricle and the lateral ventricle are also
prominent.
Cranial Nerve Nuclei (H7-2, 200-203)
1.
In order to locate the cranial nuclei and some of the cranial nerves, you should
look at the brain stem slides in reverse order. The oculomotor nucleus (T9BL
or H5-24, H5-25, H5-26) may be seen just ventral to the cerebral aqueduct and
the exiting oculomotor nerve (T9CL) fibers coming through the substantia
nigra. The oculomotor nerve (S1L) can also be seen on this slide. The
trochlear nucleus (T8AL, T8BL or H5-23) may be seen along with the fibers of
the trochlear nerve (T8AL, T8BL, T7L or H5-20). The trochlear nerve descends
in the brain stem from nucleus to its point of exit. Identify the motor nucleus of
the trigeminal nerve (T6HighL) and its principal sensory nucleus (T6HighL or
H5-18). Identify the trigeminal nerve (T6LowL). Identify the nucleus of the
abducens nerve (T5L or H5-17) along with the fibers and nucleus of the facial
nerve. The arching fibers of the facial nerve create the facial colliculus in the
floor of the fourth ventricle. Identify the cochlear nerve and the ventral and
dorsal cochlear nuclei (T4AL or H5-12) in relation to the inferior cerebellar
peduncle. Note the vestibular nuclei (H5-12) and their relationship to the sulcus
limitans. Other vestibular nuclei (T5L) can be seen. Identify the VIIIth nerve
(T5L). Identify the dorsal motor nucleus (T4AL, S1L or H5-11) of the vagus.
Identify the hypoglossal nerve (T4AL) and the hypoglossal nucleus (T3L,
T4AL, S1L or H5-10). The vagal and hypoglossal trigones in the floor of the
fourth ventricle are external manifestations of these nuclei. Identify the spinal
tract of the trigeminal nerve and its nucleus (T4AL, T4BL, T3L, T2AL, T2BL,
S3L or H5-8, H5-9, H5-10, or H5-11). The trigeminal eminence is the external
manifestation.
E.
Somatosensory Systems
Some of the sensory endings that send information toward the central nervous system
are shown in the Kodachrome slides which are described. Study these.
F.
1.
Pacinian Corpuscle. This is a specialized type of encapsulated, sensory nerve
ending called a Pacinian Corpuscle (NH20). This type of ending is found in the
dermis of the skin and in various visceral structures. It is probably responsible for
pressure reception as well as other types of stimuli.
2.
Pacinian Corpuscle. This Pacinian corpuscle (NH21) was located in the
pancreas of a cat. It has been cut in cross section so that the connective tissue
laminae can be seen.
3.
Meissner=s Corpuscle. The encapsulated nerve endings of Meissner=s
corpuscle (NH22) occurs in the dermal papilla of the skin. This papilla projects
into the epidermal layer of the skin seen on the left. Flattened cells form the
capsule.
4.
Neuromuscular Spindle. This sensory ending consists of specialized skeletal
muscle fibers called intrafusal fibers (NH24) supplied by an annulospiral nerve
ending (NH24). This ending can be seen encircling the intrafusal fibers in this
slide. These endings relay information to the central nervous system regarding
the length and rate of change of length of a muscle.
5.
Intrafusal Fibers, Cross Section. Note the intrafusal fibers (NH25) in this section
surrounded by a connective tissue capsule. Other muscle present is referred to
as extrafusal.
Somatosensory Pathways
1.
Proprioception, tactile discrimination, and stereognosis (Three Dimensional
Sense) from the body (178-179).
a.
On the brain, half brain, brain stem and sections of brain review the
gracile and cuneate fasciculi, gracile and cuneate tubercles, trigeminal
eminence, thalamus, posterior limb of internal capsule and post central
gyrus. These are all gross structures associated with the somatosensory
systems.
b.
T1AL, T1BL, T1CL, T1DL, or H5-1, H5-2, H5-3, H5-4 & H5-5 show
various levels of the spinal cord. Identify any proprioceptive fibers. Why
can they be identified? By projecting successive slides, follow the dorsal
funiculus (gracile fasciculus and also cuneate fasciculus above the
midthoracic level) to section T2AL. Are these primary or secondary
neurons? Are they crossed or uncrossed? Where are the feet and the
shoulders represented in these fasciculi? Identify the termination of the
gracile and cuneate fasciculi in the gracile nucleus and cuneate
nucleus (T2AL, T2BL, T3L, S3L or H5-8, H5-9). The gracile and cuneate
tubercles are external manifestations of these nuclei. Identify internal
arcuate fibers (T3L or H5-9) and medial lemniscus (T3L or H5-9). How
would you accurately describe the level of decussation of this tract?
2.
c.
Follow the medial lemniscus rostrally to its termination in the ventral
posterior lateral nucleus of the thalamus (T9CL or H5-30). You can
view this in the Haines atlas (H5-11, H5-12, H5-13, H5-17, H5-18, H5-19,
H5-20, H5-22, H5-23, H5-24, H5-25 & H5-26). Slide S3L or H6-2, H6-4
or H6-6 will assist you in picturing the change in position of the medial
lemniscus as it ascends. The tertiary or third order neurons for the touch
and proprioceptive pathways have their origins in the ventral posterior
lateral nucleus and project through the posterior part of the posterior
limb of the internal capsule (T9CL or H5-30) to terminate in the
postcentral gyrus (somatosensory cortex) of the cerebral hemisphere.
d.
In addition to conscious proprioception there are also fibers in this system
mediating unconscious proprioception that terminate in the cerebellum
(204-205). From the upper limb the unconscious proprioceptive fibers
traverse the fasciculus cuneatus, synapse ipsilaterally in the accessory
or lateral cuneate nucleus (T3L, T4AL or H5-9). Axons from the
accessory cuneate nucleus reach the cerebellum via the inferior
cerebellar peduncle. Unconscious proproioceptive fibers from the trunk
and lower limb reach the cerebellum in the posterior spinocerebellar tract
via the inferior cerebellar peduncle. Locate the position of posterior
spinocerebellar tracts (H5-5) on a section of the spinal cord. The origin
of these fibers is the nucleus dorsalis (of Clarke) (T1CL or H5-3) that
can be seen on the section of the thoracic cord. The primary fibers from
the lower limb enter the dorsal root, rise in the ipsilateral fasciculus gracilis
and then terminate in the nucleus dorsalis which is only found at thoracic
levels.
Pain and temperature (probably itching) from the body. (Also known as
anterolateral system 180-181)
a.
Review the pathways for pain and temperature as presented in the atlas
and from your lecture notes. At this time, concentrate your efforts on the
pain fibers which are located in spinal nerves. What receptors are
involved with these sensory modalities? What type of fiber is the primary
pain fiber? Where is its cell body located? Which portion of the dorsal
root carries pain fibers to the spinal cord? Where does the primary fiber
synapse in the spinal cord?
b.
Focus on the section of the lumbar cord. Identify the pain and
temperature fibers (T1BL or H5-2) as they enter the spinal cord from the
dorsal root. Why are they identifiable? Locate the dorsolateral tract of
Lissauer, the substantia gelatinosa and the anterior white
commissure (not labeled on lumbar cord but is on thoracic level). Locate
the position of the spinothalamic tract (labeled anterolateral system).
Are the fibers making up this tract primary or secondary? Where do they
originate? Review the sections of the thoracic cord (T1CL) and the
cervical cord (T1DL) and identify all the same structures at those levels.
Where are the pain fibers from the foot as opposed to those from the
hand in the cervical cord? Locate the spinothalamic tract (T2AL, T2BL,
T3L or H5-8, H5-9, H5-10, H5-11 & H5-12). Has it changed position?
c.
3.
Examine the position of the spinothalamic tract on succeeding slides
from Haines (H5-17, H5-18, H5-19, H5-20, H5-21, H5-22, H5-23, H5-24,
H5-25 & H5-26), and up through, and including, UNMC slides T4AL, T5L,
T6HighL, T7L, T8AL, T8BL, T9AL & T9CL. This tract lies adjacent to the
medial lemniscus in the rostral pons and can be traced forward
accompanying that structure all the way to the ventral posterior lateral
nucleus of the thalamus (H5-30).
Pain, temperature, proprioception, touch, tactile discrimination for the head
and face (184-185).
a.
The previous pathways have been concerned with general somatic
afferent (GSA) information coming from the body and entering the CNS
via dorsal roots of spinal nerves. The nerve carrying GSA fibers from the
face and anterior half of the scalp is the trigeminal nerve. As you probably
remember from Core I certain localized areas of the skin associated with
the ear and external auditory meatus and mucous membrane of the
pharyngeal wall are supplied with GSA fibers by nerves VII, IX, and X.
The GSA fibers in these nerves use the same central connections as the
trigeminal nerve. Cell bodies of the latter fibers are located in the
geniculate ganglion and ganglia of nerves IX and X. The GSA pathways
from V, VII, IX, and X make up the trigeminothalamic system.
b.
Identify the principal (main) sensory nucleus of V (T6HighL or H5-17).
Confirm that this nucleus lies at the same level as the entering trigeminal
nerve fibers. Notice that the motor nucleus is medial. Can you explain
the relative positions of these nuclei? What GSA fibers belonging to the
trigeminothalamic system terminate in the principal sensory nucleus?
c.
Locate the ascending root of V (H5-18) between these two nuclei and
notice that the ascending root of V becomes the mesencephalic tract
of V. This tract should be followed rostrally along the cerebral aqueduct
(T7L or H5-19). The nerve cell bodies (T8AL, T8BL) of origin of fibers
within this tract can be seen close to the trochlear nerve. This collection
of nerve cell bodies is the mesencephalic nucleus of V (H5-20). Where
do proprioceptive axons of these cell bodies proceed from here?
d.
Fibers carrying the sensations of pain and temperature from the face also
enter the pons at the level of slide T6, but descend in the spinal tract of
V. Follow the spinal tract and nucleus of V (T5L, T4BL, T4AL, T3L,
T2AL or H5-13, H5-12, H5-11, H510, H5-9 & H5-8) caudally. How far
does this tract and nucleus descend? If you were to cut this tract
surgically where would there be a loss of pain and temperature
sensations?
The pathways to the conscious level for pain and temperature and for touch show some
differences:
4.
5.
Pain and temperature and bifurcating touch fibers.
a.
Primary fibers proceeding caudally in the spinal tract of V will synapse on
cell bodies located in the spinal nucleus of V. Highly myelinated fibers
conveying tactile sensations descend only to a level corresponding to the
caudal third of the inferior olivary nucleus. Lightly myelinated or
unmyelinated fibers that terminate in all levels of the spinal nucleus of V
convey pain and temperature impulses.
b.
Secondary neurons having origin in the spinal nucleus of V decussate
diffusely across the medulla so no specific bundle of crossing fibers may
be seen on your slides. Crossed fibers will form the ventral
trigeminothalamic tract (T7L) on the contralateral side. This tract
ascends with the medial lemniscus (lies dorsal to it but can=t be
distinguished from it) (H5-20), but will synapse in the ventral posterior
medial nucleus of the thalamus (T9CL, T9AL or H5-30). Identify this
nucleus. The tertiary neuron of this pathway projects from the VPM
thalamic nucleus to the postcentral gyrus via the posterior limb of the
internal capsule.
Touch
a.
b.
Discriminatory touch fibers terminate in the principal sensory nucleus
(T6HighL). Secondary fibers having origin in the principal (main) sensory
nucleus of V also ascend with the medial lemniscus. Most secondary
fibers from the principal (main) sensory nucleus cross at pontine level, but
some fibers remain ipsilateral. It is therefore more difficult to lose touch
from the face than pain and temperature. These touch fibers also
terminate in the ventral posterior medial nucleus of the thalamus and
tertiary neurons will ascend through the posterior limb of the internal
capsule to the postcentral gyrus.
Because pain, temperature and touch fibers from the pharynx, external
auditory meatus and middle ear reach the brain stem in cranial nerves 7, 9
and 10 and terminate in the spinal nucleus of V, the position of those
entering fibers relative to the position of the spinal tract of V (T5L, T4BL
or H5-12, H5-13) should be investigated when viewing slides . These
somatosensory fibers will follow the same pathway as trigeminal pain
fibers.