Download Laboratory 7: Medulla MCB 163 Fall 2005 Slide #152 1) This is the

Laboratory 7: Medulla
MCB 163 Fall 2005
Slide #152
1) This is the gracile nucleus. The fibers coming in to this nucleus are the fibers of the gracile
fasciculus, which are first order and uncrossed.
2) This is the cuneate nucleus. The fibers coming into it are the fibers of the cuneate fasciculus.
There is a somatotopic organization to this structure, just as there is in the cuneate fasciculus.
3) This is the hypoglossal nucleus, which is the source of the twelfth cranial nerve. The twelfth
cranial nerve innervates the tongue, and plays a role in the reflex movements of the tongue in
swallowing, chewing, and sucking in response to gustatory stimuli and other stimuli from the
oral and pharyngeal mucosae (Barr, 151)
4) This is the solitary nucleus. It receives input from cranial nerves VII, IX, X, and the cranial
part of nerve XI (Can you give these nerves their common names?). This nucleus is
concerned with visceral sensation and taste (www.wikipedia.org). It is a major coordinator of
autonomic nervous system regulation of cardiovascular, respiratory, gustatory,
gastrointestinal, and chemoreceptive aspects of homeostasis
(http://fred.hmc.psu.edu/ds/retrieve/fred/meshdescriptor/D017552).
5) This is the spinal trigeminal tract. It is analogous to Lissauer's tract in the spinal cord. In the
trigeminal system for pain, the top of the head is represented at the bottom, and the bottom of
the head is represented at the top. Since we're fairly caudal at this level (evidence is the
presence of the gracile and cuneate nuclei, which are present near the spino-medullary
junction), the top of the head is represented here.
6) This is the spinal nucleus of V (spinal trigeminal nucleus). Think of this structure as a
continuation of the external aspect of the dorsal horn of the spinal cord (Rexed lamina I-III):
C fibers terminate here before decussating and ascending. And just like in the spinal cord,
these fibers are from the ipsilateral side of the body.
7) This is the reticular formation. Reticular means “net-like.” You can see why it is named this
now, yes? A lesion here would likely disrupt sleep/wake cycles, and can produce either
insomnia or narcolepsy.
8) This is the source of the lateral reticulospinal tract. A lesion here would do what to muscle
tone?
9) This is the pyramidal decussation, where the pyramidal tract decussates. The pyramidal tract
is also known as the corticospinal tract. These axons are from upper motor neurons on the
motor cortex, and will ultimately end in the spinal tract on alpha and gamma motoneurons as
well as interneurons. Primates are the only animals that have monosynaptic corticospinal
influence on alpha motor neurons (humans have the most developed monosynaptic
projection); rats and cats, for example, have only disynaptic influence through interneurons.
These neurons are heavily myelinated, and have a somatotopic organization, with fibers
destined for the upper spinal segments descending most medially, and fibers destined for
lower segments more lateral.
10) This is the dorsal spinocerebellar tract. Proprioceptive recepters such as muscle spindles,
golgi tendon organs, and other somatosensory receptors feed into it. This tract reaches the
cerebellum, which is typically thought of as an unconscious pathway. The equivalent of this
tract for the head itself comes from the vestibular system? The cerebellum does not project
back into the spinal cord directly, but does modify the response of the vestibulospinal and
reticulospinal tracts.
Slide #142:
1) A communicating hydrocephalus (hydrocephalus resulting from blockage of the subarachnoid
space, which is where most of the CSF leaving the fourth ventricle goes) would impact the
nuclei along the floor of the fourth ventricle. Such nuclei include the facial colliculus, the
hypoglossal nuclei, and more! (Better answers can be found in the Paxinos atlas)
2) This is the vagal motor nucleus, which is the source of the 10th cranial nerve. The conduction
velocity and degree of myelination are lower than skeletomotor axons. The nuclei
immediately around this include the solitary and the hypoglossal, which all have visceromotor
functions.
3) This is the medial longitudinal fasciculus. This tract extends from the midbrain tegmentum to
the cervical levels of the spinal cord. The MLF plays an important role in coordinating
movements of the eyes with respect to the position of the head in space. It also plays a role in
coordinating eye movements between the various oculomotor nuclei (abducens, trochlear,
oculomotor nuclei; which cranial nerves are these?), and plays a role in the vestibuloocular
reflex (which is why you can continue to read this page while shaking your head in
bewilderment).
4) The dorsal column nuclei are more caudal to this section. Now the fibers form the medial
lemniscus, which means they are Decussated, Second Order Sensory Afferent axons. These
axons represent the contralateral side of the body, and are fairly highly myelinated. Solitary
nucleus influences would be directed toward the thoracic segments of the spinal cord?
5) This is the inferior olive. This nucleus sends climbing fiber input to the cerebellum, which
innervate Purkinje cells in a complimentary manner to the mossy fibers of other
spinocerebellar axons. There is 1 climbing fiber that synapses on a Purkinje cell, while there
can be thousands of mossy fibers. Climbing fibers also cause complex spikes in Purkinje
cells.
6) This is the pyramidal tract. At this level, the axons in this tract are uncrossed. They are fast
conducting, corticospinal axons. Damage to these axons results in a loss of power and
precision of muscle movements.
7) This is the spinal trigeminal nucleus. The second order fibers project to the posterior
thalamus, the reticular formation, and the central gray, just as the neo- and paleospinothalamic fibers do.
Slide #134:
1) This is the choroid plexus, which secretes cerebrospinal fluid.
2) This is the medial vestibular nucleus.
3) This is the spinal vestibular nucleus. Together these are the source of the vestibulospinal
tract. A lesion here would cause the patient to lean to the side of the lesion.
4) This is the nucleus ambiguous. This nucleus supplies muscles of the soft palate, pharynx, and
larynx, together with striated muscle fibers in the upper part of the esophagus (Barr 149).
5) The chief source of descending influence to the lateral parts of the cerebellar hemisphere is
the cerebropontocerebellar system, which arises in the cerebral cortex. The corticofugal
impulses go through the pons to arrive at the cerebellar cortex as mossy fibers. The output of
the cerebellar cortex is directed to the deep cerebellar nuclei. The lateral-most parts send their
output through the Dentate Nucleus. A unilateral lesion to the lateral cerebellar hemispheres
would not have much of an effect on posture and locomotion, but it would have an effect on
coordinated movements of the ipsilateral limb.
6) These tiny cells are the granule cells. They are incredibly numerous; some estimates place
their number at 10 to the 8th power (100,000,000). They use glutamate as their transmitter,
and synapse on Purkinje cells as parallel fibers.