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Neuroscience MSP
Problem Set #14
1.a) Describe the three modalities of balance, and where do these modalities
integrate?
Vestibular, Visual, and Somatosensory (proprioceptive).
Labyrinth (vestibular): indicates position and movement of head.
Proprioception: for each body part, and most important for posture are
proprioceptor of leg and
neck.
Early stage convergence of input occurs at the Vestibular Nuclei. Thus this
nuclei serves as the relay for the labyrinth system, as a processing center,
and motor (reflexive) command origin.
b) In "Living without a Balancing Mechanism," the patient experiences a
shocking fall as he is blind
folded by his facecloth. What is the analogous to this experience in medical
approach to vestibular
problem?
Romberg Test. Balance requires two of the modalities to be intact.
Therefore to asses vestibular problem, one can ask a patient to close eyes
and give them a little jolt. If the patient looses
balance, it means that one of the other two modalities are impaired
(labyrinth or proprioception). This test is less indicative if it is a
Vestibular Nuclei problem.
c) What are the inputs and outputs of the vestibular system?
Inputs: Labyrinth, visual, proprioceptive, vestibulocerebellum, and parts of
spinocerebellum.
Outputs: Spinal cord, reticular formation, ocular motorneurons (3, 4, 6),
thalamus/parietal lobe, vestibulo-cerebellum.
2. Explain the two paths of vestibulospinal projections that are parallel but
different.
Utricle (static labyrinth): Info on head position --->
lateral vestibular
nucleus --> lateral vestibulospinal tract (excitatory to extensor muscles).
Ipsilateral. Rostral transection of the nuclei --> tonic hyperextensive
rigidity
Semicircular Canal: info on head movement --> medial vestibular nuclei -->
MLF
cervical cord. It turns head in response to labyrinth excitation
3. Why is VOR needed? How does it operate?
Photic information take too lung to be processed, and thus as head turns
suddenly, gaze stabilization does not occur efficiently. VOR is an elegant
reflex system that immediately recognize and process head movement, resulting
in fast and accurate gaze stabilization.
In this
mechanism head movements are immediately detected by semi-circular canals,
and eye movement in the opposite direction to the same degree takes place,
resulting in an ideal -1 VOR.
Semi-circular canal detects head acceleration. Therefore, to change from
acceleration to eye position two integrations are required (remember
calculus!). First integration from acceleration to velocity occurs on the
macula of semi-circular canal. The second integration from velocity
toposition occurs in an integrator located medial to the vestibular nuclei.
4. A 63 year-old patient arrives at your clinic with difficulty focusing his
vision on a given target. Upon physical exam, you establish that his
second cranial nerve is intact. But when asking him to “follow your
finger” caudo-rostrally and laterally you elicit an exaggerated
optokinetic nystagmus in both lateral fields. This pathological nystagmus
(pathologic because of its severity, not necessarily its presence) is
likely to be caused by an anatomical lesion (tumor, cerebro-vascular
accident, etc.). Give an anatomically based differential diagnosis of this
vestibular malfunction.
Answer: Impairment of labyrinth, vestibular nerve, vestibular nuclei, medial
longitudinal fasciculus, or cerebellum.
5. List the six ways in which nystagmus can be physiologically induced in a
normal individual.
Answer:
1. A physiological nystagmus will arise from extreme lateral
deviation of the eyes, by viewing a moving target (optokinetic), by caloric
stimulation of the pinnae, by electric stimulation of the mastoid bone
(galvanic), by rotation of the subject (rotatory) and sudden stop (postrotatory), and finally by pursuing with the hand something rotating around
the body with the eyes closed.
6.You are doing a neurology externship at the University of Miami, and a 45
year-old patient with renal cell carcinoma presents with focal neurologic
symptoms, highlighted by near-sightedness n the right eye. His internist
has established that his cancer has unfortunately metastasized to the apex
of his right lung, and is now concerned with metastasis to the brain.
Upon physical exam you notice that the right side of his face is very dry
(anhidrosis), and that the patient has lid lag (ptosis) in the right eye.
When attempting to elicit pupillary light response you notice that pupil
responses are intact in left eye (even when shining light in left eye),
but you notice that throughout the exam his right pupil is markedly
decreased in size (miosis).
Are his miosis and ptosis due to an oculomotor palsy or an EdingerWestphal nucleus lesion? Given the patient’s history, what type of lesion
could describe such findings? Is neuroimaging necessary? How would an
oculomotor palsy present? How would a lesion of the Edinger-Westphal
nucleus present?
Answer: 2. Anhidrosis, ptosis, and miosis are all due to deficiencies in
sympathetic responses, not to the parasympathetic Edinger-Westphal system.
These symptoms are attributable to Pancoast’s (Superior Sulcus) Tumor, you
don’t need to know that until next year. A carcinoma at the apex of the lung
may affect the cervical sympathetic plexus, particularly disruption of the
sympathetic interomediolateral column arising from C8-T1. A neuroimaging
study would not be informational.
An oculomotor palsy would present with double vision, ptosis,
mydriasis, and difficulty looking medially and upward. At rest, eyes would be
down-and-out.
An E-W nucleus lesion would present with mydriasis and farsightedness,
due to an inability to perform pupillary constriction and accommodate lens
via ciliary muscle contraction.
7.a) Name the four most inportant types of neurons involved in bringing about
a saccadic movement of the eye.
ans: Motor neuron, Burst neuron, Tonic neuron, Omnipause neuron
b) Describe the function of each of the four types of neurons mentioned above
as they relate to the occurence of a saccade.
ans: Motor neurons- Bring about contraction of extraocular muscles as a
function of their rate of discharge of action potentials. Three different
types of signals are produced by a motor neuron during a saccade (pulses,
steps, and slow drifts)which are determined by motor programming coming from
premotor neurons of the brain stem reticular formation.
Burst neuron- An interneuron, that synapses upon the motor neuron, and fires
a high frequency burst of discharges in order to initiate the
movement(corresponds to the pulse phase of the motorneuron's firing of action
potentials during movement.)
Tonic neuron- An interneuron, that synapses upon the motor neuron, which
fires steady levels of tonic discharges that serve to maintain the postion of
the eye after it has completed the movement (corresponds to the step phase of
the motor neuron's firing of action potentials during a saccade)
Omnipause Neuron- An inhibitory interneuron on the midline of the brainstem
which functions to inhibit the signal of the burst neuron from reaching the
motor neuron until the time comes at which a saccade is to be initiated. At
that time the omnipause neuron stops fring and this allows the signal from
the burst neuron to reach the motor neuron, initiating a saccade.
2. Match the type of eye movement in the right column with the statement in
the left column that best fits the type of eye movement which is named. Use
each answer only once.
__orientation (c)
__fixation (b)
a.eye movment velocity is matched to target
velocity.
b.involves increased muscular tone
reflexively maintained by the presence of a
target.
c.response is always saccadic
__exploration (d)
d.centrally initiated
__pursuit (a)
3.Discuss what is meant by the phenomenon of corollary discharge.
ans: Corollary discharge is term which is used to describe the manner in
which afferent information pertaining to the execution of a movement is
relayed back to the brain. This information is not only sensory coming from
proprioceptors, but also includes feedback from premotor neurons which thus
give the brain a more accurate picture of what kind of movement is truly
taking place. This information can then be taken into account by the brain as
it assesses the information it receives from sensory receptors. This
phenomenon of corollary discharge is thus indispensable to the brain during
the execution of a saccade as it collects visual information about the world
around you. It allows the brain to be informed of the impending movement
before it takes place and thus makes it possible for the brain to correctly
interpret the visual image it receives.
>7.a. Name the four most inportant types of neurons involved in bringing
>about a saccadic movement of the eye.
Usually, I will not ask questions involving naming as in 1a. I prefer a
description or explanation as in your question 1b.
>ans: Motor neuron, Burst neuron, Tonic neuron, Omnipause neuron
>
>b. Describe the function of each of the four types of neurons mentioned
>above as they relate to the occurence of a saccade.
>
>ans: Motor neurons- Bring about contraction of extraocular muscles as a
>function of their rate of discharge of action potentials. Three different
>types of signals are produced by a motor neuron during a saccade (pulses,
>steps, and slow drifts)which are determined by motor programming coming
from premotor neurons of the brain stem reticular formation.
mentioning "slow drift' is not necessary. I would not consider that as
wrong.
>
>Burst neuron- An interneuron, that synapses upon the motor neuron, and
fires a high frequency burst of discharges in order to initiate the
movement(corresponds to the pulse phase of the motorneuron's firing of
action potentials during movement.)
>
>Tonic neuron- An interneuron, that synapses upon the motor neuron, which
>fires steady levels of tonic discharges that serve to maintain the postion
>of the eye after it has completed the movement (corresponds to the step
>phase of the motor neuron's firing of action potentials during a saccade)
>
>Omnipause Neuron- An inhibitory interneuron on the midline of the brainstem
which functions to inhibit the signal of the burst neuron from reaching the
motor neuron until the time comes at which the saccade is to be
initiated.indicating the desired eye position. At that time the omnipause
neuron stops firing and this allows the signal from the burst neuron to
reach the motor neuron, initiating a saccade.
The burst neuron cannot fire until it is disinhibited. Thus the gating is
functionally upstream from the burst cell, not downstream.
>
>8. Match the type of eye movement in the right column with the statement in
the left column that best fits the type of eye movement which is named. Use
each answer only once.
>
>__orientation (c)
a.eye movment velocity is matched to
target velocity.
>__pursuit (a)
b.involves increased muscular tone
>
reflexively maintained by the presence of
a target.
>__fixation (b)
>
>__exploration (d)
c.response is always saccadic
d.centrally initiated
This is a bit ambiguous because the answer c would be appropriate for both
orientation and exploration.
>
>9.Discuss what is meant by the phenomenon of corollary discharge.
>
>ans: Corollary discharge is term which is used to describe the manner in
which afferent information pertaining to the execution of a movement is
relayed back to the brain. This information is not only sensory coming from
proprioceptors, but also includes feedback from premotor neurons which thus
give the brain a more accurate picture of what kind of movement is truly
taking place. This information can then be taken into account by the brain
as it assesses the information it receives from sensory receptors. This
phenomenon of corollary discharge is thus indispensable to the brain during
the execution of a saccade as it collects visual information about the world
around you. It allows the brain to be informed of the impending movement
before it takes place and thus makes it possible for the brain to correctly
interpret the visual image it receives.
I have edited your answer as follows:
Corollary discharge is a term which is used to describe the manner in
which afferent information pertaining to the execution of a movement is
relayed back to the brain. This information is not coming from
proprioceptors, but from premotor neurons which thus give the brain a more
accurate picture of what kind of movement is truly taking place.
Such an answer is sufficient. I would not require more.