NB9-2 - Ocular Reflexes 1 and DLA

Question 1

In which directions does applying force to the stereocilia of vestibular hairs cells cause excitation and inhibition of the associated neurons?

Answer 1

Question 2

What is this an image of?

Answer 2

A

The medulla

Question 3

Answer 3

A - vestibular nuclei

B - Spinothalamic tracts

C - Pyramids

D - Medial lemniscus

Question 4

When the head is rotated to the left or right which vestibular systems are excited and inhibited?

Answer 4

When the head is turned to the left, the left horizontal semicircular duct is excited and the right one is inhibited and vice versa.

Question 5

Describe what the vestibulo-ocular reflex (VOR) is, what section of the brain drives it, and whether or not vision is interrupted during it.

Answer 5

The VOR is a compensatory eye movement that keeps a visual target in the center of the visual field following angular acceleration (rotation) of the head. It is driven by input from the vestibular labyrinth (horizontal semicircular ducts). Vision is maintained during VOR.

Question 6

Describe the vestibulo-ocular reflex (VOR) pathway. Use turning your head to the left as an example.

Answer 6

1. Head turn to the left activates the left horizontal semicircular duct and inhibits the right one.
2. CN VIII enters the brainstem at the pontomedullary junction and synpases at the ipsilateral (left) vestibular nuclei in the medulla.
3. Axons from the vestibular nuclei decussate and synapse on the contralateral (right) abducens nerve
4. This also activates the ipsilateral (left) oculomotor nerve via the MLF

Question 7

Define Nystagmus, describe its phases and directionality.

Answer 7

Nystagmus is rhythmically alternating eye movements and it usually comes in two phases:

• Slow Phase - driven by a reflex (ie - VOR)
• Fast Phase - reset the position of an eye through saccadic movements

The directionality of nystagmus is defined by the direction in which the fast-phase movements are occurring.

Question 8

How can the VOR be examined?

Answer 8

Oculocephalic (Doll’s Eyes) maneuver - move the patients head left and right for them. If the patient’s eyes perform compensatory conjugate movements in the opposite direction, the VOR is fine. If they eyes stay fixed relative to the head, there could be a lesion to the VOR

Caloric Testing - injection of warm or cold water into the outer ear of a comatose supine patient induces eye movement away from the stimulated ear if warm water is used and towards the stimulated ear if cold water is used. If this occurs then the VOR is intact.

Question 9

Describe why caloric testing elicits eye movements.

Answer 9

The cold water causes the endolymph of the horizontal semicircular duct to descend which inhibits signalling from those cells, stimulating a head turn in the opposite direction.
The opposite is true with warm water.

Question 10

List and describe the types of vertigo.

Answer 10

Subjective Vertigo - sensation of movement of self

Objective Vertigo - sensation of movement of environment

Question 11

What is the technical name for motion sickness and what causes it?

Answer 11

Kinetosis occurs when susceptible individuals experience a discrepancy between vestibular and visual inputs. What makes individuals susceptible is unknown.

Question 12

Why can alcohol consumption cause vertigo?

Answer 12

It is caused by an interaction of blood alcohol with the endolymph of the semicircular ducts.

Question 13

Why can some antibiotics cause vertigo?

Answer 13

Some antibiotics (streptomycin) can be toxic to vestibular hair cells

Question 14

Answer 14

A

Question 15

Answer 15

D

Question 16

A right nystagmus could indicate damage in which labyrinth? Explain

Answer 16

A right nystagmus means the reflex-driven phase (slow phase) is towards the left. This means the right vestibular apparatus is functioning normally. Therefore, this could indicate a left vestibular lesion.

Question 17

Describe what the optokinetic reflex is, what section of the brain drives it, and whether or not vision is interrupted during it.

Answer 17

The optokinetic reflex si a compensatory eye movement that keeps a visual target in the center of the visual field while that target is moving.. It is driven by visual input from the magnocellular pathway. Vision is maintained during these movements

Question 18

Describe the optokinetic reflex pathway. Use an object moving to the right as an example

Answer 18

1. The magnocellular neurons (M ganglion cells) from the right eye synapses in the top two layers of the LGN and then synapses in layer IV of V1 before forming a dorsal (parietal) pathway that synapses in the parieto-occipital eye field
2. Neurons from the right parieto-occipital eye field synapse on the right subpontine nuclei
3. Neurons from the right subpontine nuclei decussate and synapse on the left vestibulo-cerebellum nuclei in the flocculonodular lobe
4. Neurons from the left vestibulo-cerebellar nuclei synapse on the left vestibular nuclei
5. Neurons from the left vestibular nuclei decussate and synapse on the right abducents nuclei
6. This causes activation of the right abducens nerve and left oculomotor nerve (via the MLF) causing both eye to track right to follow the object

Question 19

Describe the phases of optokinetic nystagmus.

Answer 19

Slow Phase - optokinetic reflex

Fast Phase - saccadic eye movement back once target is out of range

Question 20

Answer 20

B