Lecture 29: Vestibular System

Question 1

What are the different types of eye movements?

Answer 1

A. Voluntary (Conjugate eye movements)

1. Smooth pursuit: cannot be done voluntarily in the absence of a moving target
2. Optokinetic: reflex following movements elicited when large portions of the visual field moves slowly (e.g. on a train). Brainstem mediated
3. Vestibular (VOR): a variety of eye movements collectively called vestibule-ocular reflex (VRO). Stabilizes gaaze in spite of head movements or tilt
4. Saccades: a quick return movement of the eyeball in order to establish a new image on the retina

B. Voluntary disjunctive
5. Vergence: very slow movements of the eyes in the 3rd dimension (toward objects either beyond or closer than fixation plane). Disjunctive because eyes move in opposite directions

C. Involuntary conjugate
6. Fixation movements: slow drifts and very small rapid shifts. If image is stabilized on retina, it fades quickly so fixation are required for vision

Question 2

Saccadic eye movement

Answer 2

a quick return movement of the eyeball in order to establish a new image on the retina

Question 3

What is the common pathway of all the eye movements?

Answer 3

The extraocular motorneurons

• no stretch reflex
• very fast twitch time
• otherwise identical to spinal motor neurons

Question 4

Where do the extraocular motor neurons come from?

Answer 4

CN III, IV and VI

Question 5

What does superior oblique do?

Answer 5

Intort and depresses eye

Question 6

What inferior oblique do?

Answer 6

Extort and elevates

Question 7

On a slide, how do you tell you are at the level of oculomotor/edinger-westphall nucleus?

Answer 7

Look for a giant “V” shape coming ventrally from periaqueductal gray with superior colliculus on dorsal end

Question 8

What forces do the eye have to overcome to move?

Answer 8

1. Elastic restoring force

2. Viscous resistance (from fluid)

Question 9

How does eye overcome the forces to generate precise saccadic movement?

Answer 9

There needs to be both a PULSE and a STEP
1. Pulse of action potential is needed for initial movement
2. Step is needed to overcome the elastic restoring force
Lack of step = your eye will slowly move back towards the resting state
Lack of pulse = your eye slowly moves to the desired position lol
Pulse and step need to occur simultaneously and must be equal in amplitude?

Question 10

What generates the pulse for horizontal eye movements?

Answer 10

The paramedian pontine reticular formation (PPRF) near the abducens nucleus
-under control from superior colliculus and frontal eye fields

Question 11

Paramedian pontine reticular formation (PPRF)

Answer 11

generates the pulse for horizontal eye movements

• located near abducens nucleus
• under control from superior colliculus and frontal eye fields

Question 12

Where is the pulse of firing rate transmitted to?

Answer 12

1. abducens nucleus in horizontal movement

2. neural integrator

Question 13

What is the neural integrator?

Answer 13

Performs temporal integration

-produces a number, a step in firing rate, which is synaptically applied to abducens motor neuron

Question 14

What happens if you lesion frontal eye fields (FEF) and superior colliculus?

Answer 14

You can never have another saccadic eye movement

-if you lesion only one or the other, you can still generate saccades

Question 15

What does the superior colliculus mediate?

Answer 15

Express saccades, saccadic eye movements in response to sudden appearance of visual/auditory stimulus

Question 16

Is extraocular eye movement represented in M1? If not, where?

Answer 16

No

Frontal eye fields

Question 17

Aside from the lateral rectus, where do the other half of neurons in abducens nucleus go?

Answer 17

They innervate the medial rectus subdivision of the oculomotor complex on the opposite side via the MLF

Question 18

What is the function of the vestibular system? What comprises vetibular apparatus?

Answer 18

Sense motions of the head and report these to the brain
Composed of
i. cochlea
ii. utricle (linear accel)
iii. sacculus (linear accel)
iv. semicircular canals (angular acceleration
They all use hair cells

Question 19

How do statolymph (utricle, saccule) hair cells open or close in response to movement?

Answer 19

Cilia of hair cells are attached to calcium carbonate crystals (otoconia), and upon movement, shifting of cells either open or close ion channels

Question 20

How do semicircular hair cells open or close in response to movement?

Answer 20

Cilia tips are connected to something known as a CUPULA, which divides the canal down the middle, separating the endolymph.

• upon rotation, the endolymph moves, thereby displacing the cupula (proportional to head velocity)
• upon linear acceleration, the endolymph on both sides of cupula will make it so that the forces are equal and opposite (cupula doesn’t move)
• three orthogonal canals allows you to sense rotation in 3-D

Question 21

What happens when you move your head linearly at constant velocity?

Answer 21

CN 8 will only increase firing rate during ACCELERATION, but when velocity is constant, then the firing returns to normal…known as mechanical adaptation

Question 22

What is the Vestibule-ocular reflex (VOR):

Answer 22

the stability of your gaze despite head movements
Example: when you turn your head 10 degrees to the right, your eyes turn 10 degrees to the left to maintain gaze
-this works in all planes and in translational motion as well

Question 23

What is Nystagmus?

Answer 23

-fast saccade-like motion followed by a slow drift back to rest
Named for direction of fast phase
(so if you move your head to the left, it is left nystagmus even though the eyes are counterrotating to the right)

Question 24

Spontaneous nystagmus:

Answer 24

-nystagmus without motion of the world or head
Mechanism: CN 8 or one labyrinth is compromised
fast phase away from lesion to CN 8 or labyrinth

Question 25

What happens when you lesion the neural integrator?

Answer 25

You get pathologic nystagmus

Question 26

Physiological nystagmus:

Answer 26

Normal nystagmus (spinning on a stool)

Question 27

Caloric nystagmus:

Answer 27

The type of nystagmus you get when you put different temperature water in ear
COWS

Question 28

What is COWS?

Answer 28

Cold Opposite
Warm Same
Refers to how if you put cold water in the right ear, you get nystagmus to the left (opposite)
If you put warm water in the right ear, you get nystagmus to the right (same)

Question 29

Mechanism of VOR when you have a target on the subject’s left

Answer 29

1. movement of head increase firing rates of left horizontal semicircular canal
2. activation of hair cells in semicircular canal gets sent to CN 8 nuclei
3. CN 8 then sends excitatory synapses to right abducens nerve (movement of right lateral rectus)
   And inhibits the abducens motorneurons on the left
   This allows you (when you’re turning your head left) to arrive at a final position where right eye is AD ducted and left eye is AB ducted

Question 30

What happens if the eyes keep moving after initially turning?

Answer 30

Eyes will snap to the center, and then continue the counterrotation if head keeps moving

Question 31

How plastic is the Vestibulo-ocular reflex (VOR)? How can you tell?

Answer 31

It is very plastic
The way that it was proven was an experiment done in which patient wears reversing prism glasses, where eyes have to rotate in same direction as head turning…after 3 hours, your eyes adjust and rotate with the head (which is opposite to the norm…norm is that the eyes counter rotate
-if the eyes don’t turn to the side of head turning with reverse prisms, then there is a lesion of frontal lobe

Question 32

What is involved in modulating and adjusting the reflex circuits of eye?

Answer 32

Cerebellum

Modulates and adjusts the gain of reflex circuits