VF - Eye Movements 4 - Optokinetic, Vestibulo-Ocular & Vergence Pathways - Week 6

Question 1

What are the two functions of the vestibular system? Is the vestibular system alone responsible for these?

Answer 1

-Maintain balance
-Maintain stable gaze as we move
Is not solely responsible

Question 2

Define what is meant by translational and rotational motion in terms of the vestibular system.

Answer 2

Translational - movement along an axis - xyz

Rotational - rotational movement around an axis - xyz

Question 3

With what are the semicircular canals better aligned with?

Answer 3

The orbits of the eyes.

Question 4

What do semicircular canals detect?

Answer 4

Angular acceleration

Question 5

How is linear acceleration detected, and does this include gravity?

Answer 5

Sensed by the utricle and the saccule.

Gravity is also detected.

Question 6

Describe the mechanism by which the utricle and saccule operate.

Answer 6

Otoliths are tiny crystals embedded in a gel found in the utricle and saccule.
When they are displaced, they provide load on the hair cells underlying the gel, allowing them to detect linear acceleration in all possible planes.

Question 7

Describe what happens if an otolith becomes dislodged.

Answer 7

If an otolith is dislodged, it may find its way into the semicircular canal with age.
This has a rotation sensor become a gravity sensor.
The condition is called benign paroxysmal positional vertigo.

Question 8

Describe where rotation is sensed, and explain how this is done.

Answer 8

Takes place within the ampulla at the base of the semicircular canals.
Within there is a gelatinous mass with hair cell cilia within called the cupula.
When the head turns, the inertia of the endolymph distends the cupula which is sensed by the hair cells.

Question 9

Define tonic firing rate, and how rotation affects paired semicircular canals.

Answer 9

Cupula within the ampulla of paired semicircular canals fire at a basal rate, the tonic firing rate.
When one is innervated, the other is inhibited.
This is due to how the canals are arranged relative to each other.

Question 10

Define kinocilium, and the pressure required to inhibit or excite them.

Answer 10

They are the hair cilia found within the cupula.

Forward pressure excites them, and backward pressure inhibits them.

Question 11

What happens to hair cells in the cupula if rotation occurs at a constant velocity for some time?

Answer 11

The endolymph will eventually catch up with the canal, and the cupula will straighten out.

Question 12

Describe the 3 neuron arc, and where they project to.

Answer 12

• Oculomotor and abducens nerves which drive the eye muscles, project to the oculomotor/abducens nuclei respectively.
• Vestibular nucleus neuron, projecting to the oculomotor/abducens nuclei.
• Vistibulocochlear nerve, innervates the ear, projects to the vestibular nuclei, originiates from the inner ear.

Question 13

Describe push-pull organisation.

Answer 13

For a given head rotation, the signal from one canal increases basal firing rate.
At the same time, the other canal decreases in basal firing rate.

Question 14

Consider that rotation is sensed by 2 different inputs to semicircular canals.
What would happen if one canal, or its pathway, were damaged?

Answer 14

The brain would perceive a rotation, and generate an eye movement to compensate for a head motion that isnt occuring.

Question 15

Describe a test to assess an individual’s VOR.

Answer 15

To maintain gaze when the head is turned.

The head is thrust to one side suddenly, and the eye is checked to see it can maintain gaze.

Question 16

Consider a test for assessing VOR. Given this, it is suggested that the VOR depends on visual input. Describe briefly how this is thought to occur.
Name one important mechanism that this contributes to.

Answer 16

It modifies the direct VOR pathway. Cognitive inputs to the cerebellum may play a role.
Contributes to maintaining calibration of the VOR as age and disease take their toll.

Question 17

What happens to the VOR if you rotate for a long time? What sensation does this result in? What response is this known as?

Answer 17

It eventually dies out as the endolymph catches up with the semicircular canals.
It is lengthened beyond this by the indirect pathway of the VOR.
Eventually results in the sensation that youve stopped rotating, and that the world is rotating around you.
This is an optokinetic response known as optokinetic nystagmus.

Question 18

What happens to optokinetic nystagmus as you progress down to more modern phyla?

Answer 18

Gets more complex and more cortical.

Question 19

Small hand-held stimuli elicit what response, and via what inputs?

Answer 19

Mostly smooth pursuit via cortical imputs.

Question 20

Large, full-field stimuli drives what, and elicits a response from what system?

Answer 20

Drives the peripheral retina, and by extension, the accessory optic system.

Question 21

Name a key distinction between small stimuli and large stimuli?

Answer 21

Large stimuli elicit a sense of self motion, as well as OKN.

Question 22

Name downside to connecting visual imput to the vestibular system.

Answer 22

You may feel you are moving, when in reality, you are stationary.
Occurs when eyes say moving, and canals say stationary.
Vice versa, it may cause motion-sickness.

Question 23

Name two stimuli for vergence.

Answer 23

When looking at a far off object, a target at near will appear doubled.
This disparity is a stimulus for vergence.
Another is blur.

Question 24

Describe how the supraoculomotor area affects vergence access to the medial recti muscles.

Answer 24

The SOA is a region above the oculomotor nerve nucleus.

This means vergence has independent access to the medial recti, without going through the nucleus.

Question 25

What system is the OKN progressively integrated with going to modern phyla?

Answer 25

Geniculostriate system.