Vestibular System

Question 1

What does proper balance require?

Answer 1

not normally a “conscious” process, but is essential for the proper functioning of the motor activities, eye movements and normal posture.

Proper balance requires functional vestibular, proprioceptive and visual systems to be present, but only 2 of the 3 systems are essential for balance.

Question 2

Where is the vestibular apparatus located?

Answer 2

Located in the petrous portion of the temporal bone just lateral to the internal acoustic meatus along with the cochlea.

Question 3

Describe the bony and membranous labyrinths. What are the 3 main divisions? *

Answer 3

The bony and membranous labyrinths are continuous with those of the auditory system, as are the fluids contained in each (perilymph and endolymph). There are 3 main divisions:

Semicircular canals - anterior, posterior, horizontal.
Utricle - connects 3 canals - otolithic membrane (gravity & acceleration sensor*).
Saccule - continuous with cochlea - otolithic membrane.

Question 4

Describe the unique chemical make up of the endolymph. *

Answer 4

the endolymph has a unique chemical make-up which is essential for the creation of the receptor potentials (K+&raquo_space; Na+). As in the auditory system, hair cells are the primary receptors.

Question 5

Where are the hair cells of semicircular canals located? What do they consist of? *

Answer 5

The hair cells in the semicircular canals are located in a swelling (ampulla) in each canal, and consists of the ampullary crest (nerve) and the cupola, which is a gelatinous substance which covers the hair cells.

Question 6

What is the cupola?*

Answer 6

gelatinous substance which covers hair cells

Question 7

What does rotation of the head cause? What does it result in?

Answer 7

Rotation of the head causes an inertial displacement of the fluid and the cupola, which results in bending of the hair cells. The hair cells are sensitive to movements as small as 0.1 /s2 (acceleration force - NOT velocity).

Question 8

What does movement of the head cause?

Answer 8

Movement of the head results in an inertial displacement of the fluid which pushes the cupola in the opposite direction due to the weight of the fluid (endolymph) in the canal.

Question 9

What does inertial displacement cause? *

Answer 9

This also results in differential displacement of the hair cells on the left and right horizontal canals. The cortex perceives these corresponding increases and decreases in output as directional movement (left or right) with a specific “velocity.”

The greater the difference, the faster the perceived velocity.

Question 10

How are the hair cells oriented functionally? *

Answer 10

Functionally, the hair cells themselves are oriented in specific planes:
the horizontal canals are oriented towards the utricle.

the anterior & posterior canals are oriented away from the utricle.

Question 11

Describe the pairing of the canals. *

Answer 11

The anterior canal on the right is paired with the posterior canal on the left;
The posterior canal on the right is paired with the anterior canal on the left;
The horizontal canals are paired.

Question 12

Describe the polarization of the hair cells of the SSC.

Answer 12

The hair cells of the SSC are polarized both structurally and functionally:

There is ONE large kinocilium and 40-70 stereocilia per hair cell. The stereocilia are mechanically linked to the kinocilium (tip links similar to auditory system), and thus give the hair cell an orientation axis.

(The vestibular nerve has a tonic output, and the hair cells serve to modify that output. Bending of the stereocilia TOWARDS the kinocilium causes depolarization of the receptor, an increased release of the NT, and an increase in the number of AP’s in the vestibular nerve. Bending of the stereocilia AWAY from the kinocilium results in hyperpolarization and a decrease in AP’s from the vestibular nerve.
There are NO AP’s in the hair cells!
Bending of the stereocilia perpendicular to the axis has NO EFFECT.)

Question 13

What does the cupola in the saccule and utricle contain?*

Answer 13

The cupola in the saccule and utricle are also gelatinous, but contain otolithic granules (calcium carbonate crystals), which have weight, and therefore respond to gravity.

Question 14

Describe the utricle and saccule.

Answer 14

Utricle: detects upright position and head tilt; considered the primary gravity sensor; also detects horizontal linear acceleration (not velocity).

Saccule: detects general orientation and vertical linear acceleration.

Question 15

Describe the input to the CNS. Describe the 2 divisions of the vestibular ganglion.

Answer 15

Input to the CNS
Hair cell ⇨ bipolar cell (AP’s) ⇨ vestibular ganglion ⇨ CN VIII ⇨ Vestibular nuclei.

2 divisions of the vestibular ganglion:
Superior: utricle, anterior portion of saccule, horizontal & anterior SSC
Inferior: posterior portion of saccule and posterior SSC

Question 16

Describe the cerebellar influences.

Answer 16

VESTIBULOCEREBELLUM: associated with the flocculonodular lobe and vestibular nuclei.

 Receives direct vestibular input from semicircular canals and otolithic organs, as well as indirect input from the vestibular nuclei, lateral geniculate, superior colliculus & pontine nuclei.

Question 17

Describe the output of the cerebellar influences.

Answer 17

Output is projected back to vestibular nuclei, to control axial and proximal limb muscles associated with balance and posture; also controls eye movements and coordination of head & eye movements.

Question 18

Describe the medial vestibulospinal tract

Answer 18

Medial Vestibulospinal Tract (or “descending MLF”)
From the Medial Vestibular nucleus
Bilateral projection (?)
Cervical spinal cord only – medial ventral funiculus
Neck muscles
Maintains head erect

Question 19

Describe the lateral vestibulospinal tract.

Answer 19

Lateral Vestibulospinal Tract
From the Lateral Vestibular nucleus
Uncrossed projection
Entire length of the cord - ventral funiculus
Proximal limb muscles
Maintains balance by acting on the limbs

Question 20

Describe the vestibular pathways.*
Primary afferents?
Secondary Vestibular connections

Answer 20

Primary Afferents
Vestibular nuclei
Flocculonodular lobe

Secondary Vestibular connections
Cerebellum
Flocculonodular lobe, Cerebellar vermis and Fastigial nucleus
Spinal cord
Lateral and medial vestibulospinal tract
Brainstem
Ascending MLF, Reticular formation, Vestibular commisural connections
Thalamic and cortical areas.

Question 21

Describe the vestibular system and the eyes.

Answer 21

Stabilize our gaze during a head movement or when we shift gaze to a new target
The effector system is the extraocular muscles
The vestibular system, cerebral cortex, cerebellum and brainstem are all involved
These types of eye movements include:

Vestibulo-ocular reflex

Optokinetic response (& nystagmus)
Smooth pursuit
Saccadic eye movements
Vergence eye movements

Question 22

Describe the vestibulo-ocular reflex (VOR). * 
What does it do?
What happens as the head rotates?
What would happen without this reflex?
What happens when the head stops moving?
What does it attenuate with?

Answer 22

Stabilizes the image on the retina during a rotation of the head and faster than visual tracking
As the head rotates the VOR rotates the eyes with the same speed, but in the opposite direction
Without this reflex, the image would appear “smeared” upon the retina
Active almost all of the time.
If the vestibular system is damaged, then you can’t focus on objects when the head/body is in motion! You must first stop all movement.
Once the head stops moving the eyes remain in that same direction of gaze
“stabilization” occurs through the nucleus prepositus hypoglossi
tonic activation maintains the activation/activity of the involved cranial nerve nuclei (the 3rd and 6th)
Attenuates with tonic stimulation

Question 23

Describe the VOR when head is rotating to the right. *

Answer 23

Head is rotating to the right
The right horizontal canal is activated
Right vestibular nucleus is “activated’
The left 6th nucleus (via PPRF) is activated and the left lateral rectus muscle contracts
The left PPRF “activates neurons in the right 3rd nucleus and the right medial rectus contracts

…both eyes begin to move to the left

Question 24

What does flocculus do?
What can Purkinje cells do?
(relating to VOR)

Answer 24

Flocculus integrates adjustments to the gain of the V-O system:
Purkinje cells can inhibit vestibular interneurons.
Adaptive feed-forward “open-loop” control.

Question 25

What is nystagmus?*

Answer 25

1. Rhythmic back and forth movement of the eyes
2. Usually the movement is slow in one direction (“smooth”) and fast (“saccadic”) in the other
   When you induce it by spinning yourself around….
3. The VOR is generating the slow phase which helps to keep an eye on a target
   Once the eye approaches the maximum that it can turn, a saccade will then occur moving the eyes in an opposite direction and onto a new target (Optokinetic nystagmus or OKN)
4. Tested clinically by the caloric test; “COWS”

Question 26

When does nystagmus occur? What are its phases?*

Answer 26

Nystagmus occurs during sustained rotation, and has 2 distinct phases:
SLOW - opposite the direction of rotation.
FAST - returns gaze to starting position (towards rotation).
Nystagmus habituates in the dark, due to adaptation of the semicircular canals *.

Question 27

How is nystagmus coordinated in the brainstem? *

Answer 27

Nystagmus is coordinated in the brainstem by a 3 neuron arc:
Semicircular canal - Vestibular nuclei - Oculomotor neurons (III, IV, VI)
Therefore eye muscles are coordinated with semicircular canals
V-O changes the “head velocity signal” to an “eye velocity signal”.

Question 28

Describe the optokinetic system.

Answer 28

Optokinetic system:
· Responsible for rotary nystagmus, which takes over for vestibular nystagmus in light, and continues to function as long as the visual system is intact.
· long latency and slow decay; vestibular system still habituates.
· Adaptive under feed-forward “open-loop” system also.
· Reflex CAN be suppressed if head AND target are both moving.
· Visual information from cortex and pretectum integrated to facilitate reflex.

Question 29

How do we get dizzy?

Answer 29

Vestibular input without vision
While spinning in a chair with your eyes closed (the constant motion eventually results in the cupula membrane returning to its baseline) and you suddenly open your eyes

Sense of motion via the visual system, but without vestibular “confirmation” (“a disconnect”)
Looking out a car window when an adjacent car moves away (false sense of motion)

Sense of motion via the vestibular system, but without visual “confirmation” (“a disconnect”)
In the cabin of a boat during a storm (motion sickness)
Looking at the horizon (stable point) helps to minimize this effect

Question 30

What do ascending and descending fibers of the MLF control? *

Answer 30

Ascending fibers (MLF) to control eye movements.
Descending fibers (MLF) to control head & neck movements.