Vestibular Function

Question 1

In which bone is the vestibular system embedded?

Answer 1

• Temporal bone

Question 2

What are the components of the vestibular system?

Answer 2

• 3 semicircular canals
• 3 ampulla (swelling at bases of canals)
• Utricle (all canals attach to)
• Saccule

Question 3

How are the three semicircular canals oriented with respect to one another?

Answer 3

• Oriented at right angles to each other

3 dimensions

Question 4

Where are the sensory hair cells in the vestibular system?

Answer 4

In the ampulla, utricle and saccule

Question 5

What are the otolith organs?

Answer 5

The utricle and saccule

Question 6

What sort of acceleration is detected by the separate components of the vestibular system?

Answer 6

• utricle: back/front tilt and horizontal acceleration
• Saccule: vertical acceleration
• Semi-circular canals: rotational acceleration

Question 7

Where are the sensory cells of the semicircular canals located?

Answer 7

In the ampulla

• Cilia embedded inside of gelatinous cupula

Question 8

Describe the anatomy of the ampulla in regards to sensation of movement?

Answer 8

• Cilia embedded in gelatinous cupula
• Endolymph fluid on surrounds the cupula
• When the head rotates the endolymph fluid moves and exerts shearing force on the cupula which bends the cilia embedded within

Question 9

Describe how rotation is sensed by semicircular canals

Answer 9

• Head rotates
• Inertia of endolymph fluid exerts shearing force on the cupula, bending the cilia within (in opposite direction of movement)
• The cilia synapse directly with the vestibular nerve and impulses sent to brain

Question 10

Describe what happens when the head rotates at a constant velocity

Answer 10

• At first shearing force from inertia of endolymph bends the cupula
• If rotating at constant velocity for a few seconds the endolymph catches up with the speed of head rotation, no more shearing force exerted on cupula (stops bending)
• When rotation of the head stops the momentum of the endolymph fluid keeps it moving, once again bends the cupula
• Detects changes in rotational velocity, equilibrates at constant velocity

Question 11

What are the different types of cilia of the hair cells in the vestibule? How does this allow the body to determine the direction of movement?

Answer 11

• There is one large kinocilium and a set of progressively smaller stereocilia
• If the stereocilia are distorted towards the kinocilium there is an increase in depolarization and APs to the vestibular nerve
• If the stereocilia are distorted away the inverse happens and there is a decrease in discharge of APs

Question 12

Where does most of the integration of vestibular signals take place in the brain?

Answer 12

The cerebellum

Question 13

What are the sensory apparatus of the otolith organs (saccule and urtricle) know as?

Answer 13

The macculae

Question 14

Describe the anatomy of the maculae

Answer 14

• Cilia (stereocilia and kinocilium) embedded in a gelatinous otolith membrane
• Also embedded in the otolith membrane are CaCO3 crystals called otoliths
• The maculae in the utricle are oriented in the horizontal plane, in the saccule are oriented in the vertical plane

Question 15

How do the maculae sense movement?

Answer 15

• The otoliths have a higher density than the otolith membrane
• So when the head is tilted or there is vertical/horizontal acceleration the otoliths move with gravity and distort the otolith membrane
• The distortion of the membrane causes distortion of the stereocilia either to or away from the kinocilium, causes increase or decrease in APs to the vestibular nerve

Question 16

How do backward and forwards tilt move the stereocilia in regards to the kinocilium?

Answer 16

• Backwards tilt moves stereocilia towards the kinocilium, so more APs are fired
• Forwards tilt moves stereocilia away from the kinocilium, less APs

Question 17

What tracts are involved in the vestibular system reflexes?

Answer 17

the vestibulocoritcal and vestibulospinal tracts

Question 18

What are the three main vestibular system reflexes?

Answer 18

1. Tonic labyrinthine reflexes: Keep axis of head in line with body. Use information from maculae and neck proprioceptors.
2. Dynamic righting reflexes: rapid postural adjustments that are made to stop you falling when you trip. Long reflexes, involving extension of all limbs
3. Vestibulo-ocular reflexes: association between the vestibular apparatus, the visual apparatus and postural control

Question 19

What is the relationship between the vestibular system and eye movement? Between visuals and posture?

Answer 19

• afferents from the vestibular system synapse with afferent fibres travelling to the extraocular nuclei and thus influence eye movement
• visual system sends descending projections which control posture

Question 20

What is the static reflex (branch of vestibulo-ocular reflexes)?

Answer 20

When you tilt your head, the eyes intort/extort to compensate so the image stays the right way up.

Question 21

What is Dynamic Vestibular Nystagmus

Answer 21

A series of eye movements that rotate the eye against the direction of passive rotation of the head and body so that the original direction of gaze is preserved despite head rotating
(if you rotate your head but keep looking at the same thing)

Question 22

By convention, if you rotate somebody to the right, do you get a right or a left nystagmus?

Answer 22

Right nystagmus

Question 23

What is a post rotatory nystagmus test?

Answer 23

• test for vestibular function
• Rotate patient left for a while, get a left nystagmus
• During deceleration get a right nystagmus due to the momentum of endolymph fluid (speed decreases slower than the speed of the heads rotation)

Question 24

What is a caloric stimulation test?

Answer 24

• vestibular function test
• Place either warm or cold (relative to body temp) water in the ears of patient. Causes convection currents in the endolymph fluid due to the bone being thin
• If water is warm should see a nystagmus to the affected side, if cold nystagmus to opposite side

Question 25

What is labyrithitis?

Answer 25

- Acute interference with vestibular function due to infection - ANS symptoms and vertigo

Question 26

What is vertigo?

Answer 26

Perception of movement in the absence of movement

Question 27

What is Menieres disease?

Answer 27

Combination of: - Vertigo - Nausea - Nystagmus - Tinnitus (ringing in ears)

Question 28

What drug is the inner ear sensitive to damage from?

Answer 28

Streptomycin

Question 29

What happens in patients with chronic vestibular impairment?

Answer 29

- Compensation by the visual system (usually)