Vestibular function

Question 1

What is the vestibular system?

Answer 1

A sensory system essential in the control of posture and balance. Located in the inner ear.

Question 2

Describe the arrangement chambers in the inner ear.

Answer 2

3 bone lined chambers (bony labyrinth). Inside there are 3 membranous chambers (membranous labyrinth). These are embedded in the petrous part of temporal bone.

Question 3

Where are perilymph and endolymph?

Answer 3

Perilymph inside body labyrinth, endolymph inside membranous labyrinth.

Question 4

What is the vestibular apparatus?

Answer 4

• 3 x semi-circular canals (anterior, posterior and middle)
• utricle (semi-circular canals all connect to this)
• saccule

Question 5

What are the swellings at the base of the semi-circular canals and which motion do these detect?

Answer 5

Ampulla - these contain sensory hair cells.

Detect rotational acceleration.

Question 6

What are the otolith organs?

Answer 6

Utricle and saccule. These detect linear acceleration:

• saccule: side to side
• utricle: back and forward

Question 7

What is the function of the semi-circular canals?

Answer 7

To sense motion via sensory hair cells (cilia). Inside the ampulla lie sensory receptors (cristae) contained within gelatinous cupula.

When the head moves, so does endolymph which moves the cupula, which moves the cilia and this activates the cristae –> impulses send via CNVIII.

Question 8

How does the endolymph move?

Answer 8

It doesn’t want to and so produces drag. This bends the cupula, which bends the cilia in the opposite direction to movement. Endolymph will eventually catch up after a few seconds and move at a constant rate.

Question 9

What happens if you are spinning and suddenly stop?

Answer 9

Endolymph is moving at a constant velocity - if you stop spinning the endolymph will keep moving as there is momentum

–> dizziness

Question 10

What are the different types of cilia in the semi-circular canals?

Answer 10

2 types:

• Kinocilium: 1 large hair
• Stereocilia: progressively smaller, many of them

Question 11

Describe how the brain builds a 3D image of body position via the semi-circular canal cilia.

Answer 11

• Distortion of cilia in the direction of the kinocilium increases discharge of AP’s in the vestibular nerve
• Distortion of cilia away from the kinocilium leads to hyperpolarisation and decreases discharge of AP’s in the vestibular nerve

–> this information is sent to the cerebellum and is integrated (pattern of firing and inhibition) which tells the brain which position the body is in

Question 12

What are the sensory receptors of the otolith organs and how are these orientated?

Answer 12

Maculae:

• vertical orientation in saccule
• horizontal orientation in utricle

Question 13

What are the functions of the otolith organs?

Answer 13

• Maculae have a set of cilia: 1 kinocilium and a series of stereocilium
• These protrude into the otolith membrane (gelatinous)
• CaCO3 otolith crystals are embedded in the otolith membrane

The head tilts which moves the otoliths –> distorsion of jelly which moves the cilia.

Backwards tilt moves otolith in direction of kiinocilium –> depolarisation and increased AP discharge (opposite for forwards tilt)

Question 14

Where do projections via the vestibular nerve go?

Answer 14

• cerebellar centres for balance (control of postural muscles)
• vestibular centres of the medulla –> thalamus –> cerebral cortex for perception of movement and body position (kinaesthesia)
• project to descending motor pathways and extraocular nuclei

Question 15

Do vestibular nuclei receive any input?

Answer 15

Yes from proprioceptors (limb and body position), neck and eye muscles

Question 16

What are the main vestibular system reflexes?

Answer 16

1) Tonic labyrinthine reflexes
2) Dynamic righting reflexes
3) Vestibulo-ocular reflexes

Question 17

What are tonic labyrinthine reflexes?

Answer 17

Keep the axis of the head in a constant relationship with the rest of the body using information from maculae and neck proprioceptors.

Question 18

What are dynamic righting reflexes?

Answer 18

Rapid postural adjustments that are made to stop you falling when you trip. Long reflexes involving extension of limbs (cats).

Question 19

What are vestibulo-ocular reflexes?

Answer 19

a) static reflex: when you tilt your head your eyes extort or intort to compensate so that you can see the image the right way
b) dynamic vestibular nystagmus: rapid rotation of the eye against the direction of rotation of the head so that the original direction of gaze is preserved despite head rotation - restricted and when the eyeball is at the end of its range of movement it rapidly flicks back to looking straight ahead

Question 20

How do you tell the direction of a nystagmus?

Answer 20

It is in the same direction as the rapidflick back eg. R rotation, R nystagmus

Question 21

How can nystagmus be used to test vestibular function (post-rotary nystagmus)?

Answer 21

Rotate someone in a chair to the left –> left nystagmus. Will then get a right nystagmus when the chair has stopped spinning as the endolymph is playing catch up and is therefore moving the cupula in the opposite direction.

Question 22

How can nystagmus be used to test vestibular function (caloric stimulation)?

Answer 22

Testing the function of the horizontal semi-circular canal:

• inject warm or cool fluid into the inner ear
• warm fluid (44 degrees): nystagmus towards the affected side
• cold fluid (30 degrees) nystagmus away from the affected side

–> COWS (cold opposite warm same)

(Nystagmus can also be seen with lesions to peripheral or central vestibular pathways)

Question 23

What is kinetosis and why does this occur?

Answer 23

Motion sickness due to powerful and maintained stimulation of the vestibular system.

More likely to occur if visual and vestibular system are in conflict eg on a bus when its dark. Sickness signal sent to the cerebellum which causes ANS symptoms.

Question 24

What are the symptoms of kinetosis?

Answer 24

Autonomic symptoms: N/V, low BP, dizziness, sweating and pallor

Question 25

What is vertigo?

Answer 25

Interference with normal vestibular function as a result of infection (labyrinthitis) can cause vertigo:

• the perception of movement in the absence of movement; may also be nystagmus; impairment of balance and posture; very disabling

Question 26

What is Meniere’s disease?

Answer 26

Over-production of endolymph causing increased pressure:

• vertigo
• nausea
• nystagmus
• tinnitus

Question 27

Name an antibiotic that can attack the inner ear.

Answer 27

Streptomycin.

Question 28

What happens if vestibular impairment becomes chronic?

Answer 28

This can be well compensated by the visual system due to new learning circuits being set up in the cerebellum.

If subjects cannot see this causes problems.

Question 29

What can lesions of the brain lead to?

Answer 29

Nystagmus at rest.