Vestibular Function

Question 1

What is the vestibular function?

Answer 1

Sensory system essential for control of posture and balance
Found in inner ear
Series of fluid filled membranous tubes (labyrinths) embedded in temporal bone

Question 2

What does the vestibular apparatus consist of?

Answer 2

3 semi-circular canals
Utricle (semi-circular canals all connect)
Saccule

Question 3

What is the structure of the 3 semi-circular canals?

Answer 3

All at right angles
Swellings at their bases (ampulla)
Ampulla contains sensory hair cells

Question 4

What are the utricle and saccule collectively known as?

Answer 4

Maculae (Otolith organs)

Question 5

What does the utricle detect?

Answer 5

Back/front tilt

Horizontal acceleration

Question 6

What does the saccule detect?

Answer 6

Vertical acceleration

Question 7

What does the semi-circular canals detect?

Answer 7

Rotational acceleration

Question 8

What are the sensory receptors in the semi-circular canals and how do they function?

Answer 8

Hair cells within the ampulla
Hair cells are embedded in a flexible gelatinous structure (cupula) that stretches across entire width of ampulla
Becomes distorted by movement of endolymph fluid in canals

Question 9

What does the cilia of the hair cells synapse with?

Answer 9

Directly with sensory neurons of the vestibular nerve (CN VIII)

Question 10

How does detection of rotational acceleration work?

Answer 10

If skull is rotated left or right from rest
Endolymph does not move at first because of inertia
Ampulla moves instantly because it is embedded in the skull
Inertia produces drag = bends cupula (+cilia) in opposite direction to movement
Endolymph catches up eventually
Sudden stop causes endolymph to continue to move due to momentum creating sense of dizziness and continued movement

Question 11

What are the 2 types of cilia of the hair cells?

Answer 11

Single, very large kinocilium

Progressively smaller stereocilia

Question 12

What is the result of distortion of the cilia in the direction of the kinocilium?

Answer 12

Depolarisation and increased discharge of APs in vestibular nerve

Question 13

What is the result of distortion of the cilia away from the kinocilium?

Answer 13

Hyperpolarisation and decreased discharge of APs in the vestibular nerve

Question 14

How does the body make a 3D image of body position?

Answer 14

Using the pattern of firing and inhibition received by result of distortion

Question 15

Where does most integration of the sensory information from the vestibular system?

Answer 15

Cerebellum

Question 16

How is the macula orientated in the utricle?

Answer 16

Horizontal plane

Question 17

How is the macula orientated in the saccule?

Answer 17

Vertical plane

Question 18

What does the cilia of the maculae protrude into?

Answer 18

Gelatinous mass = otolith membrane

Question 19

What is embedded in the otolith membrane?

Answer 19

CaCO3 crystals (otoliths)

Question 20

How is tilt of the head detected by the macula in the utricle?

Answer 20

otoliths = greater density than endolymph = affected more by gravity
Otoliths and membrane move when head tilted = distorted jelly = moves cilia

Question 21

What does backward tilt result in?

Answer 21

Otolith moves towards kinocilium = depolarisation and increased discharge of APs

Question 22

What vertical forces does the vertically orientated macula in the saccule respond to?

Answer 22

Elevation in a lift

Provides information on orientation of head when lying down

Question 23

What are tonic labyrinthine reflexes?

Answer 23

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

Question 24

What are dynamic righting reflexes?

Answer 24

Rapid postural adjustments that are made to stop you from falling when you trip

Question 25

What are vestibulo-ocular reflexes?

Answer 25

Association between vestibular apparatus, visual apparatus and postural control
In the medullary vestibular nuclei, afferents from vestibular system synapse with afferent fibres going to extraocular nucleii = eye movement

Question 26

What is static reflex?

Answer 26

Head tilts

Eyes intort/extort to compensation

Question 27

What is dynamic vestibular nystagmus?

Answer 27

Series of saccadic eye movements that rotate the eye against direction of passive rotation
When eyeball comes to end of range of movement = eye rapidly flicks back to straight ahead

Question 28

What occurs in post-rotatory nystagmus?

Answer 28

If rotated left, during acceleration = left nystagmus
End of rotation, during deceleration = right nystagmus
Due to endolymph catching up and pushing the cupula in opposite direction

Question 29

What is caloric stimulation?

Answer 29

Test for horizontal semi-circular canal
Outer ear is washed with cold/warm fluid
Temperature difference gets through thin bone = convection currents = affect endolymph
Warm fluid = nystagmus to affected side
Cold fluid = away from affected side

Question 30

When is motion sickness most likely to occur?

Answer 30

If visual and vestibular system inputs to the cerebellum are in conflict
Cerebellum generates ‘sickness signal’ to hypothalamus causing nausea, vomiting, decreased BP, dizziness, sweating, pallor

Question 31

What is labyrinthitis?

Answer 31

Acute interference with normal vestibular function as a result of infection = all ANS symptoms + vertigo

Question 32

What is vertigo?

Answer 32

Perception of movement in the absence of movement
May be nystagmus
Gross impairment of posture and balance

Question 33

What is Meniere’s disease?

Answer 33

Vertigo, nausea, nystagmus and tinnitus
Associated with over production of endolymph causing increased pressure
Cause is unknown

Question 34

What can cause nystagmus at rest?

Answer 34

Lesions of the brainstem