1.53 The vestibular system

Question 1

What is the structure of the vestibular system?

Answer 1

Bony otic capsule - buried in lateral part of the petrous bone
Vestibule - semi circular canals
Membranous labyrinth containing endolymph surrounded by bony labyrinth containing perilymph

Question 2

What makes up the vestibule?

Answer 2

Utricle and succule - connected by utriculo-saccular duct

Question 3

What are the semi-circular canals?

Answer 3

Lateral - horizontal 
Superior 
Posterior - vertical
Perpendicular from one another 
Originate from utricle 
Bulge at one end - ampulla

Question 4

What do the utricle and saccule contain?

Answer 4

They are otolith organs and contain calcium carbonate

Question 5

What do the utricle and saccule detect?

Answer 5

Detect changes in head angle and linear acceleration

Question 6

What is the sensory epithelium called?

Answer 6

Macula
Horizontal - utricle
Verticle - saccule

Question 7

What do the vestibular hair cells have?

Answer 7

Kinocillium and stereocillia

Bending towards longest depolarises, towards shortest hyperpolarises

Question 8

What does the macula respond to?

Answer 8

Gravity or linear acceleration

Question 9

What do the semi-circular canals detect?

Answer 9

Rotational acceleration

Question 10

Where are the hair cells embedded in the semi-circular canals?

Answer 10

gelatinous cupula in ampulla

Question 11

What happens when the head rotates in the appropriate plane?

Answer 11

When head rotates in appropriate plane, canal rotates but initially fluid doesn’t owing to its inertia – bends cupola, and stereocilia of hair cells
Detects change in velocity (acceleration)

Question 12

What happens at constant velocity?

Answer 12

At constant velocity fluid eventually catches up with canal and rotates at same velocity
Cupola dragged back to upright position, so bending of hair cell stereocilia eliminated eliminated
Can only detect a change in velocity not constant velocity

Question 13

What happens when rotation stops?

Answer 13

cupola bends in the opposite direction

Vestibular axons spontaneously active and so can signal both increase and decrease in firing - even when no stimulation

Question 14

Why is the central vestibular pathway important?

Answer 14

Important in controlling head, eye and body position

Question 15

Where does the otolith organ project to and what does that allow?

Answer 15

Otolith organ output projects to lateral vestibular nucleus and via vestibulospinal tract - controls legs to maintain posture

Question 16

Where does each vestibular nucleus project into?

Answer 16

Lateral - cerebellum & limb motor neurons

Medial - medial longitudinal fasciculus - II, IV , VI and neck motor neurons and thalamus and cortex

Question 17

Where do semicircular canals project?

Answer 17

Medial vestibular nucleus

Question 18

What is the function of the vestibular ocular reflex?

Answer 18

Stabilises eyes – keeps them pointing in same direction despite head movements
Compensates for head movement with counter rotation of the eyes

Question 19

Which part of the vestibular system is important in the vestibular ocular reflex?

Answer 19

Semi - circular canals

Question 20

Is the reflex affected by the darkness?

Answer 20

No it also works in the dark - doesn’t need vision!!

Question 21

Does the left vestibular nucleus connect unilaterally or bilaterally with the nuclei of the eye muscles?

Answer 21

Bilaterally, inhibits and excites appropriate ones

Question 22

What is nystagmus?

Answer 22

Slow movement in opposite direction to rotation followed by a rapid flip back, also seen when rotation stops

defined by the direction of the fast beating

Question 23

What is acquired nystagmus and what is it caused by?

Answer 23

Eyes move without rotation

Vestibular and brainstem lesion

Question 24

What is menieres disease?

Answer 24

a disease of unknown cause affecting the membranous labyrinth of the ear, causing progressive deafness and attacks of tinnitus and vertigo.

Question 25

What could be a cause of miners disease?

Answer 25

•Cause unknown but possibly excessive endolymph production

endolymphatic hydrops