Physiology of Balance

Question 1

What is the structure of the inner ear?

Answer 1

thin walled membranous labyrinth floats inside the outer bony wall/labyrinth
perilymph/periotic fluid - separates bony labyrinth from membranous labyrinth
endolymph/otic fluid - fills the membranous labyrinth

Question 2

What is the structure of hair cells in the inner ear?

Answer 2

single motile kinocilium surrounded by numerous non-motile stereocilia

Question 3

What happens when stereocilia deflect towards the kinocilium?

Answer 3

depolarisation - increased firing of CN VIII

Question 4

What happens when stereocilia deflect away from the kinocilium?

Answer 4

hyperpolarisation - decreased firing of CN VIII

Question 5

Where is the vestibular system located?

Answer 5

bony labyrinth of the inner ear

continuous with the cochlea

Question 6

What are the names of the three semi-circular canals?

Answer 6

posterior
anterior
lateral

Question 7

How are the semi-circular canals orientated?

Answer 7

at 90 degrees to each other

Question 8

How are semi-circular canals paired?

Answer 8

horizontal ducts (in the same plane) 
anterior canal of one side with the posterior canal of the other (parallel)

Question 9

What are ampulla?

Answer 9

bulbous expansion at the base of each semi-circular canal

Question 10

What do ampulla contain?

Answer 10

sensory epithelium/crista that contains hair cells

Question 11

What do semi-circular ducts sense?

Answer 11

rotary movement/angular acceleration of the head

Question 12

Where do the hair bundles from the crista extend into?

Answer 12

cupula

Question 13

What is the cupula?

Answer 13

gelatinous mass that bridges the width of the ampulla forming a fluid barrier through which endolymph can’t circulate

Question 14

What happens when there is movement of head in the plane of one of the semi-circular canals (angular acceleration)?

Answer 14

inertia of endolymph distends the cupula away from the direction of head movement

Question 15

What is inertia?

Answer 15

desire to stay the same

Question 16

What happens in the semi-circular canals when their is linear movement of the head?

Answer 16

equal force on both sides of cupula - no displacement of hair bundles

Question 17

What are the utricle and saccule?

Answer 17

two large chambers of the vestibular system

Question 18

What is the function of the utricle and saccule?

Answer 18

sense forward and backward (linear acceleration) movement of the head

Question 19

What is the macula?

Answer 19

small flat epithelial patch in the utricle and saccule that contains supporting cells and sensory hair cells

Question 20

What do sensory hair cells in the macula synapse with?

Answer 20

sensory endings of the vestibular nerve

Question 21

Where are hair cells located in the macula?

Answer 21

embedded in the otolithic membrane

Question 22

What is the otolithic membrane?

Answer 22

gelatinous mass

Question 23

What are otoliths?

Answer 23

calcium carbonate crystals

Question 24

Where are otoliths found?

Answer 24

otolithic membrane

Question 25

What is the function of otoliths?

Answer 25

increase membrane weight and its resistance to change in motion

Question 26

What happens when the head is tilted?

Answer 26

otolithic membrane shifts its position due to gravity

bends the cilia of the hair cells

Question 27

What is the neural pathway for the vestibular system?

Answer 27

information from the vestibular system goes directly to the vestibular nuclei in the brainstem or cerebellum

Question 28

What is the vestibular nuclei?

Answer 28

main integrative centre for balance

Question 29

Where do the vestibular nuclei receive inputs from?

Answer 29

vestibular system
visual receptors
somatic receptors - particularly stretch receptors in the neck muscles that report the angle or inclination of the head

Question 30

Where do the vestibular nuclei send impulses too?

Answer 30

brainstem centres that control the extraocular eye movements and reflex movements of the neck, limb and trunk muscles

Question 31

What are the vestibulo-ocular reflexes?

Answer 31

keep the eyes still as the head moves

Question 32

What are the vestibulospinal reflexes?

Answer 32

allow the body to maintain or regain balance

Question 33

What are nystagmus?

Answer 33

involuntary eye movements that preserve eye fixation on stable objects in the visual field during angular and rotational movements of the head

Question 34

Give an example of nystagmus

Answer 34

as the head rotates the vestibulo ocular reflexes cause a slow compensatory drifting of eye movement in the opposite direction - stabilises binocular fixation point - pattern of slow-fast movement is nystagmus

Question 35

How is the direction of nystagmus clinically names?

Answer 35

for the direction of the fast phase

Question 36

When is nystagmus pathological?

Answer 36

spontaneous nystagmus that occurs without head movement