Physiology of Hearing and Balance

Question 1

what is the primary function of the Eustachian tube

Answer 1

to equalise the pressure in the middle ear to that of the external environment

Question 2

what is the 2y function of the eustachian tube

Answer 2

to drain any accumulated secretions from the middle ear

Question 3

describe how sound waves travel through the cochlea

Answer 3

stapes movement displaces the perilymph, causing vibrations in the labryinth

vibrations ascend to the apex of the cochlea in the scala vestibuli and descend in the scala tympani

specific part of the basilar membrane vibrates and the organs of Corti (inner hair cells) are bent and triggered to fire (depolarise) - this stimulates the cochlear nerve and central pathways

Question 4

how are different sounds perceived

Answer 4

differenet areas along the basilar membrane respond differently to different frequencies of sound

lower sounds vibrate closer to the apex of the cochlea, and higher closer to the base

Question 5

oscillopsia

Answer 5

occurs when we have no/problems with vestibular input - objects inthe visual field appear to oscillate

Question 6

semi-circular canals

Answer 6

sense rotational movement (eg turning head left or right)

as we rotate our head the endolymph shifts in the canals, and causes hair cells in the crista ampullaris to increase or decrease firing in each canal

the canals are sensitive to the degree and speed of rotation

Question 7

what are the sensory organs of rotation

Answer 7

crista ampullaris

Question 8

otolithic organs

Answer 8

eg urticle and saccule

detect linear acceleration and head positioning

Question 9

describe the contents of otolithic organs

Answer 9

contain calcium carbonate crystals (otoconia) attached to hair cells (kinocilium) in a viscous gel membrane

Question 10

function of otolithic organs

Answer 10

movement - head tilt or linear acceleration moves the crystals as they are heavier. this physically pulls on the hair cells and triggers an AP

the AP firing adjust to linear motion, but does not with head tilt

Question 11

Vestibulo-Ocular Reflex

Answer 11

Activation of the vestibular system causes eye movement.

This reflex functions to stabilise images on the retinas during head movement by producing eye movements in the opposite direction to head movement.

eg if move head right, right side is excited and left inhibited

Question 12

what happens to the vestibulo-ocular reflex in right vestibular neuritis

Answer 12

there would be no firing on the right side - nystagmus

Question 13

when is VOR used?

Answer 13

all the time to stabilise the eyes, those whose VOR is impaired find it very hard to read small print

Question 14

nystagmus

Answer 14

VOR impairement can cause vestibular nystagmus

• the eyes make repetitive uncontrolled movements

Question 15

which tract connects the CN involved in ocular muscles and provides motor output to eyes

Answer 15

medial longitudinal fasciculus

- connects CN III, IV, VI, VIII to integrate the movements of the eyes and the head