auditory function and balance

Question 1

how many ossicles (small bones) are present in the middle ear

Answer 1

3

Question 2

what do the middle ear ossicles do

Answer 2

transmit vibration of tympanic membrane onto cochlea of innner ear, matching impedance and reducing loss in energy as vibration goes from air to cochlea (some reflected, some transmitted; similar to violin bridge)

Question 3

what is the shape of the cochlea, and what is it filled with

Answer 3

snail-shaped organ filled with liquid

Question 4

define impedance

Answer 4

measures reluctance of system in receiving energy from a source

Question 5

define resonant frequency

Answer 5

frequency at which the impedance of the system is minimal; changes locally

Question 6

what controls tension of tympanic membrane

Answer 6

malleus and incus, which can be adjusted by tensor tympanic muscle and stapedius muscles

Question 7

in the cochlea, what does the motion of the stapes generate

Answer 7

difference in pressure between the two liquid-filled chambers

Question 8

what does the difference in pressure created by the stapes in the cochlea cause

Answer 8

basilar membrane to vibrate (part of Organ of Corti)

Question 9

what are the 4 sections of the Organ of Corti in the cochlea, and diagram

Answer 9

basilar membrane, tectorial membrane, hair cells, supporting cells

Question 10

function of basilar membrane, and how it does this

Answer 10

frequency analyser, by breaking complex sounds down by distributing energy of each component frequency along its length using active and passive (impedence) processes

Question 11

structure of basilar membrane and how related to function

Answer 11

elastic structure that vibrates at different positions along its length in response to different frequencies; impedance, and therefore local resonant frequency, of basilar membrane varies along its length

Question 12

what is present along the whole length of the basilar membrane to detect all frequencies

Answer 12

hair cells, which act as sensory receptors

Question 13

structure of hair cells

Answer 13

have a hair bundle on projecting surface, which is a cluster of modified microvilli (stereocilia)

Question 14

name of how hair cells produce electric signals

Answer 14

mechano-transduction

Question 15

process of mechano-transduction

Answer 15

motion of basilar membrane deflects stereocilia away from modiolus -> when they bend towards tallest stereocilium, K+ channels open -> K+ enters from endolymph -> changes cell internal voltage, causing hair cell to depolarise -> opens Ca2+ channels in body of hair cell -> glutamate released from base depolarises axon, creating action potential that travels towards brain

Question 16

how are stereocilia connected to hair cells

Answer 16

filamentous linkages called tip links

Question 17

function of tip links on stereocilia

Answer 17

work as small springs stretched by stereocilia’s sliding

Question 18

what are response currents in mechano-transduction a result of, concerning tip links

Answer 18

opening of ion channels activated by stretching of tip links (as tip links share location with ion channels)

Question 19

effect of external stimulus on mechano-transduction ion channels, tip links and whole of stereocilia

Answer 19

mechano-transduction ion channels open, relaxing the tip links and the whole stereocilia

Question 20

what happens to measured stiffness when mechano-transduction ion channels open and why

Answer 20

becomes negative, as healthy stereocilia actively complies with direction of stimulus (not passive -> actively swings when triggered)

Question 21

4 aspects of active process by which cochlea has sensitivity and sharp frequency selectivity

Answer 21

amplification, frequency tuning, compressive non-linearity, spontaneous otoacoustic emission

Question 22

evidence for amplification

Answer 22

particular segment of a living basilar membrane vibrates far more in response to its resonant frequency than a dead basilar membrane

Question 23

evidence for frequency tuning

Answer 23

a dead basilar membrane produces a broad response and it is not tuned for a specific frequency; a living basilar membrane instead selectively amplifies single frequencies

Question 24

evidence for compressive non-linearity

Answer 24

basilar membrane motion is augmented 100-fold during low-intensity stimulation, but amplification diminishes progressively with the increasing intensity of the stimulus (compress dynamic range of sound to limit of basilar membrane vibration)

Question 25

evidence for spontaneous otoacoustic emission

Answer 25

70% of normal humans ears emit one or more pure tones when in a quiet environment, which is only possible in healthy cochleas because of the work performed by the ear in normal conditions to counteract viscous drag in cochlea

Question 26

how is pitch determined and normal range

Answer 26

frequency (20Hz-20kHz)

Question 27

how does the basillar membrane act as a frequency analyser

Answer 27

high frequencies vibrate basilar membrane nearer to base, low frequencies vibrate membrane nearer to apex