Lecture 6 - overview of auditory system

Question 1

what is sound frequency?

Answer 1

• number of cycles per second
• frequencies from 20Hz-20kHz
• achieved by mechanics if cochlea and physiology of hair cells

Question 2

what is sound intensity?

Answer 2

• amplitude of wave from peak to peak
• achieved by firing rate of many nerve fibres

Question 3

onset

Answer 3

rapid onset is important for localising different sounds and creating a map of auditory world around us

Question 4

duration

Answer 4

• ear remains sensitive to sounds for long periods without fatigue
• never rests and is always on
• all info is encoded by hair cells and nerve fibres

Question 5

travelling of sound wave

Answer 5

sound wave travels down outer ear to tympanic membrane and through the cochlea

Question 6

what is the cochlea innervated by?

Answer 6

auditory nerve

Question 7

three compartments of the cochlea

Answer 7

scala vestibuli, scala media and scala tympani

Question 8

where does organ of corti sit?

Answer 8

on the basilar membrane

Question 9

what does scala vestibuli and scala tympani contain

Answer 9

perilymph (normal extracellular solution) resting potential of +80mV

Question 10

endocochlear potential

Answer 10

+80mV

Question 11

what does scala media contain?

Answer 11

endolymph which has a very high potassium concentration compared to normal

Question 12

where are high potassium created?

Answer 12

by cells in the stria vascularis and pump potassium in scala media

Question 13

resting potential of sensory hair cells

Answer 13

-60mV

Question 14

the result of the different potentials of the two solutions (endolymph and perilymph)

Answer 14

makes a high driving force and is vital for how they function (140mV)

Question 15

perilymph contents (normal)

Answer 15

• low potassium
• normal calcium
• high sodium

Question 16

endolymph contents

Answer 16

• high potassium
• low calcium
• low sodium

Question 17

hair cells in organ of corti

Answer 17

• inner hair cells are main sensory cells and encode all auditory information for brain
• outer hair cells do not have sensory role but are important for cochlea amplification

Question 18

tonotopic organisation of mammalian cochlea

Answer 18

cells of the base are activated by high frequency sounds and cells at the apex respond to low frequency sounds

Question 19

place-frequency code

Answer 19

the position of the active inner hair cell along the cochlea that encodes the sound frequency

Question 20

what is cochlea tonotopicity established by?

Answer 20

the basilar membrane travelling wave. wave travels along the basilar membrane that travels from base to apex

Question 21

characteristic frequency

Answer 21

sound of one frequency causes maximal movement of the basilar membrane at one location

Question 22

lower frequency sound on basilar membrane

Answer 22

• travels further along basilar membrane
• causes maximal movement towards apex
• characteristic frequency location is closer to apex

Question 23

higher frequency sound on basilar membrane

Answer 23

• travels less along the basilar membrane
• causes maximal movement towards the base
• characteristic frequency location to the base

Question 24

what is the characteristic location determined by?

Answer 24

width and stiffness of basilar membrane
- apex is wide and floppy’
- base is narrow and stiff

Question 25

inner hair cells

Answer 25

- primary sensory receptors - encode all auditory information and pass it onto nerve fibres

Question 26

what are inner hair cells defined by?

Answer 26

the stereocilia hair bundle

Question 27

what are within the stereocilia

Answer 27

mechanosensitive ion channels - called mechanoelectrical transducer channels (MET) - these channels join the taller stereocilia next to it

Question 28

inner hair cells at rest

Answer 28

- slight tension on tip links and transducer channels open creating a transducer current - resting inwards MET current is -55mV - potassium ions enter down large electrical gradient (140mV) - potassium ions enter perilymph at bottom of cell

Question 29

what keeps the perilymph and endolymph separate?

Answer 29

a tight barrier

Question 30

inner hair cells in the two solutions

Answer 30

hair bundle is in endolymph and cell body is surrounded by perilymph

Question 31

inner hair cells with excitatory stimulation

Answer 31

- large deflection of hair bundle towards the taller stereocilia. increases tension in tip links - opens MET channels and causes large MET current - depolarises hair cell (-30mV) and activates the calcium channels - inner hair cells do not fire action potentials, they respond to motion -

Question 32

depolarisation of inner hair cells potassium channels

Answer 32

activates potassium channels - potassium ions exit down concentration gradient into perilymph to help repolarise cell

Question 33

inner hair cells with inhibitory stimulation

Answer 33

- large deflection of hair bundles towards shorter stereocilia and making tip links slack - closes MET channels and turns off MET current - hyperpolarises hair cell below resting potential - potassium channels open for longer to repolarise cell (-65mV)

Question 34

Inner hair cells with sustained stimulation

Answer 34

- sustained sound moves hair bundle back and forth at sound frequency creates a cycle of membrane potential matching sound frequency and hair cell moves back and forth - generates pulses of neurotransmitter release and afferent activity - sensory information relayed to brain

Question 35

outer hair cells

Answer 35

- function as the cochlear amplifier - they shorten and lengthen in time with sound - electromotility - they do not have many afferents

Question 36

function of Prestin

Answer 36

- in the outer hair cell membrane - allows the hair cell to shorten or elongate in response to changes in membrane potential

Question 37

outer hair cell stimulation

Answer 37

- outer hair cells stimulation works the same for inner hair cells - at rest there is a resting MET current

Question 38

outer hair cell resting potential

Answer 38

around 40mV

Question 39

what causes outer hair cells to shorten

Answer 39

when they depolarise

Question 40

the combined movement of 3 rows of outer hair cells

Answer 40

- acts as a positive feedback in cochlea - increases movement of basilar membrane - increases stimulation of inner hair cell bundles - outer hair cells amplify the stimulation of inner hair cells

Question 41

how outer hair cells affect movement of basilar membrane

Answer 41

- outer hair cells electromotiltiy amplifies basilar membrane motion - basilar membrane movement is greatly increased with cochlear amplification - results in sharply tuned and highly sensitive inner hair cells

Question 42

loss/ damage of outer hair cells

Answer 42

- stimulation of inner hair bundle is weaker - severe hearing loss but not complete deafness still have basic tuning of basilar membrane