Lecture 6 - overview of auditory system Flashcards
what is sound frequency?
- number of cycles per second
- frequencies from 20Hz-20kHz
- achieved by mechanics if cochlea and physiology of hair cells
what is sound intensity?
- amplitude of wave from peak to peak
- achieved by firing rate of many nerve fibres
onset
rapid onset is important for localising different sounds and creating a map of auditory world around us
duration
- 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
travelling of sound wave
sound wave travels down outer ear to tympanic membrane and through the cochlea
what is the cochlea innervated by?
auditory nerve
three compartments of the cochlea
scala vestibuli, scala media and scala tympani
where does organ of corti sit?
on the basilar membrane
what does scala vestibuli and scala tympani contain
perilymph (normal extracellular solution) resting potential of +80mV
endocochlear potential
+80mV
what does scala media contain?
endolymph which has a very high potassium concentration compared to normal
where are high potassium created?
by cells in the stria vascularis and pump potassium in scala media
resting potential of sensory hair cells
-60mV
the result of the different potentials of the two solutions (endolymph and perilymph)
makes a high driving force and is vital for how they function (140mV)
perilymph contents (normal)
- low potassium
- normal calcium
- high sodium
endolymph contents
- high potassium
- low calcium
- low sodium
hair cells in organ of corti
- 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
tonotopic organisation of mammalian cochlea
cells of the base are activated by high frequency sounds and cells at the apex respond to low frequency sounds
place-frequency code
the position of the active inner hair cell along the cochlea that encodes the sound frequency
what is cochlea tonotopicity established by?
the basilar membrane travelling wave. wave travels along the basilar membrane that travels from base to apex
characteristic frequency
sound of one frequency causes maximal movement of the basilar membrane at one location
lower frequency sound on basilar membrane
- travels further along basilar membrane
- causes maximal movement towards apex
- characteristic frequency location is closer to apex
higher frequency sound on basilar membrane
- travels less along the basilar membrane
- causes maximal movement towards the base
- characteristic frequency location to the base
what is the characteristic location determined by?
width and stiffness of basilar membrane
- apex is wide and floppy’
- base is narrow and stiff
inner hair cells
- primary sensory receptors
- encode all auditory information and pass it onto nerve fibres
what are inner hair cells defined by?
the stereocilia hair bundle
what are within the stereocilia
mechanosensitive ion channels - called mechanoelectrical transducer channels (MET)
- these channels join the taller stereocilia next to it
inner hair cells at rest
- 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
what keeps the perilymph and endolymph separate?
a tight barrier
inner hair cells in the two solutions
hair bundle is in endolymph and cell body is surrounded by perilymph
inner hair cells with excitatory stimulation
- 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
depolarisation of inner hair cells potassium channels
activates potassium channels
- potassium ions exit down concentration gradient into perilymph to help repolarise cell
inner hair cells with inhibitory stimulation
- 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)
Inner hair cells with sustained stimulation
- 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
outer hair cells
- function as the cochlear amplifier
- they shorten and lengthen in time with sound - electromotility
- they do not have many afferents
function of Prestin
- in the outer hair cell membrane
- allows the hair cell to shorten or elongate in response to changes in membrane potential
outer hair cell stimulation
- outer hair cells stimulation works the same for inner hair cells
- at rest there is a resting MET current
outer hair cell resting potential
around 40mV
what causes outer hair cells to shorten
when they depolarise
the combined movement of 3 rows of outer hair cells
- 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
how outer hair cells affect movement of basilar membrane
- 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
loss/ damage of outer hair cells
- stimulation of inner hair bundle is weaker
- severe hearing loss but not complete deafness
still have basic tuning of basilar membrane
