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Audition - Coggle Diagram
Audition
Signal transduction
OHC and IHC differ in spiral ganglion innervation
- IHCs receive 95% afferent auditory nerve fibres - myelinated
- 5% of (unmyelinated) fibres innervate 20 OHCs each --> convergence/grouping into receptive fields
--> low threshold stimulation
Each IHC innervated by 10 nerve fibres
- reduced sensitivity as stereocilia dont touch tectorial membrane
- more discrete than OHC - acuity to small f differences
- respond more to velocity of basilar membrane rather than displacement --> adaptation
Efferent innervation arrives from superior olive via cholinergic and divergent fibres
- presynaptic termination on IHCs
- more extensive and direct onto OHCs -> direct function contrl
EE: Bekesy, 1930s - Human cadaver measurements
- sound causes travelling wave of displacement along basilar membrane from base --> apex
- high f: peak = near cochlea base
- lower f: peak shifts towards apex
- increase in width and decrease in thickness
OHC electromotility amplifies their sensitivity and sharpens frequency tuning
- basilar membrane response to weak sounds is amplified --> lowering the threshold for activation
- Mossbauer technique(gamma ray measurements to qunatify vibrations) --> peak v sharp
Transmembrane p.d drives oscillations in cell length
- VG conformational changes in the motor protein
- 70,000 per second - actin-myosin couldn't achieve this
EE: Liberman et al, 2012 - Prestin KO mice
- OHCs isolated from 5/7 wk old prestin KO and control
- voltage clamp records changes:
decrease in length of control cells when depolarised
no change in motility of KO cells --> loss of amplification and otoacoustic emissions
Otoacoustic emissions = sounds generated within inner ear
- dervied from OHC motility
- spontaneous or evoked by external stimulation
- used for screening hearing loss in newborns
CC: Tinnitus - derived from OHC motility?
- imaginary sounds heard as continous high pitched whistling noises
- feedback too large; unwanted oscillation
- signifies importance of olivocochlear bundle in modulating
Encoding of different sound wave properties
- broadening of tuning curves and saturation of responses with increasing intensity --> resolution becomes poorer above threshold
Tonotopicity = point of innervation along basilar membrane; topographic variation within nerve
- perceived pitch dependent on which fibres active
-timbre encoded by relative activity of different fibres
Increases in intensity increase firing rate up to 30-50dB above fibre threshold
- ganglion cells sensitivity to glutamate varies --> fire at different intensities
Two tone suppression: 2nd frequency supresses excitatory tone
- due to cochlear amplification and basilar membrane movement
- lateral inhibition: sharpens selectivity
Phase locking enables encoding of low f by timing of action potentials
- increased probability of firing during certain stages of cycle
- dont fire at every cycle but firing triggers at same interval
- when combined a greater frequency encoded
Auditory structure
Outer ear
Functions:
- protect tympanic membrane(lies in ext. auditory canal, highly sensitive movement)
- pinna catches sounds waves and deflects into ext. auditory meatus
Pinna
- boosts certain sounds by concha and ear canal
- localisation role
Middle ear
Muscles: stapedius and tensor tympani
- increase stiffness of bones upon contraction --> dampens transmission
- contraction activates by intense low f sounds e.g. own speech, enabling focus on ext. sounds
Air filled cavity
- connects to nasopharynx via Eustachian tube
- lower tube closed normally: opening leads to equilibration of pressure across tympanic membrane
- 3 bones in cavity: malleus, incus, stapes
Stapes couples movement of eardrum to cochlea membrane
- cochlea = fluid filled; force to move tympanic membrane increases
- tympanic 30x > stapes footplate; concentrates force
- lever system: decreasing lever arm(displacement) bone to bone; force amplified
Inner ear
Sound arrival for analysis and transduction
- pass through outer ear across tympanic membrane: amplified through stapes, malleus and incus into the oval window
- round window vibrates in opposite manner to oval window; allowing cochlea fluid movement
Encased with bone and cochlea coiled
- outer chambers(V and T) filled with perilymph
- inner(M) filled with endolymph
- V and M separated by basilar membrane - organ of Corti located
Organ of corti contains hair cells, supporting cells and nerves
- hair cells: receptors attached to rigid reticular membrane
- stiff apical stereocilia vary in height and tip links
- three rows of OHCs within sterocilia embedded on tectorial membrane
- single layer of IHCs in stereocilia just short of tectorial membrane
Endolymph establishes endocochlear potental
- perilymph similar conc to CSF: 7K+ 140Na+
- endolymph similar to intra: 150K+, 1Na+
- stria vascularis filters endolymph; transporting K+ into scala media
- high conc gradient due to tight junctions
- +80mV potential (80 in endolymph, 0 in perilymph) across basilar membrane
- +125mV(intra = -45) across cell membrane
Stria vascularis = multi layered
- main site of energy usage; highly vascularised
- basal cells linked tightly by Claudin 14 and 9 preventing K+ backflow
- EEL: Claudin 14 mutation ==> deafness
- electrical potential: reduce energy usage and reduces vasculature nearby -> pulsatile movement doesn't dirsupt
Transmission
- Stapes footplate touches oval window of scala vestibuli - lymph incompressible --> round window on scala tympani must bulge out
- Pressure gradient across scala media and endolymph --> basilar membrane vibrates
- Vibration/displacement of membrane:
- bends OHC sterocilia against tectorial membrane
- IHCs move through endolymph displacing sterocilia
- Tip links connecting cilia upon stretch open transduction channels
- composed of CDH23 and PCDH15 dimer --> Ca2+ dependently linked
- anchor proteins can move up/down cilium -> sensitivity adjustment
CC: Anchor protein mutation
- CHD23 mutation causes deafness
- KO mice = deaf
- K+ influx enters along electrochemical gradient
- endolymph + 80mV
- intracellular -45mV
- Cell depolarisation
- opening of VGCC leading to the release of glutatamate at the base of the hair cell
- postsynaptic afferent nerve --> excited
EE: Russell et al, 1986 - Measurement of HC sensitivity
- 1-3 day postnatal mice cochlea dissection; organ of Cortis cultured for 2-7 days
- transmembrane p.d measured - glass electrodes - stereocilia displacement monitor
- 2nm displacement by 0.8mV stimulus(most sensitive)
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Sound basics
Components of sound
Pitch is our perception of frequency
- frequency = cycle rate; pitch = high or low
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Human limits
f: 20Hz to 20,000Hz decreasing with age and exposure to noise exposure --> death of hair cells
Higher frequency loss first - sat, that
Amplitude: 0-80 dB w/o damage, 120 dB - discomfort, 130 dB - damage and pain