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Neurobiology of Hearing Salamanca, 20th May 2015
The cochlea: mechanics and transduction Jonathan Ashmore Neuroscience, Physiology and Pharmacology University College London
Not all tuning mechanisms use the same cellular design
BM Hair
cell frogs
reptiles
BM Hair birds
cell
Sound Soundinput
Sound Soundinput
BM Hair
cell mammals
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In non-mammals, hair cells are electrically tuned: Neural tuning = intrinsic hair cell tuning
Found in frog hair cells (Hudspeth et al); Turtle hair cells (Fettiplace et al.)
In mammals, neural tuning = basilar membrane tuning
Post-mortem
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The mammalian cochlea is a mechanical spectrum analyser
B A
Stapes Basilar membrane
Fluid Fluid in motion at Round rest window
BM stiffness high low
Does the spiral contribute to cochlear performance? Only in a minor way.
high frequencies
low frequencies
Sound enters from middle ear From Manoussaki et al., Phys Rev Letts E 2006
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Stapes Basilar membrane
Fluid Fluid in motion at Round rest window
d 2 x dx Simple m i h i k x f (t) , oscillator i dt 2 i dt i i i
N 2 j i d xi dxi dxi dxi1 dxi1 Gi mi j 2 hi Ui (yi ) si 2 ki x Gi aS (t) j1 dt dt dt dt dt
x = BM displacement ‘undamping’ y = tectorial membrane displacement
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What in silico models of the cochlea do: Inform intuition! Natural framework for functional genomics
They indicate that there may be : Cochlear gradients of the mechanotransduction channel Cochlear gradients of K channel densities A small degree of longitudinal coupling along the cochlea
From Robles & Ruggero, 2001
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Raw data from interferometry
BM velocity
normalised velocity (gain)
i.e. At low sound levels, there is amplification. The system is non-linear.
The original data: Sellick et al (1983)
40 dB gain
Methods: used Mossbauer effect , radioactive Co 57 chip on BM better: laser interferometry with reflectance from BM
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The mechanics of the basilar membrane
From mechanics to neural signal
tectorial membrane
basilar membrane
nanometres!
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Hair Cells
Inner hair cells (IHCs): one row, ~3500 total
Outer hair cells (OHCs): 3-5 rows, ~12000 total
A cochlear amplifier – depends on outer hair cells
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Sound
transduction in stereocilia
IHC OHC
glutamate ACh
Auditory nerve fibres build audiograms
Tone level for extra action potential dB SPLpotential action dB for extra level Tone
FrequencykHz in kHz
From guinea pig, Evans 1972
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Each auditory nerve fibre responds best to a single frequency
Auditory nerve fibres are sharply tuned and have varying response thresholds.
Nerve fibers also show phase-locking (temporal coding) up to about 3000 Hz.
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