The Cochlea: Mechanics and Transduction Jonathan Ashmore Neuroscience, Physiology and Pharmacology University College London

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The Cochlea: Mechanics and Transduction Jonathan Ashmore Neuroscience, Physiology and Pharmacology University College London

21/05/2015 Neurobiology of Hearing Salamanca, 20th May 2015 The cochlea: mechanics and transduction Jonathan Ashmore Neuroscience, Physiology and Pharmacology University College London [email protected] 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 1 21/05/2015 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 2 21/05/2015 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 3 21/05/2015 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 4 21/05/2015 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 5 21/05/2015 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 6 21/05/2015 The mechanics of the basilar membrane From mechanics to neural signal tectorial membrane basilar membrane nanometres! 7 21/05/2015 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 8 21/05/2015 Sound transduction in stereocilia IHC OHC glutamate ACh Auditory nerve fibres build audiograms Tone level for extra action potential dB SPLpotential action dB forextra level Tone FrequencykHz in kHz From guinea pig, Evans 1972 9 21/05/2015 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. 10 .

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Macromolecular and Electrical Coupling Between Inner Hair Cells in the Rodent Cochlea
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