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How does the cochlea change a mechanical sound wave into an electrical impulse?

After the mechanical conduction of the sound stimulus from the environment to the external ear and middle ear, the transmitted sound energy from the oval window to the inner ear is transduced into an electrical by the organ of corti.

The organ of corti extends across the surface of the basilar membrane from the spiral limbus situated axil to the osseous spiral laminal (osl) to the Claudius cells that lie between the edge of sensory region and outer anchorage of the basillar membrane.

The transmitted sound pressure waves in cochlear fluid set up waves of motion along the perilymph and then the basilar membrane. The hair cells in the organ of corti detect this basilar membrane motion. The entire basilar membrane doesn’t vibrate simultaneously. Instead of a specific area along the basilar membrane move variably in response to the different frequency of sound resulting in tonotopical organization. Thus lower frequency stimulating the hair cell in the apical region while the higher frequency stimulating the hair cells in apical region where the higher frequency stimulating the hair cells towards basal area.

The motion of the basilar membrane –organ of corti complex driver the deflection of the stereocilia of the hair cells. This result in shearing force between the tectorial membrane and hair cells.

Deflection in the direction of increasing sterociliary height (Kinocilium) depolarize the outer hair cells by causing the channels to open and allow influx of K+ into the cells whilst opposing deflection hyperpolarize the hair cell by closing the channels.

The magnitude of the receptor potential produced by depolarization is increased by the high positive EP (+80ml) which together c the resting membrane potential of hair cells (around -70mv ),result in a much greater potential than normally present even in neurons of 150mv b/w endolymph and the inside of hair cells. This results in triggering the neural signal and transmitted along the auditory nerve and higher auditory pathway for central processing.

 

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