Mechanism of transduction

Middle ear

Sound waves funneled into the ear cannel by the pinna vibrate the eardrum, which in turn vibrates the three tiny bones in the middle ear. Sound pressure is transferred from the eardrum to the 15-30 times smaller oval window. By increasing the force and decreasing the surface area, amplication hence occurs, allowing adequate pressure for movement of cochlear fluid to enable sound detection.

Inner ear

How does the basilar membrane respond to the sound?

After amplication in the middle ear, perilymph is pushed within the cochlea from base at the oval window toward the apex, then back to the base at the round window below through a small hole called helicotrema. The fluid movements consequently deflect the basilar membrane. Because the membrane is wider at the apex and stiffer at the base, high-frequency sounds only deflect the base, but low frequencies travel further down to displace the apex. This forms the basis of topographical mapping of frequency. 

How are the neural signals generated?

Displacement of the basilar membrane causes the stereocilia of hair cells inserted into the tectorial membrane to bend. Depending on the bending direction, ion channels in the hair cells, identified as TRPA1 channels, are either stretched open or close. Opening and closure of the transduction channels regulate the potassium conductance in hair cells. The hair cells hyperpolarize if the channels are closed and depolarize if the channels are open due to potassium influx. This mechanism relies on fine filamentous structure knows as tip links, run in parallel to the plane of bilateral symmetry, connecting the tips of adjacent stereocilia. The tip links provide the means for rapidly translating hair bundle movement into a receptor potential.

When the hair bundle is deflected toward the tallest sterocilium, cation selective channels open near the tips of the sterocilia. Opening of the channels causes an influx of potassium ions, hence a rapid depolarization of the entire hair cell. The depolarization in turn leads to calcium influx through voltage-gated calcium channels at the base of the hair cell. The calcium triggers the release of neurotransmitter from synaptic vesicles also located at the base of the hair cell. This stimulates the afferent nerve fibres and the signal thus passes along to the brain.

Depolarisation of a hair cell. Image courtesy of openlearn.open.ac.uk/mod/resource/view.php?id=263162  under a Creative Commons license.