3.7.1 Summary of Sections 3.4 to 3.7
Hair cells are found in the organ of Corti and run the length of the basilar membrane. They transform mechanical energy into neural signals.
When the basilar membrane vibrates in response to sound, hair cells located at the site of maximal vibration on the basilar membrane are stimulated. This means that the mechanical properties of the membrane allow the auditory system to distinguish one frequency from another by the location on the membrane that is maximally excited by a particular frequency. Hair cells located at the place of maximum excitation respond, allowing the auditory system to extract information about the frequency of a sound.
Auditory transduction occurs when the basilar membrane moves up and down and the cilia of the inner hair cells rub against the tectorial membrane. The bending of the cilia produces an electrical response in the hair cells. Most of the transduction current is carried by potassium ions, potassium being the cation with the highest concentration in the endolymph bathing the hair bundle.
Displacement of the stereocilia towards the tall edge results in an influx of cations and a depolarisation of the hair cell. Displacement of stereocilia towards the short edge results in hyperpolarisation of the hair cell. Depolarisation of the hair cell allows calcium ions to enter the cell leading to the release of transmitter from the presynaptic terminals on the hair cell.
Outer hair cells help amplify vibrations of the basilar membrane.