Hair cells are the sensory cells of the auditory system in all vertebrates. In mammals, the hair cells are located within the cochlea's organ of Corti. They derive their name from the tufts of stereocilia that protrude from the apical surface of the cell, a structure known as the hair bundle. Mammalian hair cells come in two functionally distinct types -- the outer and inner hair cells. Modern research has shown that outer hair cells are motile and responsible for stimulus compression and amplification, whereas the inner hair cells are responsible for transforming the input sound stimulus into an electrical signal that is relayed to the brainstem and auditory cortex via the auditory nerve. Although the hair cell itself does not fire an action potential, it does form synapses with auditory nerve fibers that do.

The deflection of the hair-cell stereocilia opens mechanically gated ion channels that allow any small positively charged ions (primarily potassium and calcium) to enter the cell. The influx of positive ions makes the receptor potential of the cell more positive, which has specific effects depending on the exact type of hair cell.

Sensory Function of Hair Cells

In mammalian inner hair cells, the receptor potential triggers the release of neurotransmitter, often glutamate, at the basal end of the cell, via the opening of voltage gated calcium channels. The neurotransmitter diffuses across the synapse and binds to a receptor located on one of numerous auditory nerve fibers that contact each inner hair cell. In this way, the mechanical sound is converted into an electrical signal.

Amplification by Hair Cells

In mammalian outer hair cells, the receptor potential drives voltage-dependent changes in hair-cell length, a process termed somatic electromotility. Calcium flowing into the cell also actively moves the hair bundle, a process known as active hair-bundle motility. Both of these processes thus convert electrical signals back into mechanical movement, forming an electromechanical feedback loop that amplifies the ear's response to sound.

These motor and sensory functions of hair cells are not limited to mammalian outer and inner hair cells, respectively, but instead are common to all vertebrates, except for somatic electromotility, which is unique to mammals. The molecular biology of hair cells has seen considerable progress in recent years, with the identification of the motor protein underlying somatic electromotility (prestin) and a candidate for the mechanoelectrical transduction channel itself (TRPA1).