Hair Cells

The sensory cells of hearing, about 15,000 in each human cochlea

Overview

Hair cells are the sensory receptors of the cochlea, named for the bundles of stereocilia on their upper surface. Each human cochlea contains roughly 15,000 of them, arranged in one row of about 3,500 inner hair cells and three rows of about 12,000 outer hair cells. Inner hair cells are the true sensors: they convert vibration into neurotransmitter release, and about 90 to 95 percent of auditory nerve fibres connect to them. Outer hair cells act as the cochlear amplifier, changing length in response to sound through the motor protein prestin, which increases sensitivity and sharpens frequency tuning and gives rise to otoacoustic emissions. Deflection of the stereocilia opens mechanotransduction channels linked by tip links. Each hair cell responds best to a characteristic frequency set by its position, with high frequencies at the base and low frequencies at the apex. Mammalian cochlear hair cells do not regenerate, which is why their loss causes permanent hearing loss, whereas birds and fish can regenerate them, a difference that drives research into regenerative therapies.