Hair cell

Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. In mammals, the auditory hair cells are located within the spiral organ of Corti on the thin basilar membrane in the cochlea of the inner ear. They derive their name from the tufts of stereocilia called hair bundles that protrude from the apical surface of the cell into the fluid-filled cochlear duct. The stereocilia number from fifty to a hundred in each cell while being tightly packed together and decrease in size the further away they are located from the kinocilium. The vertices of all hair bundles point away from the center of the cochlea because bundle deflection only in the direction of the longest stereocilia leads to increased possibility of mechanotransduction. Hair cells are organized tonotopically. Hair cells at the base of the cochlea respond best to high-frequency sounds, while those at the apex respond best to low-frequency sounds. The cilia are taller in the apex and shorter in the base, maintaining an orderly gradation along the cochlea's length. The physical and functional properties of these structures vary systematically along the tonotopic axis of the auditory system. For example, hair bundles might be longer or shorter, or synapses might be more or fewer or have different properties, depending on whether they are associated with high-frequency or low-frequency regions. Mammalian cochlear hair cells are of two anatomically and functionally distinct types, known as outer and inner hair cells. Damage to these hair cells results in decreased hearing sensitivity, and because the inner ear hair cells cannot regenerate, this damage is permanent. Damage to hair cells can cause damage to the vestibular system...