<P> In the inner ear, stereocilia are the mechanosensing organelles of hair cells, which respond to fluid motion in numerous types of animals for various functions, including hearing and balance . They are about 10--50 micrometers in length and share some similar features of microvilli . The hair cells turn the fluid pressure and other mechanical stimuli into electric stimuli via the many microvilli that make up stereocilia rods . Stereocilia exist in the auditory and vestibular systems . </P> <P> Resembling hair - like projections, the stereocilia are arranged in bundles of 30 - 300 . Within the bundles the stereocilia are often lined up in several rows of increasing height, similar to a staircase . At the core of these hair - like stereocilia are rigid cross-linked actin filaments, which can renew every 48 hours . These actin filaments face their positive ends at the tips of the stereocilia and their negative ends at the base and can be up to 120 micrometres in length . Filamentous structures, called tip links, connect the tips of stereocilia in adjacent rows in the bundles . The tip links are made up of nearly vertical fine filaments that run upward from the top end of a shorter stereocilia to its taller neighbor . Tip links are analogous to tiny springs, which, when stretched, open cation selective channels thus allowing ions to flow across the cell membrane into the hair cells . They also are involved in the force transmission across the bundle and the maintenance of the hair bundle structure . </P> <P> As acoustic sensors in mammals, stereocilia are lined up in the organ of Corti within the cochlea of the inner ear . In hearing, stereocilia transform the mechanical energy of sound waves into electrical signals for the hair cells, which ultimately leads to an excitation of the auditory nerve . Stereocilia are composed of cytoplasm with embedded bundles of cross-linked actin filaments . The actin filaments anchor to the terminal web and the top of the cell membrane and are arranged in grade of height . As sound waves propagate in the cochlea, the movement of endolymph fluid bends the stereocilia . If the direction of movement is towards the taller stereocilia, tension develops in the tip links, mechanically opening transduction channels near the tips . Cations from the endolymph flow into the cell, depolarizing the hair cell and triggering the release of neurotransmitters to nearby nerves, which send an electrical signal to the central nervous system . </P> <P> In the vestibular system, the stereocilia are located in the otolithic organs and the semicircular canals . Hair cells in the vestibular system are slightly different from those in the auditory system, in that vestibular hair cells have one tallest cilium, termed the kinocilium . Bending the stereocilia toward the kinocilium depolarizes the cell and results in increased afferent activity . Bending the stereocilia away from the kinocilium hyperpolarizes the cell and results in a decrease in afferent activity . In the semicircular canals, the hair cells are found in the crista ampullaris, and the stereocilia protrude into the ampullary cupula . Here, the stereocilia are all oriented in the same direction . In the otoliths, the hair cells are topped by small, calcium carbonate crystals called otoconia . Unlike the semicircular ducts, the kinocilia of hair cells in the otoliths are not oriented in a consistent direction . The kinocilia point toward (in the utricle) or away from (in the saccule) a middle line called the striola . </P>

Stereocillia on the organ of corti are arranged