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The ability to orient with respect to gravity, to detect motion, and to hear all depend on specialized mechanoreceptors called hair cells that sense movement and vibration. They are found in fishes and amphibians, in which they detect movement of the surrounding water, and in many invertebrates, in which they sense gravity and other forces acting on the animal. They are also found in the ears of terrestrial vertebrates, in which they sense sound, body orientation, and motion.

In all these cases, hair cells sense mechanical vibrations. These vibrations move small, nonmotile, hairlike projections from the surface of the hair cell called stereocilia. In turn, the motion of stereocilia causes a depolarization of the cell’s membrane by opening or closing ion channels. Despite their name, stereocilia are more similar to microvilli than to cilia since cilia can move on their own but stereocilia cannot. Hair cells themselves do not fire action potentials but instead, when depolarized, release neurotransmitters that alter the firing rate of adjacent neurons.

Unlike chemoreceptors, hair cells are generally similar in structure; there is not a specific hair cell for a particular pitch of sound or body orientation. Thus, for hair cells to provide information about mechanical stimuli, they must interact with other structures within the sensory organ in which they reside. These more elaborate structures within the sensory organ influence how individual hair cells respond to physical motion, influencing the information they provide.