Hair cells are the sensory receptor cells of hearing and balance. They are as important for the senses of the inner ear as chemoreceptors are for taste and smell; as photoreceptors are for vision; and as the mechanoreceptors of the skin, muscles and joints are for touch and body sense. Hair cells are specialized mechanoreceptors that convert the mechanical stimuli associated with hearing and balance into neural information for transmission to the brain. The conversion of one type of energy to another is called transduction and hair cells are mechanotransducers.
Sensory hair cells are not in any way related to body hair. Their name derives from the fact that they have a collection of around 100 thin cellular projections at one end of the cell. These projections (stereocilia) are similar to smaller projections (microvilli) associated with the surface of cells that line the gut or upper respiratory pathways. Individual stereocilia are long and packed with a stiff filament-like molecule called "actin". The actin serves as a type of skeleton (called a cytoskeleton because it is inside the cell). A stereocilia bundle is located at the "apical" end of the cell so that the cells appear to have hair in photomicrographs, hence the name hair cell. The hair style varies between the different hearing and balance organs. Hearing hair cells have something like a 50s crew cut while some balance hair cells possess a style that borders on punk rock. The different hair styles reflect differences in function between the hair cell organs.
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