Stereocilia consist of actin filaments within a tubular membrane. Because the filaments are crossbridged, each stereocilium behaves like a stiff rod which pivots at its base. The adequate stimulus for the hair cell is displacement of the hair bundle. The hair cell is a mechanoreceptor, producing an electrical signal, or receptor potential, in response to mechanical stimulation of its hair bundle. It is the relative motion between the hair cell and the auxiliary structure specific to the receptor organ that provides the displacement.
Hair cells are extraordinarily sensitive to cilia displacement.
At threshold detection levels in humans, the auditory or the vestibular systems are operating at the same order of magnitude of displacement as that of thermal motion. Although it has been possible to learn a considerable amount about the operation of hair cells in the mammalian inner ear, a more nearly complete picture of the transduction process has emerged from in vitro studies of hair cells from lower vertebrates. Under these conditions it is possible to insert a glass microelectrode into a hair cell and thereby record the intracellular electrical events when the hair bundle is deflected. In a quiescent cell, the resting membrane potential is around 60 mv. As in the case in neurons, the electrical signals in the hair cell originate from the flow of ionic currents across the membrane through specific pores, or ion channels.