The defining feature of hair cells is the hair bundle (left), the organelle of sensory transduction formed by ordered arrays of stereocilia. Hair cells sense bundle deflection producing a receptor potential, i.e. a time dependent modulation of their membrane potential. This is caused by the opening of ion channels located on the stereocilia and sensitive to mechanical deformation (mechano-sensitive channels). It is currently believed that mechano-sensitive channels are attached to one or possibly both ends of thin filaments, called tip-links (right), which
connect stereocilia of adjacent rows. Bundle deflection stretches the tip-links, which in turn increases the channel open probability. The modulation of the cell membrane potential is caused by the influx of ion currents, carried mostly by potassium ions, through open mechano-sensitive channels.
The driving force for this transduction current is provided by the large potential difference across the apical membrane of the hair cell. This is due to the stereocilia projecting into endolymph, a fluid that fills the space between the top surface of the organ of Corti and the Reissner's membrane (see Anatomy section). The endolymph is a potassium-rich extracellular fluid whose standing potential is about 80 mV more positive than perilymph, a fluid similar, and continuous with, cerebrospinal fluid that fills the spiral canal and surrounds the basolateral membrane of the hair cells. The intracellular potential of inner hair cells, referred to perilymph, is about -45 mV, whereas that of the outer hair cells is about -70 mV. Thus, the driving force for current is a potential difference comprised between 125 mV and 150 mV.
Inner hair cells (IHCs) are contacted mostly by afferent nerve fibers that leave the organ of Corti to reach the cochlear nuclei in the brainstem. IHCs are thought to be the main sensory receptors of the cochlea that signal the vibration of the organ of Corti to the
brain. The animation at left visualizes the activation of an IHC by the fluid viscous drag applied to its stereocilia by the oscillation of the tectorial membrane. Outer hair cells are the target of abundant efferent innervation and possess a unique type of motility. They convert receptor potentials into cell length changes at acoustic frequencies (see Physiology section). The animation at right visualizes the activation of the outer hair cell motor driven by the motion of the tectorial membrane into which the tips of the tallest stereocilia are inserted. A second class of sensory receptors, the outer hair cells couple visco-elastically the reticular lamina to the basilar membrane (see Anatomy Section) through their supporting Deiters' cells (see Capacitance/Viscosity in Index).
See what happens to your stereocilia while you happily dance in a discotheque! (see also Gale et al. 2004).
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