Hair Cells

The auditory and the vestibular systems use hair cells (HCs) as their sensory receptors. HCs are neu roepithelial cells characterised by the presence of a bundle of microvilli-like structures that protrude from their apical surface, called stereocilia. The displacement of stereocilia, which is caused by acoustic stimuli in the cochlea or head movement in the vestibule, is converted into a depolarising inward current by mechanoelectrical transducer (MET) channels located at their tip. The depolarisa tion of HCs opens voltage-dependent Ca2+ channels at their basolateral synaptic active zones, which are functionally coupled to glutamate-containing vesicles at specialised ribbon synapses. There is also evidence for a nonquantal synaptic transmis sion at the vestibular HCs, likely involving direct postsynaptic depolarisation by K+ exiting the cells. In mammals, HC loss causes irreversible balance and hearing impairment because these cells do not regenerate.