Figure 1

MET and ionic currents in the hair cell. Each hair cell is equipped with a mechano-sensory hair bundle, i.e., a tuft of stereocilia that emanates from the apical surface of the cell. Stereocilia are arranged in rows of increasing height, with neighboring stereocilia being interlinked by fine filaments, the so-called tip links. The hair bundle is immersed in K⁺-rich endolymph. In contrast, the basolateral membrane of the hair cell is in contact with perilymph, which is characterized by a low K⁺ and a high Na²⁺ ion concentration. Upon deflection of the hair bundle toward the largest row of stereocilia, tension in the tip links increases. This elicits the opening of mechanically gated ion channels (MET channels) that are located near the tips of stereocilia. As a result, K⁺ ions rush into the hair cell, giving rise to an inward MET current (I_MET, green arrow). The basolateral membrane of the hair cell comprises several types of ion channels, associated with specific ionic currents. Shown are DRK K⁺ (I_DRK), inwardly rectifier (I_K1), K⁺/Na²⁺ h-type current (I_h), Ca²⁺ (I_Ca), and Ca²⁺-activated K⁺ BK currents (consisting of the steady I_BKS and transient I_BKT currents). Red arrows indicate the directions of ionic currents.