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 (IMET, green arrow). The basolateral
membrane of the hair cell comprises several types of ion channels,
associated with specific ionic currents. Shown are DRK K+
(IDRK), inwardly rectifier
(IK1), K+/Na2+ h-type current
(Ih), Ca2+ (ICa), and
Ca2+-activated K+ BK currents (consisting of the
steady IBKS and transient IBKT
currents). Red arrows indicate the directions of ionic currents.