Mechanoelectrical and Voltage-Gated Ion Channels in Mammalian Vestibular Hair Cells

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Abstract
Mammalian vestibular afferents respond robustly to head movements at low frequencies and provide input to reflexes that control eye, head and body position. Vestibular organs have distinctive regions and hair cells: Type II cells receive bouton afferent endings and type I cells receive large calyx afferent endings. In the rodent utricle, type II cells are broadly tuned to frequencies between 10 and 30 Hz. Other recent data suggest that otolith organs function in this frequency range, which is higher than previously imagined. Some of the tuning derives from adaptation of the transducer current, which is best fitted with a double exponential decay with time constants of ∼4 and 40 ms. Further tuning is provided by basolateral conductances, principally outwardly rectifying, voltage-gated K+ conductances. The kinetics of the K+ currents tend to vary with location in the sensory epithelium and therefore may contribute to regional variation in afferent physiology. Type I hair cells have a large, negatively activating K+ conductance, gK,L, that confers a very low input resistance and therefore attenuates the receptor potential. This may reduce nonlinearity in the receptor potential, a possibly useful feature for the motor reflexes served by the vestibular system. On the other hand, the small receptor potentials together with unusually negative resting potentials are hard to reconcile with calcium-mediated quantal transmission. This problem may be overcome by factors that inhibit gK,L’s activation at resting potential. Also, the calyx may support nonquantal transmission.