Hair cell mechanotransduction is based on a finely tuned machinery residing in the hair bundle, the hair cell's receptive organelle. The machinery consists of a transduction channel, an adaptation motor, the tip link, and many other components that reside in the stereocilia. The transduction channel is connected to and opened by a gating spring, for which there are several molecular candidates. The interplay among the motor, the spring, the channel, and the tip link assures that the channel is always working at its most sensitive point of this machine, allowing very fast responses to a force stimulus. This chapter addresses the mechanisms and molecular components underlying mechanotransduction, adaptation, and motility in the hair bundle. Bundle deflection in the excitatory direction is thought to increase tension in the tip link, which leads to an opening of the mechanoelectrical transduction channel, located close to the insertion site of the tip link.
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