Active Mechano-Sensitivity by Hair Cells in the Inner Ear

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Video 1: Spontaneaous hair-bundle oscillations. Two hair bundles from the inner ear (saccule) of the bullfrog are imaged from the top under the objective of an optical microscope. The movie is in real time.

Video 2: Hair-bundle compliance and friction from gating of mechanoelectrical transduction channels in the hair bundle. Force is applied to a hair bundle by moving the base of an attached flexible fiber. A back-and-forth triangular motion of the fiber results in a force-position relation that displays hysteresis as the result of energy dissipation. There is a range of positions where the slope of the relation is lower and the height of the cycle is larger, corresponding to lower  stiffness and higher friction of the hair bundle, respectively. Upon application of a channel blocker (the aminoglycoside gentamicin), the two long branches of the cycle become linear and the hysteretic cycle shrinks. These observations suggest in return that gating of the channels reduces the stiffness and increases the friction of the hair bundle. This effect is  interpreted as an inherent consequence of hair-bundle mechanosensitivity from direct mechanical activation of ion channels by force. See Bormuth et al (2014) PNAS 111:7185.