Since the amplitude, and hence the mechanical energy, of airborne sounds is tiny, the cochlea mechanically amplifies the incoming vibrations. The motors which supply this mechanical amplification are the outer hair cells. Like inner hair cells, they use stretch receptors associated with the stereocilia at their tips to sense vibrations and convert them to electrical currents. But only in outer hair cells are these currents used to control length changes which parallel, and reinforce, the incoming mechanical vibration. The video below, which was recorded in the laboratory of Prof. Jonathan Ashmore, shows an isolated guinea pig outer hair cell to which a whole cell patch electrode has been attached. Through the pipette, an alternating current signal is injected, and the resulting motor response is observed under a microscope. The alternating current signal is also played to a loudspeaker, so we can hear the signal that the outer hair cell receives.

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