Does the Apical Organ of Corti act as an Electromechanical Transistor?

The inner ear not only transduces sound-induced vibration into electrical signals, it also amplifies weak sound to boost its detection. The actuators of this active process are sensory outer hair cells in the organ of Corti, whereas the inner hair cells ensure that the sound-induced signals reach the auditory nerve. However, how the outer hair cells modulate the stimulus to the inner hair cells remains unclear. Using theoretical modeling and experimental measurements, we find that the stimulation of the inner hair cells depends on the resting length of the outer hair cells. When these are in an elongated state, sound causes minimal stimulation of inner hair cells, whereas outer hair cell contraction leads to large amplification of sound-evoked motion. This novel mechanism for regulating the sensitivity of the hearing organ is analogous to an electromechanical transistor, and could be important for our ability to listen selectively to speech-frequency sounds.