More whistles and bells for fly hearing

Amplification of sensory input is a common theme for almost all sensory modalities. Once a sensory signal is received, be it a photon or a molecule of saccharin, the receptor cell will append an amplifying second-messenger cascade onto the initial transduction event. Amplification of sound by hair cells within the cochlea also follows this rule, but with an important caveat: the hair cells produce no second-messenger cascade for the amplification (1); instead, they pump kinetic energy back into the hearing organ to magnify the sound (2).

How this energy is put back into the system is somewhat controversial. Considerable evidence indicates that the outer rows of hair cells contract and expand their somas in response to plasma membrane voltage changes, thereby contributing mechanical energy to the already vibrating basilar membrane on which they reside (3–5). But there is also mounting evidence that another portion of the hair cell is not passively along for the ride on a bucking basilar membrane. The organelle that receives the incoming stimulus, the hair bundle, also seems to play an active role. Vestibular hair bundles can amplify mechanical stimuli (6) by harnessing the energy released during opening and closing of the mechanically activated transduction channel (7). It remains to be seen whether the hair bundles in the cochlea also contribute to the motions of the basilar membrane, but it is clear that these organelles can serve as a source of amplification. Hearing organs appear to be unique among sensory organs in their ability to generate signals that they were meant to detect. In a surprising report in …