Hair bundles: nano-mechanosensors in the inner ear

Hair cells of the inner ear detect mechanical stimuli by deflections of the hair bundle, which open tension-gated transduction channels in the cell membrane to admit cations from the surrounding fluid. Recent experiments have shown that the hair bundle has an active response and is not just a passive elastic structure. Indeed, spontaneous oscillations of the bundle have been observed in the absence of a stimulus. We have proposed the general concept of 'self-tuned criticality' to explain why such oscillations occur, and how they help the ear to hear. According to this idea, when working normally each hair cell is maintained at the threshold of an oscillatory instability. Poised on the verge of vibrating at a characteristic frequency, a hair bundle is especially responsive to weak periodic stimuli at that frequency. The cellular basis of the activity and the tuning mechanism remain to be established. We have put forward two alternative models. In the first, oscillations are generated by molecular motors within the hair bundle and self-regulation is accomplished by a feedback involving calcium ions which enter through the transduction channels. In the second, oscillations are generated by the interaction of calcium ions with the transduction channels, and tuning to the critical point is accomplished by a set of myosin-1C motors which are attached to the channels. In both cases, the combination of motor protein activity and calcium dynamics provides an active amplifier which enables the ear to detect faint sounds.