Hair bundles: nano-mechanosensors in the inner ear

Abstract

Hair cells of the inner ear detect mechanical stimuli by deflections of the hairbundle, which open tension-gated transduction channels in the cell membraneto admit cations from the surrounding fluid. Recent experiments haveshown that the hair bundle has an active response and is not just a passiveelastic structure. Indeed, spontaneous oscillations of the bundle have beenobserved in the absence of a stimulus. We have proposed the generalconcept of ‘self-tuned criticality’ to explain why such oscillations occur,and how they help the ear to hear. According to this idea, when workingnormally each hair cell is maintained at the threshold of an oscillatoryinstability. Poised on the verge of vibrating at a characteristic frequency, a hairbundle is especially responsive to weak periodic stimuli at that frequency.The cellular basis of the activity and the tuning mechanism remain to beestablished. We have put forward two alternative models. In the first,oscillations are generated by molecular motors within the hair bundleand self-regulation is accomplished by a feedback involving calcium ionswhich enter through the transduction channels. In the second, oscillationsare generated by the interaction of calcium ions with the transductionchannels, and tuning to the critical point is accomplished by a set ofmyosin-1C motors which are attached to the channels. In both cases, thecombination of motor protein activity and calcium dynamics providesan active amplifier which enables the ear to detect faint sounds.