TRANSDUCTION IN VESTIBULAR SENSE ORGANS

Alan M. Brichta, School of Biomedical Sciences, Faculty of Health, University of Newcastle, Callaghan, N.S.W.

Head movements stimulate mechanoreceptors in mammalian peripheral vestibular organs. Because of the head's large inertial mass, the frequency sensitivity of the peripheral vestibular organs was thought to range from zero to below 5 Hz, the measured upper frequency range of normal head movements. Consequently, elements that collectively form peripheral vestibular organs, (cupula and otolithic membrane, hair cells, and primary afferents) were rarely tested for their capacity to follow frequencies greater than 5 Hz. Accumulating evidence however, from whole animal experiments and studies of hair cells in isolated vestibular epithelial preparations strongly suggest that peripheral organs are capable of processing stimulus frequencies much higher than 5 Hz. Little is known about the mechanical properties of elements involved in the early stages of vestibular transduction, for example, how is the cupula or otolithic membrane tethered to hair cells and how does this ultimately influence frequency response. Nevertheless, it appears that these properties are not impediments to vestibular transduction as recent experiments in rhesus monkeys have shown that otolithic organs can provide input to ocular reflexes at linear acceleration frequencies > 20 Hz. These results suggest that subsequent elements in the transduction process are also capable of transducing these higher frequencies, including vestibular hair cells. Moreover, measurements of transduction currents in mouse utricular hair cells show an adaptation response to sustained displacement steps that is fast enough to contribute to an oscillation with a characteristic frequency close to 40 Hz. In addition, whole-cell recordings of voltage-gated conductances associated with the basolateral membrane of vestibular hair cells suggest that some are tuned to frequencies above 10 Hz. Together these recent data suggest that models of peripheral vestibular transduction must now recognise that various elements in the transduction cascade are able to follow higher frequencies than previously thought.