Microscopic observation of cochlear hair cell motility

Inner and outer hair cells were dissociated from mammalian organ of Corti and maintained using cell culture techniques. Micro-video recording was used to study motile responses in outer hair cells that were evoked by intra- and extracellularly injected electric currents and by iontophoretic application of acetylcholine. When intracellular current injections were made in the synaptic end of cells, the cells became shorter in length and wider in diameter in response to depolarizing currents and longer and narrower in response to hyperpolarizing currents. Transcellular stimulation with alternating currents could result in either length increase or decrease and in some cells the direction of the evoked response depended on the stimulus frequency. Iontophoretic application of acetylcholine to the synaptic end of outer hair cells also evoked shape changes. Inner hair cells and other cells from the organ of Corti were non-motile. The evoked motile response of outer hair cells may produce a portion of the mechanical energy required for the sharp tuning and low threshold of mammalian hearing and suggests that outer hair cells have an effector as well as a sensory function.