Hearing

Intracellular current administration evokes rapid, graded, and bidirectional mechanical responses of isolated outer hair cells from the mammalian inner ear. The cells become shorter in response to depolarizing and longer in response to hyperpolarizing currents in the synaptic end of the cell. The cells respond with either an increase or decrease in length to transcellular alternating current stimulation. The direction of the movement with transcellular stimuli appears to be frequency dependent. Iontophoretic application of acetylcholine to the synaptic end of the cell decreases its length. The microarchitecture of the organ of Corti permits length changes of outer hair cells in a manner that could significantly influence the mechanics of the cochlear partition and thereby contribute to the exquisite sensitivity of mammalian hearing.

Hair cells of the guinea pig cochlea were preserved for electron microscopic examination by fixing in glutaraldehyde without the use of osmium. An extensive array of cross-links was seen between the stereocilia, by both scanning and transmission electron microscopy. The stereocilia were linked together laterally, particularly near their apical ends, by links running approximately at right angles to the long axis of the stereocilia. One set joined stereocilia of the same row, and another set joined stereocilia of the different rows, holding the tips of the shorter stereocilia in towards the longer stereocilia of the next row. In addition, the tip of each shorter stereocilium on the hair cell gave rise to a single, upwards-pointing link, which ran up to join the taller stereocilium of the next row. We suggest that distortion of this link would give rise to sensory transduction. On this basis, we are able to explain the V shape of the rows of stereocilia on outer hair cells. Within the rows, the three-dimensional arrangement of the stereocilia was different from that seen conventionally. Rather than standing parallel, the stereocilia of the different rows tapered in together at the tips, presumably held by the laterally-running cross-links. In addition, a membrane roughness, particularly pronounced in the region of the stereocilium which gives rise to the cross-links, was seen. However, the lateral and basal surface membranes of the hair cell, and the membranes of the internal organelles, had a more conventional appearance.

A new auditory phenomenon has been identified in the acoustic impulse response of the human ear. Using a signal averaging technique, a study has been made of the response of the closed external acoustic meatus to acoustic impulses near to the threshold of audibility. Particular attention has been paid to the waveform of the response at post excitation times in excess of 5 ms. No previous worker appears to have extended observations into this region. The response observed after about 5 ms is not a simple extension of the initial response attributable to the middle ear. The oscillatory response decay time constant was found to change from approximately 1 ms to over 12 ms at about this time. The slowly decaying response component was present in all normal ears tested, but was not present in ears with cochlear deafness. This component of the response appears to have its origin in some nonlinear mechanism probably located in the cochlea, responding mechanically to auditory stimulation, and dependent upon the normal functioning of the cochlea transduction process. A cochlear reflection hypothesis received some support from these results.