Non-linearities in the responses of turtle hair cells

1. Intracellular recordings were made from single cochlear hair cells in the isolated half-head of the turtle. Receptor potentials were recorded while the ear was stimulated with high-intensity tones in order to examine the cochlear non-linearities which shape the hair cell responses.2. The size of a hair cell's voltage response to a tone burst was reduced, abolished and then reversed by steady depolarizing currents of increasing strength. The average current needed to produce reversal was about 0.3 nA, the reversal potential being close to zero with respect to the scala tympani.3. Short current pulses injected on the peaks and dips of the receptor potential showed that the membrane resistance and time constant were decreased on the depolarizing phase of the receptor potential. These changes were not due to non-linearity in the hair cell's current-voltage curve in the absence of acoustic stimulation. The results are consistent with the idea that the transducer causes the cell to depolarize by increasing the membrane conductance to ions with an equilibrium potential close to zero.4. Saturated receptor potentials from poorly tuned cells exhibited a pronounced asymmetry, with the maximum depolarizing excursion being several times the maximum hyperpolarizing excursion. This asymmetry was not seen in sharply tuned cells. It is proposed that the asymmetry is present in the transducer conductance change and in sharply tuned cells is reduced in the receptor potential by subsequent filtering.5. For high sound pressures which produced close to a saturated response, the hair cell voltage wave form displayed a number of non-linear features dependent upon the frequency of stimulation relative to the characteristic frequency (c.f.). The most prominent feature occurred at very low frequencies where the potential exhibited damped oscillations on the depolarizations and hyperpolarizations; these ;ringing frequencies' lay above and below the c.f. of the cell respectively.6. The ;ringing frequencies' varied with the c.f. of the cell but for a given cell were largely independent of the frequency of stimulation. The ;ringing frequencies' could be changed by injecting steady currents into the cell during acoustic stimulation; depolarizing currents increased the ringing frequencies and hyperpolarizing currents decreased the frequencies.7. The hair cell's response to a continuous test tone at the c.f. of the cell could be suppressed by simultaneous addition of a second tone whose sound presure was comparable to, or greater than, the test tone. The degree of suppression varied with the intensity and frequency of the second tone, and was maximal close to the c.f. of the cell. The sound pressure required to produce a constant suppression as a function of frequency was sharply tuned, and the tuning of the suppression showed similarities to the frequency selectivity of two-tone suppression described in the auditory nerve.8. An attempt was made to reconstruct the main features of the receptor potential at high intensities.