Receptor potentials of lizard cochlear hair cells with free-standing stereocilia in response to tones.

Intracellular potentials were recorded with micropipettes from hair cells with free-standing stereocilia in the cochleae of anaesthetized alligator lizards. Wave forms of intracellular responses to click stimuli were classified into three types: hair cells, supporting cells, and untuned cells. We studied primarily the responses of hair cells to tonal stimuli. For most frequencies, f, and levels, P, of tone-burst stimuli, the response envelope of the receptor potential increases monotonically at the tone-burst onset, and decreases monotonically at tone-burst offset. Overshoot in the envelope of the response at the onset and offset of tone bursts is observed only for tone bursts of low f, high P, and short (approximately equal to 1 msec) rise/fall time. The steady-state response to tones consists of a positive (depolarizing) d.c. component, V0, plus a.c. components (e.g. a fundamental component, V1, second harmonic, V2, and third harmonic, V3). The magnitudes of a.c. and d.c. components are functions of f and P, and show three characteristics: frequency selectivity, non-linearity, and low-pass filtering. The receptor potential is frequency selective. The frequency selectivity of V0 and V1 components was measured by means of iso-voltage (iso-V0 and iso-V1) contours. Iso-V0 and iso-V1 contours are V-shaped: the maximum sensitivity occurs at a characteristic frequency (c.f.). The shapes of these contours near the c.f. depend on the values of V0 and V1 at which the contours were measured and are sharper for lower values of V0 and V1. The mean slopes of the low- and high-frequency sides of these contours are: -45.0 and +85.1 dB/decade for iso-V0 contours (n = 26), and -33.6 and +103.8 dB/decade for iso-V1 contours (n = 28). The receptor potential has non-linear properties. The magnitudes and phase angles of V0, V1, V2, and V3 receptor-potential components were measured as a function of P for different f. The slopes of level functions (the dependence of log V0 and log magnitude of V1 on log P) were measured at low levels for different f. For values of f differing from c.f. by more than a half-octave, the slope for V0 is between 1 and 2 with a mean of 1.3; the slope for V1 is about 1, i.e. magnitude of V1 increases approximately linearly with P. For frequencies near c.f., the slopes for V0 and V1 are approximately 0.8 and 0.5, respectively, indicating the presence of a compressive non-linearity.(ABSTRACT TRUNCATED AT 400 WORDS)