Generator potentials and spike initiation in auditory fibers of goldfish.

Responses were recorded extra- or intracellularly from single, small afferent fibers of the goldfish saccule, that is, S2 fibers, to clarify how the spontaneous and sound-evoked firings in these fibers are initiated from generator potentials. Spontaneously active units randomly selected from the saccular nerve of five goldfish (total 78 units) were classified by the coefficient of variation (CV) of interspike intervals into irregular (59 units; CV greater than 0.3), intermediate (10 units), and regular types (9 units; CV less than 0.23). The irregular type showed a burst (38 units) or random (21 units) pattern of firing in spontaneous activity. In cases where a clear generator potential could be recorded in response to each sound wave, spontaneous generator potentials could also be observed in the absence of stimulus sound. These spontaneous potentials were irregular in amplitude and time course, and often contained components much slower than the sound-evoked generator potentials. The sound-evoked generator potentials in S2 fibers were produced with a delay of 0.4-0.85 ms following each sound wave, and had a time course comparable to the sound-evoked excitatory postsynaptic potentials (EPSPs) of large S1 fibers. They also shared other properties such as adaptive decline in amplitude, incremental and decremental responses, and off-suppression. The mean amplitude of the sound-evoked generator potentials in S2 fibers was linearly related to the rate of afferent firing. There was no apparent difference in the action potential threshold amplitude for spontaneous and sound-evoked generator potentials. It may be concluded that the generator potentials that underlie the spontaneous and sound-evoked firing of S2 fibers are produced by the release of transmitter by hair cells.