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One class of models of hair cell synaptic function that has been investigated in recent years consists of one or more reservoirs of synaptic material connected to other reservoirs and/or the synaptic cleft by means of diffusion paths. One such general model is considered here, comprising two reservoirs and a global source of synaptic material connected in series and releasing material into the synaptic cleft by a diffusion path characterized by an intensity-dependent permeability. The explicit form of the solution of the model for a sudden onset of stimulation is derived: The solution comprises two exponentially decaying terms plus a constant. This solution is shown to have a unique inverse. This allows the determination of the parameters of the model directly from experimental data from auditory-nerve fibers in the Mongolian gerbil. The behavior of the derived model parameters with variation of stimulus intensity is demonstrated, and implications for synaptic function are discussed.

The responses of single auditory-nerve fibers of Mongolian gerbil were studied using tonal stimuli. The peristimulatory adaptation of firing rate in response to tone bursts presented in quiet and during a background stimulus is described quantitatively. The total transient response which can be produced to the onset of a tone burst, whether presented in quiet or as an intensity increment, is limited and appears to demonstrate a form of conservation. Specifically, the total numbers of spikes produced by the rapidly adapting component, and the slower short-term adaptation component, are proportional at all intensities, and are limited for each fiber. Furthermore, when an incremental stimulus is presented on a background, the total transient response to the background and to the increment is limited and depends upon the final intensity, not the background intensity. When the presumed underlying synaptic drive is derived by removing the effects of refractoriness from the spike train, the same conservation of the transient response components, and proportionality between rapid and short-term components, are observed.

No abstract

In response to stimuli of constant sound intensity, auditory-nerve firing rates are maximum at onset, and then decay or adapt towards a steady-state rate. The early decay appears to be made up of two exponential components, rapid adaptation and short-term adaptation, with time constants on the order of 5 and 50 ms, respectively. A variety of data will be presented and reviewed that generally, albeit indirectly, support the conclusion that both phases of adaptation have a common, synaptic, origin. The data demonstrate the following: adaptation is not present in the hair cell response, adaptation compresses the input-output characteristic, adaptation has an asymmetrical effect on responses to changes in intensity, adaptation does not qualitatively alter response variability, adaptation is minimally affected by stimulus rise time, adaptation is not caused by neural refractoriness, and adaptation is a complex function of sound frequency. A previously described [R. L. Smith and M. L. Brachman, Biol. Cybern. 44, 107–120 (1982)] model of adaptation, based on hypothesized properties of synaptic depletion, is consistent with most of the results. [Work supported by NSF and NIH.]