A diffusion model of the transient response of the cochlear inner hair cell synapse.

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.