The static and dynamic properties of the posterior canal afferent synapse were examined in the isolated frog labyrinth by intracellularly recording the EPSP rate at rest and during sinusoidal rotation. Experiments were performed in the presence of TTX to prevent afferent spike discharge. A statistical method measuring the variance, skewness and power spectrum of the fluctuations in membrane potential gave a reliable estimate of the amplitude, frequency of occurrence and shape of the EPSPs even if summated and fused in the bulk discharge. During rotation the change in EPSP frequency is approximately sinusoidal, indicating a close functional relationship between stimulus (acceleration) and receptor response. Regression analysis between acceleration and response reveals that the EPSP frequency during excitation and inhibition is in some units linearly related to the stimulus, while in others it is a linear function of the stimulus logarithm. The excitatory response is found to be systematically larger than its inhibitory counterpart and is also affected by adaptation. The results suggest that many properties of the spike discharge (asymmetry, adaptation, linear or nonlinear intensity functions) are of presynaptic origin, whereas rectification (transient silencing of spike rate) arises at the encoder
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