Figure 7.

A refractory period follows subthreshold excitatory inputs. A, EPSG spike threshold was measured in the multicompartment model as a function of interstimulus interval, as in Figure 6 B. The initial input was set to 90% of threshold. Direct comparisons with real EPSG amplitudes at different input frequencies (dashed horizontal lines) show that threshold often rises well above physiological EPSG amplitudes when eighth nerve rate is elevated. B, Depolarizing inhibition dramatically increased EPSG spike threshold in the multicompartment model. During a time period after the coincidence summation window, small inhibitory conductances raised spike generation threshold well above physiological EPSG amplitudes. During larger inhibitory conductances, spikes could not be evoked (between upward arrows), indicating an absolute refractory period despite the fact that no action potential was fired.