Figure 6.

Subthreshold inputs increase EPSC threshold and cause accommodation. A, The amplitude of EPSC1, modeled in current clamp, was fixed, whereas the amplitude of EPSC2 (middle traces) was varied at each interstimulus interval point until an action potential was evoked (bottom traces). Depolarizing inhibition was simulated with long-duration square pulses in conductance clamp, with a reversal potential of −37 mV (top traces). The horizontal dashed line in the middle traces over EPSC2 indicates control EPSC threshold. B, C, EPSC spike threshold was plotted as a function of interstimulus interval after an initial subthreshold input (EPSC1) for NM neurons (n = 12; mean ± SEM). Control threshold is marked by a dashed line. Within the coincidence summation window (very small interstimulus intervals; less than ∼0.5 ms), temporal summation was close to linear and EPSC2 spike threshold was low. Beyond the coincidence summation window EPSC2 spike threshold increased dramatically before returning to control levels after ∼3 ms. Depolarizing inhibition enhanced this effect (open symbols). D, E, Voltage threshold accommodation was strong and followed the same function as ESPC2 spike threshold after a subthreshold input. Depolarizing inhibition also enhanced this effect.