Figure 4.
Frequency dependence, signaling regimes, and synaptic transfer functions of facilitating synapses. (A1) Average EPSPs (4 sweeps) recorded in interneuron (30 Hz). Membrane potential, −89 mV. (A2) Simulated synaptic response (30 Hz). Postsynaptic potential is computed by using a passive membrane mechanism τmem⋅dV/dt = −V + (Rin⋅Isyn(t)) with Rin of 1 GΩ and τmem = 60 ms (V = voltage, Isyn = synaptic current). (B) Steady-state EPSP amplitudes vs. presynaptic AP frequency. Each point represents average of 20–30 EPSPst. Solid line, model prediction (membrane nonlinearity not accounted for). Same synaptic connection as in A. Dashed line, inverse relationship with frequency. Peak frequency marked as θ, limiting frequency as λ. λ is determined when the model fit of the experimental data deviates 10% from the 1/f curve. (C) Net depolarization as a function of presynaptic frequency (product of EPSPst, presynaptic frequency and membrane time constant, 60 ms) illustrating the continuum of signaling regimes from supra-linear to sub-linear. (D) Simulated postsynaptic current (Im) generated by Poisson AP trains with a sequence of instantaneous transitions from 0 to 80 Hz. Model parameters, Ase = 1,540 pA, U = 0.03, τrec = 130 ms, τfacil = 530 ms. The trace represents the average of 500 “sweeps.”