In this program, xo is the facilitating epsp, and x2 is the basic response (unfa-cilitated). The time course of facilitation is determined by the (f, g) LPF with natural frequencies c and d r/ms. Figure 4.1-3 illustrates the response of the model to three impulses spaced 4 ms. Trace 1 = x1, 2 = x2 (basic response), 3 = xo (facilitated epsp), 4 = ff (the facilitation factor). If the input pulses are spaced farther apart, there is less facilitory effect. This is shown in Figure 4.1-4, where the pulse spacing is 10 ms; each xo(t) response is affected by ff, but in 10 ms, the g(t) response has almost died out, so the facilitation is minimal. Facilitation is interesting because it provides a mechanism whereby a jump in the instantaneous pulse frequency in the presynaptic signal (e.g., by inserting an extra spike) produces a disproportionate postsynaptic response.
Antifaciltation, as the name suggests, is where successive epsps in response to an input pulse train grow progressively smaller than the basic response epsp. It is easy to interpret antifacilitation as a fatigue phenomenon, such as might be caused by temporary depletion of the neurotransmitter caused by its release rate exceeding its replacement rate in the bouton. As in the case of facilitation, if the input pulses occur with a long period, the antifacilitating response has time to decay, and the epsps evoked are close to the basic response. A Simnon program, ANTIFAC.t, modeling antifacilitation is show below:
continuous system ANTIFAC " 3/10/99 Use Euler integration w/ DT = .001. STATE x1 x2 f g " Antifacilitating psp.
DER dx1 dx2 df dg TIME t
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