This section considers various linear and nonlinear dynamic models for chemical synapses described in Section 1.3, and the generation of simulated "ballistic potentials" associated with epsps and ipsps. As has been seen, the epsps and ipsps are summed both in time and in space (over the dendrites of a neuron and/or its soma where synapses occur) and the resultant summed depolarization seen at the SGL is what the neuron responds to in producing its output spike train. Characteristic of all chemical synapses is a small delay between the arrival of the presynaptic spike and the onset of the psp. This delay has four significant components: (1) a delay associated with the presynaptic release of neurotransmitter; (2) a time for the neurotransmitter to diffuse across the 20 to 40-nm synaptic cleft; (3) a time for the transmitter molecules to bind to the ion channel receptor proteins; and (4) a time required for ion channels to be gated open. The total delay between the arrival time of the presynaptic action potential peak at the bouton and the onset of the corresponding epsp or ipsp can range from 0.1 to 1 ms, depending on the preparation, the temperature, etc. (In most simulations, it is acceptable to ignore this small synaptic delay because it can be included in the model by increasing the rise time constant of the psp.)
The sections below illustrate simple Simnon computer models for (1) a linear, two-pole ballistic filter to generate psps, (2) a nonlinear model of a synapse showing facilitation in the generation of a series of psps, and (3) another nonlinear model showing antifacilitation of synaptic fatigue in the generation of a series of psps.
Facilitation is a phenomenon where each succeeding psp is larger than the previous one, providing the presynaptic (input) pulses are closely spaced in time. The physiological reasons for this phenomenon are complex, and will not be covered here. If the times between the successive input pulses are long enough, the facilitory effect from each previous input pulse dies out, and the "basic response" of the synapse to each input pulse is seen.
Antifacilitation is a behavior also seen in the generation of psps from an input pulse train. In this case, however, if the time between input pulses is short enough, each successive input pulse causes a progressively smaller psp than the basic response. Antifacilitation can be viewed as a fatiguing of the synapse (i.e., one or more of its components).
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