Thus synapses modulate their strength by two- to threefold through use by a natural spike train in a deterministic manner. As has been well established, hippocampal synapses behave probabilistically and release a single vesicle-full of neurotransmitter at random and infrequently. When the single synapse is studied, the probability of release varies two- to threefold, but the average size of the postsynaptic response, when one occurs, is invariant. This means that all of the response variability seen with random spike trains occurs because the synapse is continually adjusting its reliability in response to the history of its use but that the postsynaptic sensitivity to neurotransmitter remains constant. The ongoing modification of synaptic strength is therefore a presynaptic phenomenon.
Facilitation is an increase in release probability that follows use of a synapse.
The effects of earlier synaptic uses add together and are remembered by the synapses for about half a second with facilitation. Augmentation is a second and larger increase in release probability that endures for about 10 seconds. Similarly, larger synapses deplete more slowly than smaller ones. Thus the size of a synapse strongly influences the pattern of synaptic strength regulation shown by the synapse. One complication in describing the ongoing regulation of synaptic strength by the history of use is the fact that the plasticity mechanisms differ significantly from one synapse to the next.