Figure 11. EPSPs in a pyramid to horizontal O-A interneurone connection generated by brief spike trains of different durations and recorded at different postsynaptic membrane potentials.

The averaged EPSPs each include between 60 and 120 single sweeps. On the left, EPSPs elicited by similar, brief trains of 3 presynaptic spikes were recorded at 4 different postsynaptic membrane potentials (-61, -69, -72 and -78 mV). Averaged EPSPs decreased in amplitude and duration with hyperpolarization to -78 mV and with depolarization to -69 mV from -72 mV. This latter decrease in amplitude may, however, be an overestimate as at -69 and -61 mV the largest single sweep events elicited postsynaptic action potentials (see Fig. 12) and were excluded from the analysis. Paired pulse and brief train facilitation are, however, apparent and relatively similar at all membrane potentials. On the right, responses to brief 3 and 4 spike trains of different durations are compared at two membrane potentials (-61 and -78 mV). At -61 mV averaged responses to 3 spike trains of two different durations are compared. Facilitation of second and third EPSPs appear to decline with increasing interspike interval. At -78 mV averaged responses to trains of 4 spikes in which the timing of the third and fourth (but not the second) presynaptic spikes are different, are compared. Facilitation of the third and fourth EPSPs appears to decline with increasing interspike interval.