Fig. 5.
Latency histograms of release events. (A) Histogram of PSC latencies following 86 laser stimulations from 19 single-site recordings (6 autapses and 13 MLI-MLI synapses), measured from the time of the laser pulse (laser energy range: 0.03–5.3 μJ) and displayed at low (Top; 10-ms bins; Freq, frequency) and intermediate (Middle; 3-ms bins) time resolutions. The intervals were fitted by maximum likelihood with a two-exponential probability density function (red curves superimposed on the histograms) with means of 4.0 ms and 319.6 ms and with areas 60% and 40% of the total. (Bottom) Histogram of latencies to the first quantal event shown with high time resolution reveals an initial silent period of 0.4 ms (laser pulse durations ≤0.2 ms). (B) Analysis applied to a restricted dataset with high laser energies >0.2 μJ (59 laser stimulations, from 6 autapses and 8 MLI-MLI synapses). The two exponential probability density function time constants are 4.3 ms and 340.0 ms. A total of 61% of the area is associated with the short latency component, corresponding to an average 1.76 ± 0.07 events per stimulus in the fast component. This represents an uncorrected estimate of the RRP size for a single-site synapse. (C) Schematic of a small synapse with a single active zone (brown) and associated postsynaptic receptors (green). Synaptic vesicles (yellow) bind to docking sites (blue) before exocytosis. In this view, the RRP is represented by the number of docked vesicles, which varies from stimulus to stimulus, and its value is limited at a given synapse to the total number of available docking sites. Thus, RRP = ND * f has a mean of ND * F and maximum ND, where ND is the number of docking sites and f is their occupancy at any time, with the mean value F.