Figure 2.

Transmitter release time course. A. In response to an AP-like stimulation (a 1–2 ms depolarizing pulse), the cumulative release (the number of vesicles releasing neurotransmitter) can be determined by N Pr. The real AP has a repolarizing phase after depolarization. When the pulse duration is prolonged, all the vesicles release neurotransmitter, and the cumulative release should be N (P_r = 1). The release time course should be fitted by an exponential if Pr is homogeneous. From top, presynaptic potential, cumulative release, and schematic drawing of vesicle release are shown. The schematic drawing illustrates the case for N = 3 and Pr = 0.3 or 1. Adapted from Ref. 69. B. The time course of release at the calyx of Held. The presynaptic terminal was depolarized from −80 mV to 0 mV for 50 ms (after a 2 ms pulse to +70 mV to activate Ca currents maximally). The EPSC was observed at the postsynaptic side, and the cumulative release was estimated by the deconvolution method and was fitted by a dual exponential, which suggests that Pr is heterogeneous among synaptic vesicles. From top, presynaptic membrane potential (V_pre), presynaptic Ca currents (I_Ca), EPSC, and cumulative release, schematic drawing of vesicle release, are shown. FRP and SRP are the fast releasing pool and slowly releasing pool of synaptic vesicles, respectively. N: number of vesicles in each pool. P_AP: release probability per an AP. Reproduced from Sakaba and Neher (2001), with permission from Elsevier.²⁶⁾