A, presynaptic membrane with active zone (AZ), calcium channel cluster, and three readily releasable vesicles. The location of cluster and vesicles, as well as the number of vesicles on the active zone, represent one example of random placement at multiple active zones. Drawings are to scale. B, as A but viewed from the top and with superimposed false colour coding of [Ca2+] on the membrane around the channel cluster. The concentrations reflect a simulation for the control case at time = 0.60 ms (see E). C, as B but viewed from the side, with an additional, vertical panel to show [Ca2+] in the plane perpendicular to the membrane (time = 1.15 ms). Dashed circle indicates a vesicle that has already fused. D, variable channel-to-vesicle distances result in [Ca2+] transients with variable peaks and thus heterogeneous release probability. A–D are excerpts from a video ‘The Ca2+ signal controlling phasic transmitter release’, available from the authors or at:
http://sunny.mpimf-heidelberg.mpg.de.
E, AP (trace 1, time course only) and whole-cell Ca2+ current (as predicted by the Hodgkin–Huxley Model; trace 2, left axis). Red traces show two simulated [Ca2+] transients (right axis, each normalised to the same amplitude) the average across all vesicle positions (trace 3) and at 200 nm from the channel cluster (trace 4). The first vertical dashed line from left indicates the time of peak AP (0.54 ms), and the second line indicates the time of peak Ca2+ influx (0.92 ms). F, predicted, average release rate of calyx (trace 1, right axis) and predicted EPSC (trace 2, left axis). For comparison, a measured, AP-evoked EPSC () is also shown (trace 3; shifted by ∼400 μs to the left to allow comparison of the time course). The vertical dashed line indicates the time of the peak of the predicted release rate (1.03 ms) (E and F after ).