A voltage ramp was applied to obtain ΔF of Rhod-FF as a proxy synaptic Ca2+ influx. The V1/2 of synaptic Ca2+-influx was calculated from a modified Boltzman function (see Materials and Methods) fitted to ΔF/F0. (mean ± SD, n = 55 synapses; individual synapses are color coded) (see Appendix Fig S7 for individual fits). 