Voltage command (top), corresponding whole-cell Ca2+-influx (middle) and the functional fluorescence responses (bottom) (bandstop filtered at 33.3 Hz) from Rhod-FF and iGluSnFR, respectively. A modified Boltzmann function (see Materials and Methods, R2 = 0.81) was fitted to a Rhod-FF fluorescence trace in response to a voltage ramp (C-C'). iGluSnFR-AUC was calculated per depolarization voltage and used for a Boltzmann fit (D', R2 = 0.92). The voltage of half-maximal activation (V1/2) and the dynamic range (10-90%) of synaptic Ca2+-influx and glutamate release were calculated from the fits and depicted as circle and bar in C' and D', respectively. The obtained fits from a Ca2+ "hotspot" (C') and from glutamate release (D') were plotted against each other in a voltage range from -57 mV to -17 mV in 1 mV increments. A power function was fitted to the 25% of the maximum iGluSnFR-AUC (R2 = 0.99) to obtain the m-estimate. (ruptured patch-clamp, 10 mM intracellular EGTA, 5 mM [Ca2+]e) (See also Fig EV4) 