Figure 6: Estimates of nanodomain Ca²⁺ amplitude from Ca_V2.2/TN–XL responses.

(a) Average Ca_V2.2/TN–XL responses to nanodomain Ca²⁺ signals, corrected to reflect signal of active channels alone. Top, black trace plots average whole-current, rescaled to open probability P_O by normalizing peak current during 30-mV pulse to 0.69. Grey trace, exponential fit to P_O waveform. Middle, cyan trace plots average _C, corrected CFP signal. Red trace, forward transform prediction of experimental _C using Ca_spike=60 μM. Bottom, Green trace plots average , corrected FRET-ratio signal. Red trace, forward transform prediction of experimental using Ca_spike=60 μM. Grey traces, transform prediction with Ca_spike as labelled. (b), Nanodomain Ca²⁺ signals input to forward transform for prediction of Ca_V2.2/TN–XL responses in (a). Top, P_O waveform reproduced from (a). Middle, Schematic of stochastic Ca²⁺ pulses in nanodomain. Bottom, forward transform box with parameters in Table 1. (c) Ca_spike estimation criteria. Presumed Ca_spike values plotted versus fold-fitting error (sum of squared deviations between fit and data, normalized to sum with Ca_spike=60 μM). Cusp of plot denotes Ca_spike=60 μM as best fit. Shaded zone specifies approximate confidence region, where fits yield errors within 25% of minimum.