Figure 5. Hodgkin–Huxley model of Ca²⁺ influx in endbulb terminals.

A, rapid activation (Aa) and deactivation (Ab) kinetics of I_Ca(V). Smooth red curves represent single exponential fits. Aa, activation of I_Ca(V) elicited by depolarizing voltage steps from V_h = –80 mV to various V_m values (τ_A = 0.71, 0.46, 0.40, 0.31 and 0.32 ms for steps to –10, 0, 10, 20 and 30 mV, respectively). Ab, deactivation of I_Ca(V) after a 10 ms depolarization to 0 mV after stepping back to various V_m values (τ_D = 0.49, 0.32, 0.20, 0.15 and 0.12 ms for steps to –30, –40, –50, –60 and –70 mV, respectively). B, time constants τ_m measured from deactivation (–70 to –20 mV, eqn. 3, filled circles) or activation (–10 to 40 mV, eqn. 4, open circles) of I_Ca(V). The smooth line represents the time constants predicted from the rate constants of the HH-model (τ_m = 1/(α_m+β_m)). C, activation (α_m) and deactivation (β_m) rates of the gate m plotted as a function of V_m. Continuous lines are single exponential fits (eqns (7) and (8)), with α₀ = 2.03 ms⁻¹, V_α = 26.5 mV and β₀ = 0.22 ms⁻¹, V_β = 19.8 mV. Potentials >20 and <−60 mV were excluded from the fit. D, comparison of measured and simulated I_Ca(V) elicited by a 1 ms depolarization to 0 mV. The assumed time course of V_m (top panel) was obtained by digitally filtering a 1 ms step with a single-pole filter (time constant τ = R_s×C_m, where R_s and C_m represent uncompensated series resistance and whole-cell capacitance, respectively). The activation parameter m² peaked at ∼0.71 (middle panel). Except for a small deviation at the end of the deactivation time course, the simulated I_Ca(V) (red trace) closely matched the measured current (black trace, bottom panel).