Fig. 1.

(A) Measures of action potential (AP) produced by 1-s current injection, shown below the voltage trace. An inflection on the descending limb of the AP is demonstrated. Threshold for initiation of the AP. RMP = resting membrane potential; AP₅₀ = AP duration at 50% of peak. (B) Determination of afterhyperpolarization (AHP) dimensions after an AP initiated by a 2-ms current injection, shown below the voltage trace. AHP amplitude is measured from RMP to peak. AHP₅₀ = AHP duration at 50% recovery; AHP_τ = time constant for AHP recovery. (C) Measures of Ca²⁺ current (I_Ca) response to voltage command in the form of an AP waveform. Inward I_Ca is measured at peak and as the integral of total inward current (Ca²⁺ charge transfer) during AP waveform.