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^Cais measured at peak and as the integral of total inward current (Ca²⁺charge transfer) during AP waveform.