Negative shifts in SSA and positive shifts in SSI promote earlier upstroke time (t _up ) and a larger contribution from the shifted Na ⁺ channels to the total Na ⁺ charge (Q _Na ) in single cells. (A and B) Contour maps of the (A) action potential t_up and (B) shifted Q_Na fraction are shown as functions of $Vactshift$ and $Vinactshift$⁠, for different values of the shifted channel fraction (f_Na). Contours have (A) 0.05 ms and (B) 0.1 (unitless) spacing. The o and x denote the baseline and nominal shifted Na⁺ channel values, respectively.