Addition of a subpopulation of I _Na with shifted biophysical properties results in earlier AP upstroke and larger I _Na current in a single cell. (A and B) Transmembrane voltage (V; top and middle; magenta) and I_Na (bottom; blue/red, magenta) are shown as functions of time for a single cell with (A) 50:50 and (B) 30:70 distribution of baseline:shifted I_Na subpopulations. The corresponding voltage and I_Na curves are shown for the homogeneous (only baseline I_Na) single cell in black. In A, I_Na scaled by a factor of 1/2 is shown for comparison with I_Na from the baseline/shifted single cell (dashed black line).