FIG. 9.

Comparison of spontaneous and paced electrical activity. The time series for the (a) lateral segment voltage, and the sum of the currents on the lateral segment and junctional membrane for (b) $I_{K1}$, (c) $I_{Na}$, and (d) $I_{si}$ for spontaneous (green, $f_{K1}=0.9$) and paced (gray) electrical activity. Spontaneous activity exhibits gradual depolarization between beats and smaller magnitude $I_{Na}$, while paced activity exhibits a rapid action potential upstroke and large magnitude $I_{Na}$.