FIG. 4.

Weak to moderate dependence on fraction of sodium current at the junctional membrane $f_{Na}$ in identical ephaptically coupled cells. (a) The curve of Hopf bifurcation (HB) points $(f_{K1},vcl)$ shifts slightly over a large range of $f_{Na}$ [cf., Fig. 3(f)]. (b) Period of automaticity as a function of $f_{K1}$ for fixed cleft volume $vcl=0.38\mu {\text{m}}^3$ does not significantly vary with $f_{Na}$. Increasing $f_{K1}$ shortens the period duration for fixed $f_{Na}$. Inset shows that voltages (cleft on top, lateral segment below) for a fixed value of $f_{K1}$ are similar even with modest differences in period duration. (c) The corresponding bifurcation diagrams of voltage vs $f_{K1}$ for $vcl=0.38\mu {\text{m}}^3$ also minimally vary, suppressing stability details as in Fig. 3(b). (d) As a function of $vcl$ for fixed $f_{K1}=0.885$, decreasing the cleft volume shortens the period. Increasing $f_{Na}$ increases period; however, for larger $f_{K1}\gg 0.885$, the variation is minor in the period duration. Inset shows that APs are similar for all five values of $f_{Na}$ with fixed $vcl=0.418\mu {\text{m}}^3$.