FIGURE 7.

Changes in the unstable regions of the I_bias-V₀ curve during inhibition of I_Ca,L, I_Kr, or I_Ks. Displacements of Hopf (H1, H2) and saddle-node (SN1, SN2) bifurcation points in the I_bias-V₀ curve with decreasing g_Ca,L, g_Kr, or g_Ks at an interval of 0.001 are shown for the I_K1-removed BP system. Values of V₀ and I_bias at the bifurcation points were plotted on the potential (top) and current (bottom) coordinates, respectively, as functions of the conductance. The path of the control V₀ at I_bias = 0 (i.e., EP1 in Fig. 6) is also shown on the potential coordinates, intersecting the locus of H2 at g_Ca,L = 0.134 (labeled HB). Note that the unstable region exists between H1 and H2 (or H1 and SN2), where spontaneous oscillations occur. A codimension two saddle-node bifurcation at which the loci of H1 and H2 merge together, i.e., the unstable region disappears occurred at g_Ca,L = 0.126 (labeled SNc2).