FIGURE 5.

Bifurcation structure of the model HVM during g_K1 decreases. (Left) Steady-state I/V relations for g_K1 = 0 − 1, depicted at an interval of 0.1 with [K⁺]_i fixed at 140 mM. The zero-current crossings, corresponding to EPs, are designated by EP1, EP2, and EP3. (Middle) A bifurcation diagram for g_K1 with the steady-state (EP1–3) and stable periodic (MDP, POP) branches (top), and CL plotted as a function of g_K1 (bottom). EPs were determined by the algebraic equations (Eqs. 65–69). The model cell dynamics were computed by numerically solving the differential equations (Eqs. 55–63) for 10 min at each g_K1, which was reduced at an interval of 0.001, with the initial [Na⁺]_i at g_K1 = 1 set equal to 6.14 mM. The saddle-node bifurcation point at which EP2 and EP3 merge together and disappear is located (labeled SNB at g_K1 = 0.154). (Right) Steady-state [Ca²⁺]_i (top) and [Na⁺]_i (bottom) at EPs as functions of g_K1. The minimum (Min) and maximum (Max) of [Ca²⁺]_i, as well as average [Na⁺]_i, during the BP oscillations are also plotted. Note that the steady-state [Na⁺]_i at EP3 reduces with decreasing g_K1.