Fig. 3.

Self-coupled model with I_pump. (A) Voltage and Na⁺ traces (Left) with an expanded and color-coded burst (Right). (B) V-Ca²⁺ bifurcation diagrams for fixed Na⁺ values; red, blue, and green trajectories correspond to A. Stable (unstable) points are drawn with solid (dashed) lines, and periodic orbits (maximal and minimal voltages) are shown with circles. The branch near −20 mV is a curve of spiral points, featuring a change in stability at the AH bifurcation from which the periodic orbits emanate. The branches at lower voltages that appear in the Center (b) and Right (c) parts of B, consist of unstable saddles (dashed lines) and stable nodes (solid lines), which intersect at a SNIC bifurcation. (C) Expanded view near the SNIC; a, b, and c refer to Left (a), Center (b), and Right (c) in B, superimposed without trajectories. (D) The burst cycle in Na⁺-Ca²⁺ state space, together with the SNIC and AH curves. Na⁺ is plotted such that lower values on the y axis reflect lower excitability, due to higher Na⁺ evoking greater I_pump (outward) current.