Fig. 3

Bifurcation Diagrams of Fast AP Subsystems. PP) Bifurcation diagram of the fast subsystem Eq. (10) of model PP with the potassium gating variable x as continuation parameter. The solid and dashed black lines denote stable and unstable fixed points of Eq. (10). At the subcritical Hopf bifurcation H, unstable limit cycles emerge that subsequently terminate at a saddle-homoclinic bifurcation HC. The red dashed lines show the maximum and minimum voltage values of the unstable limit cycles. PV) Bifurcation diagram of the fast subsystem Eq. (9) (appended by the equations for h, m and j from Eq. (5)) of model PV, α=1 and β=1, with the potassium gating variable x as continuation parameter. The solid and dashed black lines denote stable and unstable fixed points. At the subcritical Hopf bifurcation H, unstable limit cycles are born that turn into stable ones at a saddle node of cycles bifurcation before they terminate at a saddle-homoclinic bifurcation HC. The red dashed lines show the maximum and minimum voltage values of unstable limit cycles, the red solid lines show the extreme voltage values of stable limit cycles. UP) Bifurcation diagram of the fast subsystem Eq. (10) of model UP with the potassium gating variable x as continuation parameter. The solid and dashed black lines denote stable and unstable fixed points of Eq. (10) that annihilate each other at saddle-node bifurcations. As opposed to PP) and PV), neither a Hopf nor a homoclinic bifurcation does exist, and neither stable nor unstable limit cycles are detected. Still, model UP features chaotic EAD dynamics, see Fig. 1UP