Fixing [Ca] = 0.022. (a) The bifurcation diagram of V with respect to the parameter I for the somatic subsystem in Eqs. (1)–(4). The black S-shaped curve consists of the stable equilibrium points (the solid curve) and the unstable ones (the dashed curve). The curves which are green and blue represent the stable and unstable periodic orbits, respectively. (b) Two parameter bifurcation diagram of the fast subsystem in Eqs. (1), (2), and (4) with respect to the slow variable h, and the external forcing current I.