Phase planes for the existing model by Hancock et al. (2022) . (a) Phase plane for C-clock isolated by fixing L-type current I_L; Z = cytosolic [Ca²⁺], Y = store [Ca²⁺]. (b) C-clock phase plane with full clock coupling. (c) Phase plane for M-clock isolated by fixing Z; V = membrane potential, h = L-type inactivation gate state. (d) As in c, but with zero Z or, equivalently, knock-out of anoctamin-1 Cl⁻ channels (Ano1-KO). With gate fn. h_∞(Z) half saturated at K_h = 4 µM as in Fig. 3, the h-nullcline is invariant over the Z-range 0–2.92 µM. More generally (Hancock et al., 2022), APs can also be initiated by either low Z raising the non-zero parts of the h-nullcline (thence eventually h, thence L-type channel opening), or store calcium release (CICR) causing Ano1 current hence causing the V-nullcline peak to drop (dynamically lowering the L-type V-threshold), or both. Panels a, c, and d reprinted from Figs. 4 a; and Fig. 7, c and d, of Hancock et al. (2022), with permission from Computers in Biology and Medicine.