Figure 4: Typical Sinoatrial Node Membrane Action Potentials (Red Trace) and the Timing of Membrane Clock and Ca²⁺ Clock Components.

The phases of the action potentials are labelled including phase 4, the resting phase in which diastolic depolarisation (DD) takes place, the key to automatic pacemaker activity. MDP = maximum diastolic potential; DD = diastolic depolarisation; ICa₂T = T-type voltage-dependent Ca²⁺ current; ICa₂L = L-type voltage-dependent Ca²⁺ current; I_NCX = sodium-calcium exchange current; I_K2 = delayed rectifier potassium current; I_f = funny current; SERCA = sarco-endoplasmic reticulum ATPase; LCRs = local Ca²⁺ releases. From Monfredi et al.³⁷ Reproduced with permission from The American Physiological Society.