Phase 4 (Baseline): K+ channels are open, as K+ currents primarily determine the resting membrane potential of cell
Phase 0 (Fast depolarization): Voltage-gated Na+ channels open in response to gap junction depolarization. Na+ flows into the cell which opens more Na+ channels; each channel is open only briefly and cannot open again until repolarization occurs
Phase 1 (Notch): transient K+ channels open and rapidly leave and repolarize the cell, setting the membrane potential for phase 2.
Phase 2 (Plateau): L-type Ca2+ channels open and calcium enters the cell near the sarcoplasmic reticulum. Ca2+ binds to the SR (via ryanodine receptors), triggering lots more Ca2+ to be released into the cell; K+ is still leaving the cell
Phase 3 (Repolarization): Ca2+ channels start to close and the slow K+ channels continue to leave the cell, causing repolarization. Na+ channels will recover from depolarization from phase 0 as the membrane potential becomes more negative until it reaches the resting membrane potential