Phase 0 is called Phase of depolarization. Resting membrane potential is the voltage difference between the inside of the membrane and outside of the membrane. Intracellular environment of the cell has negative RMP (-90mV). When cell is appropriately stimulated, positive charges such as sodium and calcium will go into the cell and increase the RMP. When the membrane potential will reach to its threshold (-70mV), the voltage gated sodium channels opens and rapid influx of sodium (positive ions) occurs which will increase the membrane potential to +10mV. This event is called depolarization. Class I antiarrhythmic like procainamide blocks phase 0 in fast response fibers.
Phase 1 is called overshoot and clinically insignificant. Sodium channels are inactivated during this phase. Overshoot develops because of slow potassium current going out of the cell and chlorine coming into the cell. No drugs act on this phase.
After the cell is depolarized, depolarized-sensitive calcium and potassium channel starts opening. The level of intracellular potassium (positive charge ion) is higher than extracellular potassium and the level of extracellular calcium (positive charge ion) is higher than intracellular calcium. For a brief duration, potassium is going out and calcium is coming in the cell and keep balance in the electromagnetic voltage across the cell membrane (phase 2).
As the time passes, delayed-rectified potassium channel opens and heavy efflux of potassium occur from the intracellular to extracellular environment and RMP return back to -90mV (due to concentration dependent efflux of positive ions). This phenomenon is called repolarization (phase 3).
After the repolarization phase, Na+/K+ ATPase pump will send 2 sodium ions out of the cell in exchange for 3 potassium ions. This will help to replete ion concentration inside and outside the cell (phase 4).
This whole process of depolarization and repolarization is called as action potential. When the cell is undergoing depolarization, some of these sodium will go to the next cell via gap junction and resting membrane potassium reaches threshold of adjacent cell and leading to its depolarization and repolarization. This cell will stimulate another adjacent cell and the action potential pass over the entire myocardium.
Next: Action Potential in SA and AV Node.