Em = resting membrane potential of the cell, -90 mV. Maintained by the Na-K ATPase, which pumps K+ INTO the cell and Na+ OUT, as well as the Na-Ca exchanger, which pumps calcium OUT of the cell.
Et = threshold potential, -65 mV
Phase 0 = resting cell is brought to Et → rapid depolarization with rapid INFLUX of Na+ via activation of voltage-dependent sodium channels, as well as slower INFLUX of Ca2+ due to delayed opening of calcium channels
Phase 1 = slow repolarization to membrane potential of 0 mV due to inactivation of voltage-dependent sodium channels (i.e. sodium influx stops) + transient EFFLUX of K+
Phase 2 = plateau followed by early repolarization. Slow INFLUX of Ca2+ electrically balances the EFFLUX of K+
Phase 3 = late repolarization via continued EFFLUX of K+. Ca2+ channels are closed
Phase 4 = return to resting membrane potential via the Na-K ATPase and Na-Ca exchanger
Makielski, JC, MD, FACC. Myocardial action potential and action of antiarrhythmic drugs. In: UpToDate. Lévy, S, MD (Ed). UpToDate, Waltham, MA. (Accessed on May 10, 2015).
Defined by: Kathryn Kirkpatrick, MD