Generation of the fast action potential in myocardial cells

Schematic representation of the ionic events during the different phases of the fast action potential. The resting membrane potential (phase 4) is maintained by an energy-dependent ATPase sodium-potassium mechanism which pumps potassium (green spheres) into the cell in exchange for sodium (magenta spheres) which is extruded. Rapid depolarization during phase 0 is characterized by opening of voltage-dependent sodium channels and rapid entry of sodium ions into the cell. This is followed by delayed opening of slow calcium channels and influx of calcium ions (blue spheres). The calcium channels remain open during phase 1 (early repolarization) and there is also opening of potassium channels and efflux of potassium ions which results in late repolarization (phase 3) and a return to resting membrane potential (phase 4).