ACTION POTENTIAL:

Phase o:  Cell is stimulated, cell menbrane is more permeable to sodium ions.

Phase 1:  Brief period of rapid repolarization:  sodium influx ends in fast channels: potassium flows out

Phase 2:   Plateau phase: influx of calcium and sodium through slow channels which sometimes occurs in Phase 0 and Phase 1:   Responsible for the refractory period during whic the depolarized stae is maintained.  Calcium’s role is related to the contractility of the heart.

Phase 3:  Negativity restored due to increased loss of intracellular potassium.

Phase 4:  Return to resting membrane potential (-90 mV).  Normal distribution of Na+ and K+ are restored.

Need a review of cardiac action potentials?  These instructional videos are fabulous!