<prompt> Plateau of cardiac action potential is due toOpening of voltage-activated fast sodium channelsOpening of voltage-activated potassium channelsOpening of voltage-activated slow calcium channelsOpening of voltage- activated calcium -sodium channels.  <response>: Ans. c Opening of voltage-activated slow calcium channels Ref. Ganong Physiology 22ndpg. Chapter 29.In contrast to skeletal muscle:1. Cardiac muscle action potential has a plateau, which is due to Ca2 influx2. Cardiac nodal cells spontaneously depolarize, resulting in automaticity3. Cardiac myocytes are electrically coupled to each other by gap junctions Slow influx of calcium is responsible for the plateau phase of the action potential.Myocardial action potentialPhase 0Rapid depolarizationRapid sodium influx; these channels automatically deactivate after a few msPhase 1Early repolarizationEfflux of potassiumPhase 2PlateauSlow influx of calciumPhase 3Final repolarizationEfflux of potassiumPhase 4Restoration of ionic concentrationsResting potential is restored by NaK ATPase; there is slow entry of Na into the cell decreasing the potential difference until the threshold potential is reached, triggering a new action potential. 