<prompt> The resting membrane potential depends mainly on gradient ofSodiumPotassiumChlorideCalcium  <response>: Ans. b Potassium. Ref. Ganong Physiology, 21st ed., 55, 56Myocardial action potentialPhase 0Rapid upstrokeVoltage-gated Na-channels open.Phase 1Initial repolarizationInactivation of voltage-gated Na balances K effiux.Phase 2PlateauCa2 influx through voltage-gated Ca2 channels balances K efflux.Ca2 influx triggers myocyte contraction.Phase 3Rapid repolarizationMassive K efflux due to opening of voltage gated slow K channels and closure of voltage-gated Ca2 channelsPhase 4Resting potentialHigh K permeability through K channels 