CARDIAC MUSCLE CELL ACTION POTENTIALPHASE 0: DEPOLARIZATION Sodium rapidly into cell (calcium slowly into cell)PHASE 1 Sodium channels closePHASE 2: PLATEAU PHASE Potassium rapidly out of cell Calcium slowly into cell Calcium from extracellular space and sarcoplasmic reticulum = plateauPHASE 3: RAPID REPOLARIZATION Calcium channels close Potassium rapidly out of cell Potassium and sodium ion positions reversedPHASE 4: RESTING POTENTIAL Leaky potassium channels Sarcolemma impermeable to sodiumLong absolute refractory period in cardiac muscle cells: phase 0 to phase 3 Second action potential cannot be initiated Protective mechanism against tetanus (state of maximal contraction)
