<prompt> Phase 1 of Myocardium AP is mainly due to:K effluxNa channel blockageCa2 effluxNone of the above  <response>: Phase 0 depolarization, fast sodium channels open.Phase 1 initial repolarization, fast sodium channels close. Phase 2 plateau, calcium channels open and fast potassium channels close.Phase 3 rapid repolarization, calcium channels close and slow potassium channels open.Phase 4 resting membrane potential averages about 90 millivolts.Key Concept:Phase 1 of myocardium AP is due to K opening inactivation of Na channel. Phase 2 of myocardium AP starts with Ca entry.Long duration of myocardium AP is due to phase 2. 