Describe AP generation with the heart compared to skeletal: skeletal has depolarization (Na+); repolarization and hyperpolarization (K+); cardiac conduction system - prepotential (sodium); threshold and depolarization (Ca++); repolarization (K+); muscle cell contraction - depolarization phase- Na+ ions; early repolarization-K+ channels; plateau phase-Ca++ channels; K+ leaves to complete repolarization;