<prompt> Phase of cardiac action potential, where calcium channels close slow potassium channels open :Phase IPhase IIPhase IIIPhase IV  <response>: In cardiac action potential Phase O - Rapid upstroke - due to fast sodium channels- Influx of NaPhase I - Initial repolarization - fast Na channels - closePhase II - Plateau - due to calcium influx Phase III - Rapid repolarization - Calcium channels close slow potassium channels open Phase IV - Resting membrane potential - - 90 mv 