Review Note

Cardiac muscle cells - action potential - phases

• Phase 0 (depolarization): Fast Na+
channels open, membrane potential becomes more positive, Na+rapidly
flows into the cell and depolarizes it.
• Phase 1 (initial repolarization): Fast Na+channels close, cell begins to repolarize, and K+ ions leave the cell through open K+ channels.
• Phase 2 (plateau): Ca+2 channels open and fast K+ channels close.
Ca+2 enters the cell. Action potential reaches a plateau. Sustained cardiac contraction occurs here.
• Phase 3 (rapid repolarization): Ca+2 channels close and slow K+ channels open. K+ ions rapidly exit the cell, ends the plateau and returns cell membrane potential to its resting level.
• Phase 4 (resting membrane potential): About -90 mV, established by Na+ -K+ pump