There are five phases of the cardiomyocyte action potential:

1. Phase 0: rapid depolarization "upstroke"
2. Phase 1: brief repolarization
3. Phase 2: plateau phase
4. Phase 3: rapid repolarization
5. Phase 4: RMP @ -90 mV

Phase 0: Rapid Depolarization

• Feedback loop: Na+ channels begin to open in response to depolarization, which causes depolarization…
• Threshold potential: -70mV
  • When the MP reaches this value, enough fast Na+ channels have opened to generate a self-sustaining inward Na+ current
• These fast Na+ channels are only open for a few milliseconds, then are inactivated preventing further influx

Phase 1: Brief Repolarization

• Returns the MP to neutral (0 mV)
• Caused by the outward flow of K+ ions through transiently activated K+ channels

Phase 2: Plateau Phase

• Long plateau phase is mediated by the balance of
  • Outward K+ currents through VG delayed rectified K+ channels
  • Inward Ca++ currents through L-type Ca++ channels
    • Begin to open during phase 0 (rapid depolarization) at -40 mV
    • Slower activation and channels remain open longer than Na channels
• Nearly net zero current
• Ca++ ions that enter are critical for the initiation of myocyte contraction
• Phase ends as Ca++ channels are gradually inactivated and the charge efflux of K+ begins to exceed the charge influx of Ca++

Phase 3: Rapid Repolarization

• Continued outward K+ flow is primarily responsible
• Positive ions that entered the cell (Na and Ca) must be actively pumped to retain the ion concentration gradients
  • Sarcolemmal Na/Ca exchanger
  • Sarcolemmal Ca++ ATPase
  • Na/K ATPase