Phase 2 of the cardiac cell action potential corresponds to what?

• A. The Resting Potential
• B. The Absolute Refractory Period
• C. Plateau Phase
• D. The Effective Refractory Period

Answer: (D)

Phase 2 is the plateau phase, a period of sustained depolarization where calcium entry through L-type calcium channels roughly balances potassium efflux. This keeps the membrane potential around 0 mV for an extended time, allowing a strong, coordinated contraction of the cardiac muscle and giving the heart time to fill.

Refractoriness is a separate concept that describes when the cell can respond to a new stimulus. The absolute refractory period occurs when Na+ channels are inactivated and no stimulus can trigger another action potential. The effective refractory period is the later portion of refractoriness during which a stronger-than-normal stimulus can evoke a new impulse, and this window overlaps with the plateau and early to mid-recovery of repolarization. The resting potential corresponds to phase 4, when the cell is quiet between beats.

So, the specific phase held for phase 2 is the plateau phase due to the sustained calcium influx, while refractoriness describes when excitability is possible, which spans later phases.