Which phase of the cardiac action potential corresponds to the plateau period?

• A. Phase 0
• B. Phase 1
• C. Phase 3
• D. Phase 2

Answer: (D)

The plateau period is the phase where the inward calcium current balances the outward potassium current, keeping the cell membrane depolarized for an extended time. As L-type calcium channels open, Ca2+ enters the cell and sustains depolarization, so the membrane potential stays around a depolarized level for several hundred milliseconds. This prolonged depolarization also triggers calcium-induced calcium release for contraction, linking electrical activity to force generation.

In contrast, the rapid upstroke phase is driven mainly by a quick sodium influx that rapidly depolarizes the cell; the early transient outward current causes a brief notch after the upstroke; and the final repolarization phase is when potassium efflux dominates, returning the membrane to its resting potential.