The plateau phase of action potential is due to:

There are 5 phases of action potential. The plateau phase corresponds to phase 2 of action potential. Plateau phase is due to slow influx of calcium ions resulting from opening of sarcolemmal L type calcium channels and also due to closure of a distinct set of K+ channels called the inward rectifying K+ channels. Phase 0: Rapid depolarization: The initial rapid depolarization and the overshoot is due to the opening of voltage gated Na channels and rapid influx of Na ions. Phase 1: Initial rapid repolarization: It is due to closure of Na+ channels and opening of K+ channels resulting in transient outward current. Phase 2: Plateau: It is due to slow influx of calcium ions resulting from opening of sarcolemmal L type calcium channels and also due to closure of a distinct set of K+ channels called the inward rectifying K+ channels. Phase 3: Repolarization: It result from closing of Ca2+ channels and opening of two types of K+ channels. Phase 4: Resting potential: It is maintained by a resting K+ current, the largest contributor to which is the inward rectifying K+ current. Ref: Textbook Of Medical Physiology By Khurana page 248.