Inward rectifier channels contribute to which aspects of the action potential?

• A. Contributes to late phase 3 repolarization and maintains phase 4 negative potential
• B. Initiates phase 0 depolarization
• C. Sustains the plateau phase
• D. Produces early repolarization

Answer: (A)

Inward rectifier potassium channels mainly set and stabilize the resting membrane potential while also contributing to the final stage of repolarization. These Kir2.x channels are open when the cell is near its resting voltage, allowing K+ to help keep the membrane very negative (phase 4). As the action potential repolarizes toward the potassium equilibrium potential, these channels support the finishing portion of repolarization (late phase 3) by allowing K+ to exit, helping the cell return to its negative resting level. They are not the drivers of the rapid upstroke (which is Na+ influx), nor the main sustainers of the plateau (which rely on Ca2+ influx and other K+ currents), nor the primary cause of early repolarization. Thus, their role aligns with contributing to late phase 3 repolarization and maintaining a negative phase 4.