Which phase of the non-pacemaker action potential is the resting potential with high K+ currents and low Ca++ and Na+ currents?

• A. Phase 4 resting potential
• B. Phase 0 rapid depolarization
• C. Phase 2 plateau
• D. Phase 1 initial repolarization

Answer: (A)

The key idea is the resting phase of a non-pacemaker cardiac myocyte. At rest, the cell’s membrane is highly permeable to potassium through inward rectifier channels, which drives the resting membrane potential to a negative value (around −90 mV). Because voltage-gated sodium and calcium channels are closed in this state, their inward currents are minimal, so there’s little depolarizing current. This is why the cell stays at a stable, negative resting potential with high potassium conductance and low calcium and sodium currents.

In contrast, depolarization (rapid upstroke) is driven by a swift influx of Na+, the plateau phase arises from a sustained calcium influx, and repolarization follows mainly from potassium efflux. The described condition—resting at a stable negative potential with dominant K+ permeability and minimal Ca2+ and Na+ currents—matches the resting phase.