Which statement describes Em?

• A. The actual electrical potential difference across the cell membrane at a moment
• B. The equilibrium potential
• C. The resting potential
• D. The action potential

Answer: (A)

The main idea is that Em represents the actual voltage across the cell membrane at a given moment. It is the instantaneous transmembrane potential produced by the combined conductances and ion gradients at that time, so it can rise and fall as ion channels open or close during the cardiac cycle. The equilibrium potential, by contrast, is a theoretical voltage that would exist if only one ion could cross the membrane, serving as a reference for that ion’s driving force but not describing the real, multi‑ion membrane voltage. The resting potential is the stable, baseline Em when the cell is not actively firing, reflecting leak currents and steady ion gradients. The action potential is the rapid, transient departure from resting Em during excitation, including depolarization, plateau, and repolarization. Since Em is the actual instantaneous membrane voltage, that description best matches what Em is.