In a pacemaker action potential, the effective refractory period (ERP) spans which phases?

• A. Phase 0 and Phase 3
• B. Phase 1 and Phase 2
• C. Phase 0 and Phase 4
• D. Phase 2 and Phase 3

Answer: (A)

The main concept is the refractory period, the time during which a cell cannot be re-excited because key ion channels remain inactivated. In pacemaker cells, the upstroke (phase 0) is Ca2+-dependent, and there is no plateau phase like in other cardiac tissues. The cell stays non-excitable through the depolarization and most of the repolarization, i.e., from the start of phase 0 to the end of phase 3, until those channels recover. Only after repolarization is complete can another impulse trigger, which is why the effective refractory period spans phase 0 and phase 3. The diastolic depolarization of phase 4 marks the return toward excitability, not the refractory interval, and phases 1 or 2 aren’t typically present in pacemaker action potentials.