The ERP in non-pacemaker action potential spans which phases?

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

Answer: (A)

Understanding the effective refractory period means recognizing the time after an action potential during which a new stimulus cannot trigger another action potential. In non-pacemaker ventricular-type cells, this refractory window runs from the onset of the rapid upstroke (Phase 0) through the early and mid-repolarization periods (Phases 1, 2, and 3). Phase 4 is the resting membrane potential, when the cell is normally excitable again, so it is not part of the refractory period. Therefore, the ERP spans Phases 0 to 3. The other options fail because they omit or misplace phases involved in refractoriness, or include the resting phase that is not refractory.