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

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.

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.