The portion of an action potential during which no impulse can initiate depolarization is called the

• A. Absolute refractory period
• B. Effective refractory period
• C. Relative refractory period
• D. Refractory period of diastole

Answer: (B)

The main idea is how neurons and cardiac cells control when they can respond to stimulation, which is governed by refractory periods. The moment you’ve just had an action potential, there’s a period during which no impulse can start another depolarization at all — that’s the absolute refractory period. It lasts while the voltage-gated sodium channels remain inactivated right after the upstroke, so no matter how strong a stimulus is, you can’t get a new action potential. After this, the relative refractory period begins, during which a strong enough stimulus can evoke depolarization but the response is weaker or abnormal because some channels haven’t fully recovered. The term effective refractory period is used in some contexts to describe when a propagating impulse cannot be conducted, which overlaps with the later part of the absolute period and the early part of the relative period, but the phrase that matches “no impulse can initiate depolarization” is the absolute refractory period.