The absolute refractory period is the period of time when a stimulus of sufficient strength can initiate an Action Potential.

• A. True
• B. False
• C. Only during phase 0
• D. Only in atrial tissue

Answer: (B)

In cardiac tissue, the absolute refractory period is the time when no stimulus can trigger a new action potential because the voltage-gated sodium channels are inactivated. Right after the initial depolarization, these channels cannot reopen, so even a very strong stimulus cannot cause another impulse. As repolarization proceeds and more Na channels recover from inactivation, the cell enters the relative refractory period, during which a stronger-than-normal stimulus may elicit a new action potential.

The statement describes a period during which a stimulus of sufficient strength can initiate an action potential, which actually defines the relative refractory period, not the absolute one. Also, refractory behavior is not restricted to phase 0 and occurs in both atrial and ventricular tissue.