What event must be reached to open voltage-gated Na+ channels during an action potential?

• A. Threshold voltage is reached
• B. Resting potential is maintained
• C. Repolarization completes
• D. Hyperpolarization occurs

Answer: (A)

Reaching the threshold voltage is the trigger that opens voltage-gated Na+ channels. At rest, these channels are closed, so no Na+ enters. A strong enough depolarizing stimulus must bring the membrane to a specific level, the threshold, where the activation gates swing open. Once opened, Na+ rushes in, amplifying depolarization in a positive feedback loop and driving the rapid upstroke of the action potential. After the peak, the channels inactivate and K+ exits to repolarize. If the membrane never reaches threshold, the channels stay closed and no action potential occurs; resting potential being maintained or hyperpolarization occurring later are outside the initial opening event, and repolarization completes after Na+ entry has ceased.