Which phase of the action potential is affected by local anesthetics?

Which phase of the action potential is affected by local anesthetics?

• A. Depolarization phase
• B. Repolarization phase
• C. Hyperpolarization phase
• D. Resting potential

Answer: (A)

Local anesthetics primarily block the depolarization phase. They bind to voltage-gated sodium channels in the nerve membrane, preventing the fast influx of Na+ that normally causes the membrane potential to rise rapidly after resting. Without sufficient Na+ entry, the neuron cannot reach the threshold to trigger an action potential, so initiation and propagation of the nerve impulse are blocked. This effect is more pronounced in active fibers because the channels are in open or inactivated states where the drug binds more readily (use-dependent blockade). Repolarization depends mainly on potassium efflux, hyperpolarization is the brief overshoot after the spike, and resting potential is set by leak channels and ion gradients; these are not the primary targets, so the main action of local anesthetics is to stop the upstroke of the action potential.

Local anesthetics primarily block the depolarization phase. They bind to voltage-gated sodium channels in the nerve membrane, preventing the fast influx of Na+ that normally causes the membrane potential to rise rapidly after resting. Without sufficient Na+ entry, the neuron cannot reach the threshold to trigger an action potential, so initiation and propagation of the nerve impulse are blocked. This effect is more pronounced in active fibers because the channels are in open or inactivated states where the drug binds more readily (use-dependent blockade). Repolarization depends mainly on potassium efflux, hyperpolarization is the brief overshoot after the spike, and resting potential is set by leak channels and ion gradients; these are not the primary targets, so the main action of local anesthetics is to stop the upstroke of the action potential.