Which statement correctly describes the action of a complete sodium channel blocker?

Which statement correctly describes the action of a complete sodium channel blocker?

• A. It prevents phase 0 depolarization
• B. It increases calcium influx
• C. It enhances potassium efflux
• D. It accelerates conduction velocity

Answer: (A)

The main idea is that the rapid upstroke of the action potential (phase 0) is driven by a large inward Na+ current through fast voltage-gated sodium channels. A complete sodium channel blocker stops this current, so the rapid depolarization cannot occur. Without phase 0 depolarization, the cell’s conduction slows dramatically or is blocked, which is why this action is described as preventing phase 0 depolarization. It wouldn’t cause an increase in calcium influx, because the initial depolarization and upstroke are dominated by sodium entry, not calcium. It also wouldn’t enhance potassium efflux; potassium currents assist repolarization later, and blocking sodium channels doesn’t actively push more K+ out. And it certainly wouldn’t speed up conduction velocity; by eliminating the fast Na+ current, the speed of the upstroke—and thus conduction—falls.

The main idea is that the rapid upstroke of the action potential (phase 0) is driven by a large inward Na+ current through fast voltage-gated sodium channels. A complete sodium channel blocker stops this current, so the rapid depolarization cannot occur. Without phase 0 depolarization, the cell’s conduction slows dramatically or is blocked, which is why this action is described as preventing phase 0 depolarization.

It wouldn’t cause an increase in calcium influx, because the initial depolarization and upstroke are dominated by sodium entry, not calcium. It also wouldn’t enhance potassium efflux; potassium currents assist repolarization later, and blocking sodium channels doesn’t actively push more K+ out. And it certainly wouldn’t speed up conduction velocity; by eliminating the fast Na+ current, the speed of the upstroke—and thus conduction—falls.