Sodium channel activation in fast response cardiac cells occurs during which phase of the action potential?

Sodium channel activation in fast response cardiac cells occurs during which phase of the action potential?

• A. Phase 0
• B. Phase 1
• C. Phase 2
• D. Phase 3

Answer: (A)

Activation of the voltage-gated sodium channels drives the rapid upstroke of the action potential in fast response cardiac cells. When the membrane depolarizes toward threshold, these channels quickly open, allowing a large inward Na+ current that shifts the voltage rapidly from resting levels toward positive values. This swift, steep depolarization is the phase 0 upstroke. The channels then quickly inactivate, leading into the brief notch of phase 1 and the subsequent plateau and repolarization phases. So the opening of the sodium channels—their activation—occurs during the rapid depolarization phase.

Activation of the voltage-gated sodium channels drives the rapid upstroke of the action potential in fast response cardiac cells. When the membrane depolarizes toward threshold, these channels quickly open, allowing a large inward Na+ current that shifts the voltage rapidly from resting levels toward positive values. This swift, steep depolarization is the phase 0 upstroke. The channels then quickly inactivate, leading into the brief notch of phase 1 and the subsequent plateau and repolarization phases. So the opening of the sodium channels—their activation—occurs during the rapid depolarization phase.