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

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Multiple Choice

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

Explanation:
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.

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