Action Potential: When K+ Channels Are Open and Na+ Channels Are Closed

Voltage-gated K⁺ channels open and Na⁺ channels closed describes repolarization (the falling phase after the peak). The reverse, Na⁺ mostly open and K⁺ mostly closed, is the rising phase (depolarization).

Match the phase to the channels

• Resting: both voltage-gated types closed.
• Rising phase: Na⁺ channels open, K⁺ still mostly closed. Na⁺ rushes in.
• Peak: Na⁺ channels inactivate, K⁺ channels begin opening.
• Falling phase: Na⁺ channels closed (inactivated), K⁺ open. K⁺ leaves.
• Undershoot: K⁺ channels still open, since they close slowly. Membrane dips below rest.

Why K⁺ lags

K⁺ channels activate more slowly than Na⁺ channels, so they are only open after the peak.

Common traps

• Calling the peak the Na⁺-open point. At the peak Na⁺ channels are inactivating.
• Calling the undershoot depolarization. It is hyperpolarization.
• Confusing inactivated with closed at rest. Inactivated channels cannot reopen until the membrane repolarizes.