All voltage-gated Na⁺ channels open.

Enough voltage-gated Na⁺ channels open to allow a second AP at the same threshold.

Only half the number of voltage-gated Na⁺ channels in the membrane open.

Voltage-gated Na⁺ channels begin to recover in a random stochastic process.

Enough voltage-gated Na⁺ channels open to allow a second AP at a threshold about 50% lower than prior to the first AP.