Action Potential Phases and Refractory Periods

1. The phases of the action potential

The action potential is a brief (1–2 ms) all-or-none voltage spike with named phases. The resting phase is the stable −70 mV baseline. Depolarization is the rising phase, when voltage-gated Na⁺ channels open and Na⁺ enters. Overshoot is the portion where V_m is positive (above 0 mV), approaching E_Na (≈ +60 mV). Repolarization is the falling phase, when Na⁺ channels inactivate and K⁺ exits. Hyperpolarization, also called the undershoot, is the brief dip below rest caused by K⁺ channels staying open after V_m returns.

2. Sodium channel gating

The voltage-gated sodium channel has two gates. The activation gate is closed at rest and opens rapidly at threshold, admitting Na⁺. The inactivation gate is open at rest but closes more slowly after activation, stopping Na⁺ entry. The channel cannot reopen until the membrane repolarizes and the gates reset — the basis of refractoriness.

3. Voltage-gated potassium channels (delayed rectifier)

Voltage-gated potassium channels open in response to depolarization but with a delay (the delayed rectifier): they open slowly and stay open through repolarization. Their K⁺ efflux drives the membrane back down and produces the undershoot.

How it works

1. A graded input depolarizes the membrane to threshold.
2. Na⁺ activation gates open and Na⁺ rushes in (depolarization → overshoot).
3. Na⁺ inactivation gates close while delayed-rectifier K⁺ channels open.
4. K⁺ efflux repolarizes the membrane and briefly hyperpolarizes it.
5. Na⁺ channels reset during the absolute then relative refractory periods.
6. The cell returns to rest, ready to fire at a limited maximum rate.

Common confusions

Memory aids

"Open, close, leak out, reset." Sodium activation gates open (upstroke), inactivation gates close (peak), potassium leaks out through delayed rectifiers (downstroke), then everything resets during the refractory period.

Quick review

Topic Recap

The action potential is an all-or-none spike produced by a fast Na⁺ influx (activation gate) terminated by Na⁺ inactivation, with a delayed K⁺ efflux (delayed rectifier) restoring the membrane and briefly hyperpolarizing it (undershoot). The absolute and relative refractory periods, set by sodium-channel recovery, cap firing frequency and guarantee one-way propagation.

Knowledge Check

1. What ion and channel states produce the rising phase of the action potential?
2. Why can no stimulus fire a second action potential during the absolute refractory period?
3. What causes the undershoot (hyperpolarizing afterpotential)?
4. How do refractory periods limit firing frequency and enforce directionality?
5. How do the activation and inactivation gates differ?

Answers and Rationales

1. Voltage-gated Na⁺ channels with activation gates open drive Na⁺ influx; positive feedback opens more channels. Rationale: Na⁺ entry toward E_Na is the upstroke.
2. Na⁺ channels are inactivated and cannot reopen until the membrane repolarizes; no stimulus can reopen an inactivated channel. Rationale: inactivation is a voltage-reset, not a force to overcome.
3. Delayed-rectifier K⁺ channels stay open after V_m reaches rest, so continued K⁺ efflux drives V_m below −70 mV. Rationale: the K⁺ conductance outlasts the spike.
4. A new spike cannot start until Na⁺ channels recover, capping the rate; and the just-fired region is refractory, so current spreads only forward. Rationale: refractoriness is the physical basis of both limits.
5. The activation gate is closed at rest and opens quickly on depolarization; the inactivation gate is open at rest and closes slowly after activation. Rationale: opposite states, different time scales.