Action Potential Phases: Depolarization, Repolarization, and Recovery

Neurons transmit information by generating brief electrical spikes called action potentials. An action potential is a rapid, self‑propagating change in the membrane potential that travels along the axon, allowing the signal to reach synaptic terminals. The event is divided into three sequential phases: depolarization, repolarization, and recovery (also called hyperpolarization). Understanding each phase clarifies how ion gradients and voltage‑gated channels convert a small stimulus into an all‑or‑none electrical signal.

Why the Three Phases Matter

Each phase reflects a distinct set of ion movements that together ensure the neuron can fire repeatedly without loss of signal fidelity. The phases also set the limits for how fast a neuron can fire again, because the membrane must reset before the next stimulus can trigger another action potential.

1. Depolarization – The Rising Phase

At rest, a typical neuron has a membrane potential of about –60 mV (the resting membrane potential). When excitatory inputs raise the membrane potential to the threshold (≈ –55 mV), voltage‑gated sodium (Na⁺) channels open rapidly.

• Na⁺ influx: Sodium ions rush into the cell down their electrochemical gradient, driven by both concentration and electrical forces.
• Positive feedback: The influx makes the inside less negative, opening even more Na⁺ channels (a classic positive‑feedback loop).
• Peak voltage: Within ~1 ms, the membrane potential overshoots to about +30 mV, the “overshoot” of the action potential.

This rapid rise is the depolarization phase, and it is the only part of the action potential that can be triggered by an external stimulus. The voltage‑gated Na⁺ channels then enter an inactivated state, preparing them for the next phase.

Source: StatPearls outlines the opening of voltage‑gated Na⁺ channels during depolarization [1]; OpenStax describes the threshold and overshoot [2]; Kenhub details the depolarization step [3]; LibreTexts confirms the rapid Na⁺ influx as the cause of depolarization [4].

2. Repolarization – The Falling Phase

As Na⁺ channels inactivate, voltage‑gated potassium (K⁺) channels open, but more slowly. Potassium ions now leave the cell, driving the membrane potential back toward the negative resting value.

• K⁺ efflux: Outward K⁺ flow removes positive charge from the interior.
• Closure of Na⁺ channels: Inactivated Na⁺ channels stop the inward Na⁺ current.
• Repolarization speed: The membrane potential typically returns to near‑resting levels within 2–4 ms.

The combined effect of closing Na⁺ channels and opening K⁺ channels produces the repolarization phase. This phase also defines the absolute refractory period, during which another action potential cannot be initiated because Na⁺ channels are still inactivated.

Sources: StatPearls and OpenStax both explain the role of K⁺ channels in repolarization [1][2]; Kenhub lists repolarization as the falling phase caused by K⁺ efflux [3]; LibreTexts repeats this mechanism [4].

3. Recovery (Hyper‑polarization) – Returning to Rest

After repolarization, K⁺ channels often remain open a little longer, allowing excess K⁺ to leave the cell. This creates a brief “undershoot” where the membrane potential becomes slightly more negative than the resting level, typically around –70 mV**. This is called hyper‑polarization or the recovery phase.

• Excess K⁺ efflux: The membrane potential may dip below the resting value.
• Na⁺/K⁺ pump activity: The Na⁺/K⁺ ATPase restores the original ion distribution over seconds, re‑establishing the resting gradient.
• Relative refractory period: During hyper‑polarization, a stronger-than‑normal stimulus can trigger another action potential, but the threshold is higher than usual.

Recovery ensures the neuron is ready for the next signal while preventing immediate re‑excitation that could cause uncontrolled firing.

All four sources note a brief hyper‑polarization after repolarization, describing it as a result of continued K⁺ conductance and pump activity [1][2][3][4].

Putting the Phases Together – A Timeline

This timeline shows how quickly each phase occurs and which ion channels dominate at each moment.

Concrete Example: The Squid Giant Axon

The classic experimental system for studying action potentials is the Loligo (squid) giant axon, which is >0.5 mm in diameter and easily recorded from. In this axon:

• Resting potential ≈ –65 mV.
• Threshold ≈ –55 mV.
• Peak depolarization ≈ +30 mV.
• Hyper‑polarization trough ≈ –75 mV.

When a brief current injection raises the membrane to threshold, Na⁺ channels open within ~0.1 ms, generating the rapid upstroke. K⁺ channels open a millisecond later, pulling the voltage back down. The subsequent undershoot lasts ~1 ms before the Na⁺/K⁺ pump restores the original ionic distribution. This model matches the textbook description in all four sources, confirming that the three‑phase pattern is universal across vertebrate and invertebrate neurons.

Common Misconceptions

Self‑Check Questions

1. What ion movement initiates depolarization?
   Answer: Influx of Na⁺ through voltage‑gated Na⁺ channels.
2. Why does the membrane become more negative than the resting potential during recovery?
   Answer: Because K⁺ channels stay open longer, allowing excess K⁺ to leave the cell (hyper‑polarization).
3. During which phase is the absolute refractory period observed?
   Answer: During the repolarization phase, when Na⁺ channels are inactivated.

Next Steps in Learning

After mastering the three phases, explore how action potentials propagate along myelinated versus unmyelinated axons, and how synaptic transmission translates the electrical signal into chemical neurotransmitter release.