MCAT topics → Nervous and Endocrine Systems → Action Potential (Depolarization, Repolarization, Refractory Periods)

Action Potential (Depolarization, Repolarization, Refractory Periods)

MCAT trap: Attributes repolarization to the Na+/K+ pump instead of voltage-gated K+ channel opening. Repolarization is caused by voltage-gated K+ channels opening and K+ flowing out; the Na+/K+-ATPase restores gradients on a slower timescale.

The action potential is the electrical signal neurons use to transmit information, and on the MCAT it is one of the most heavily tested mechanisms. You need the full sequence cold — resting, depolarization, repolarization, hyperpolarization, return to resting — and critically, which specific ion channel does what and when. The most common wrong answer on this topic: students say repolarization is driven by the Na⁺/K⁺-ATPase. It is not. Repolarization happens because voltage-gated K⁺ channels open and K⁺ flows out. The pump is too slow and plays no role in the millisecond waveform.

What makes this topic brutal for most students is the channel timing. There are two voltage-gated channels (Na+ and K+) and three distinct gating events (Na+ activation, Na+ inactivation, K+ activation/deactivation), and the MCAT loves to exploit confusion between them. Students who vaguely remember 'Na+ in, K+ out' fall apart when a question asks why the absolute refractory period is absolute, or what causes the undershoot below resting membrane potential. Those answers require knowing the specific gate states — not just which ion moved.

The three biggest traps: confusing the Na+/K+-ATPase with voltage-gated K+ channels during repolarization, mixing up the mechanisms of absolute vs. relative refractory periods, and thinking a stronger stimulus makes a bigger action potential. All three of these misconceptions appear in wrong answer choices on the real exam. If you can explain why each one is wrong, you're in good shape.

Common misconceptions

Common mistake
Wrong: Repolarization during an action potential is caused by the Na+/K+-ATPase pumping Na+ out.
Right: Repolarization is caused by voltage-gated K+ channels opening and K+ flowing out; the Na+/K+-ATPase restores gradients on a slower timescale.
The Na+/K+-ATPase is a pump that works on a slow, continuous timescale to maintain concentration gradients — it is not fast enough to drive the rapid repolarization that happens in milliseconds during an action potential. Repolarization is caused by voltage-gated K+ channels opening in response to depolarization, allowing K+ to flow out down its electrochemical gradient and drive the membrane voltage back negative. The pump matters for long-term gradient maintenance, but it plays no role in the moment-to-moment shape of the AP waveform.
Common mistake
Wrong: The absolute refractory period occurs because K+ channels are still open and the threshold is elevated.
Right: The absolute refractory period occurs because voltage-gated Na+ channels are inactivated (inactivation gate closed) and cannot be reopened regardless of stimulus strength.
The absolute refractory period is absolute because voltage-gated Na+ channels are physically inactivated — a separate inactivation gate (the 'ball and chain' h-gate) swings shut and blocks the channel regardless of how strong a stimulus is applied. This is categorically different from the relative refractory period, where Na+ channels have recovered but K+ channels are still open, making it harder (but possible) to reach threshold. If you confuse these two, you'll get the mechanism questions backwards: absolute = Na+ channel inactivation, relative = residual K+ channel opening.
Common mistake
Wrong: A stronger stimulus produces a larger-amplitude action potential.
Right: Action potentials are all-or-none; stimulus strength is encoded by firing frequency, not AP amplitude.
The all-or-none law means that once a stimulus reaches threshold, the action potential fires with a fixed, stereotyped amplitude — the membrane depolarizes to approximately +30–40 mV every single time, regardless of whether the stimulus was barely above threshold or extremely strong. Stimulus strength is instead encoded by firing *frequency*: a stronger stimulus causes the neuron to fire more action potentials per second, not bigger ones. If you see an answer choice saying a stronger stimulus produces a higher-amplitude AP, eliminate it immediately.
Common mistake
Wrong: Hyperpolarization after an action potential is caused by Na+ channels reopening.
Right: Hyperpolarization (undershoot) occurs because voltage-gated K+ channels remain open briefly after the membrane returns to RMP, allowing excess K+ efflux.
Hyperpolarization (the undershoot below resting membrane potential) has nothing to do with Na+ channels — by that point in the AP, Na+ channels are inactivated or closed. The undershoot happens because voltage-gated K+ channels open slowly and close slowly: they're still open briefly after the membrane has already returned to RMP, allowing extra K+ to leave and push the membrane even more negative than resting. The undershoot ends when those K+ channels finally close and the membrane drifts back to RMP.
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What the exam tests

  1. Know the four phases of an action potential in order — depolarization (Na+ rushes in), repolarization (K+ flows out), hyperpolarization/undershoot (K+ channels still open past RMP), and return to resting membrane potential — and which ion movement drives each phase.
  2. Understand the gating behavior of voltage-gated Na+ and K+ channels: Na+ channels activate fast and then inactivate (two separate gates), while K+ channels activate more slowly and simply deactivate — the exam tests whether you know the timing difference and what state each channel is in during each phase.
  3. Distinguish absolute from relative refractory periods mechanistically: the absolute refractory period is caused by Na+ channel inactivation (inactivation gate physically blocks the channel — no stimulus can fire another AP), while the relative refractory period is caused by K+ channels still being open (threshold is elevated but a strong enough stimulus can fire another AP).
  4. Apply the all-or-none law correctly: action potentials either fire fully or not at all once threshold is reached, and a stronger or weaker stimulus does not change the amplitude of the AP — stimulus intensity is encoded by the *frequency* of action potentials, not their size.
  5. Read a voltage-vs-time trace and correctly label each phase, identify when specific channels are open or closed, and predict consequences of pharmacological channel blockade (e.g., what happens if voltage-gated Na+ channels are blocked with tetrodotoxin).

Can you avoid these mistakes?

A student says repolarization happens because the Na+/K+-ATPase kicks in and pumps Na+ out. What's wrong with this explanation, and what actually causes repolarization?
A neuron is in its absolute refractory period. You apply a stimulus twice the normal threshold intensity. Does it fire? Explain your answer in terms of specific channel gate states — not just 'it can't fire.'
On a voltage-vs-time trace, you see the membrane voltage dip below −70 mV after the action potential before returning to resting potential. What is this phase called, what ion movement causes it, and which channels are responsible?
If two stimuli of different intensities both exceed threshold, how does the nervous system encode the difference in intensity — and what does *not* change between the two resulting action potentials?

Related topics

Resting Membrane Potential and Ion Gradients Neuron Structure (Dendrite, Axon, Myelin) Saltatory Conduction and Conduction Velocity Chemical and Electrical Synaptic Transmission

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