Action Potential Stages

Why This Matters

The action potential is the fundamental language of the nervous system. It's how neurons communicate across distances, whether your brain is commanding your fingers to type or your sensory neurons are signaling pain. Exams expect you to understand more than just voltage values. You need to grasp ion channel dynamics, electrochemical gradients, and the feedback mechanisms that make neural signaling both rapid and reliable. This topic connects directly to synaptic transmission, neural coding, and pharmacology (since many drugs target specific phases of the action potential).

When you study these stages, focus on the why behind each phase: which ions are moving, which channels are responsible, and how each stage sets up the next. Don't just memorize that resting potential is $-70 \text{ mV}$. Know that it exists because of the sodium-potassium pump and leak channels working together. Understanding the mechanism means you can reason through novel questions, predict drug effects, and explain your thinking on free-response questions.

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Setting the Stage: Establishing Membrane Potential

Before a neuron can fire, it must maintain a stable electrochemical gradient. This "ready state" is actively maintained by ion pumps and selective membrane permeability, creating the potential energy that powers the action potential.

Resting State

• Membrane potential sits at approximately $-70 \text{ mV}$. This negative charge reflects the unequal distribution of ions across the membrane.
• The sodium-potassium pump ($\text{Na}^+/\text{K}^+$-ATPase) actively transports $3 \text{ Na}^+$ out for every $2 \text{ K}^+$ in, maintaining the concentration gradient at the cost of ATP. That 3-for-2 ratio itself contributes a small net negative current, but the pump's bigger role is sustaining the concentration gradients that drive ion flow through leak channels.
• Voltage-gated $\text{Na}^+$ channels are closed, while $\text{K}^+$ leak channels remain open. Because the membrane at rest is far more permeable to $\text{K}^+$ than to $\text{Na}^+$, potassium diffuses outward down its concentration gradient, leaving behind negative charges. This is the dominant reason the resting potential is negative.

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Initiation: Reaching Threshold

The transition from rest to action potential depends on reaching a critical voltage. This threshold mechanism ensures that neurons respond in an all-or-none fashion. Weak stimuli don't produce weak action potentials; they produce none at all.

Depolarization

• A threshold potential of approximately $-55 \text{ mV}$ must be reached for an action potential to fire. This is the point of no return.
• At threshold, enough voltage-gated $\text{Na}^+$ channels open that inward $\text{Na}^+$ current exceeds outward $\text{K}^+$ leak current. $\text{Na}^+$ rushes down its electrochemical gradient into the cell.
• A positive feedback loop then drives further depolarization: entering $\text{Na}^+$ depolarizes the membrane further, which opens more $\text{Na}^+$ channels, which admits more $\text{Na}^+$. This is why the rising phase is so rapid.

What gets the neuron to threshold in the first place? Typically, graded potentials from synaptic inputs or sensory receptor currents summate at the axon hillock. If their combined depolarization reaches $-55 \text{ mV}$, the action potential initiates.

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The Spike: Rapid Voltage Change

The rising phase and peak represent the most dramatic voltage changes in the action potential. The speed of this phase, often less than 1 millisecond, depends on the density and kinetics of voltage-gated sodium channels.

Rising Phase

• Membrane potential shoots toward $+30 \text{ mV}$ as $\text{Na}^+$ influx overwhelms the resting negative charge.
• This is a regenerative process: depolarization spreads to adjacent membrane regions, opening more voltage-gated $\text{Na}^+$ channels and enabling propagation along the axon.
• Neurons with higher densities of $\text{Na}^+$ channels or faster channel kinetics show steeper rising phases and can conduct signals more quickly.

Peak

• Maximum depolarization reaches approximately $+30 \text{ mV}$. The membrane briefly becomes positive inside relative to outside. The potential doesn't reach the $\text{Na}^+$ equilibrium potential (about $+60 \text{ mV}$) because $\text{Na}^+$ channels don't stay open long enough, and $\text{K}^+$ channels are beginning to open.
• $\text{Na}^+$ channel inactivation begins as the inactivation gate (a ball-and-chain-like structure on the channel's intracellular side) swings into the pore. This is a distinct conformational state from simply being closed. It automatically limits how positive the membrane can become.
• Voltage-gated $\text{K}^+$ channels start opening with a slight delay, setting up the transition to repolarization. This delay is critical for the action potential's shape.

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Recovery: Restoring the Gradient

After the spike, the neuron must return to its resting state. This recovery involves both passive ion flow through open channels and the continued work of the sodium-potassium pump to restore original ion distributions.

Repolarization

• $\text{K}^+$ efflux drives the membrane potential negative as voltage-gated $\text{K}^+$ channels reach peak conductance. Potassium flows out of the cell down both its concentration gradient and the now-positive electrical gradient.
• $\text{Na}^+$ channels remain inactivated during this phase, preventing any competing sodium influx and ensuring the membrane repolarizes cleanly.
• Membrane potential returns toward $-70 \text{ mV}$. This phase resets the electrical state, though ion concentrations haven't yet been fully restored by the pump.

Hyperpolarization (Undershoot)

• Membrane potential temporarily dips to approximately $-80 \text{ mV}$, more negative than resting potential.
• Delayed $\text{K}^+$ channel closing causes this undershoot. These channels opened in response to depolarization but close slowly after repolarization, so $\text{K}^+$ continues to leave the cell past the resting potential.
• The increased threshold for subsequent firing during this phase contributes to the relative refractory period and helps ensure unidirectional propagation of action potentials.

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Protection: Ensuring Signal Fidelity

The refractory period isn't just recovery time. It's a functional mechanism that shapes how neurons encode information. By limiting firing frequency and ensuring unidirectional propagation, the refractory period transforms action potentials into a reliable signaling system.

Refractory Period

• Absolute refractory period occurs while $\text{Na}^+$ channels are inactivated. No stimulus, regardless of strength, can trigger another action potential. This corresponds roughly to the rising phase, peak, and most of repolarization.
• Relative refractory period follows as $\text{Na}^+$ channels recover from inactivation but hyperpolarization persists. A stronger-than-normal stimulus can trigger firing during this window, but the threshold is elevated.
• Unidirectional propagation is guaranteed because the membrane region that just fired is in its absolute refractory period and cannot immediately fire again. This forces the action potential to travel forward along the axon, away from the region that already depolarized.

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Quick Reference Table

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Self-Check Questions

1. Which two phases both involve $\text{K}^+$ efflux, and what distinguishes their effects on membrane potential?

2. A neuron is at $-60 \text{ mV}$. Will an action potential fire? What must happen if it doesn't?

3. Compare $\text{Na}^+$ channel inactivation at the peak with $\text{Na}^+$ channel closure at rest. Why does this distinction matter for the refractory period?

4. If a drug blocked voltage-gated $\text{K}^+$ channels, which phases would be affected and how would the action potential shape change?

5. Using your knowledge of the refractory period, explain why action potentials propagate in only one direction along an axon.