Anatomy and Physiology 2e · The Nervous System and Nervous Tissue
The Action Potential
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In 30 seconds
Neurons carry information over long distances as brief, self-regenerating electrical signals called action potentials — rapid, temporary reversals of the voltage difference across the neuron's membrane. This voltage difference is the Membrane potential Voltage difference across the cell membrane Full entry →, and the action potential is the "language" the nervous system uses to send a signal from a fingertip to the spinal cord or from the brain to a muscle.
At rest, a neuron is polarized: its inside is more negative than its outside, a value commonly taught as about −70 mV (millivolts). This resting potential exists because ion pumps and leak channels keep ions unevenly distributed across the membrane. When a stimulus pushes the membrane potential to a Threshold The potential that must be reached to trigger an action potential Full entry →, voltage-gated ion channels snap open and the potential swings sharply positive, then back negative, in about 1–2 milliseconds. Because the event either happens fully or not at all, action potentials are all-or-none. The nervous system encodes information not by changing the size of each spike, but by changing how many spikes fire per second and along which pathways.
Why this matters
The action potential is the fundamental unit of nervous system signaling, so nearly everything in neuroscience builds on it. Every sensation, thought, movement, and reflex depends on these voltage spikes. Understanding them explains why a local anesthetic numbs a region (it blocks the ion channels that generate the signal), why demyelinating conditions slow or stop nerve conduction, and why seizure medications target ion channels. For nursing and health-science students, action-potential mechanics are classic exam territory: threshold, refractory periods, and ion movement appear on board exams in many forms, and the same ion-channel logic underlies the electrocardiogram.
The college version
Core Concepts
The resting membrane potential: the neuron's "charged battery"
Three mechanisms work together at rest:
- The Na⁺/K⁺ ATPase pump moves three sodium ions (Na⁺) out of the cell and two potassium ions (K⁺) in, building concentration gradients.
- Leak channels let small amounts of K⁺ (and some Na⁺) drift down their gradients, which actually sets the resting voltage.
- Large intracellular anions (proteins, phosphates) cannot cross the membrane, keeping the interior negative.
The result is a commonly taught resting potential near −70 mV — a reference concept, not a fixed constant.
Ion channels: the gates that change the voltage
- Leak channels are always open and set the resting potential.
- Voltage-gated channels open in response to voltage changes. The stars of the action potential are the voltage-gated Na⁺ and K⁺ channels.
- Ligand-gated channels open when a chemical messenger binds (next topic).
The voltage-gated Na⁺ channel has two gates: an activation gate that opens on Depolarization Membrane potential becomes less negative (Na⁺ in) Full entry → and an inactivation gate that plugs the channel a fraction of a millisecond later. This design produces the Refractory period Window when firing is impossible or harder Full entry →.
Depolarization: the rising phase
A stimulus opens some Na⁺ channels; Na⁺ rushes in (driven by its concentration gradient and the electrical attraction of the negative interior), making the inside less negative. At threshold — commonly taught as about −55 mV — the change becomes self-amplifying: more Na⁺ channels open, more Na⁺ enters, the membrane depolarizes further. This positive-feedback loop drives the potential sharply upward, overshooting toward the Na⁺ equilibrium potential (commonly taught as near +60 mV). The overshoot is brief because Na⁺ channels inactivate almost immediately.
Repolarization and hyperpolarization: the falling phases
Two events end the rising phase:
- Na⁺ channel inactivation stops Na⁺ entry.
- Voltage-gated K⁺ channels open more slowly, letting K⁺ flood out. Losing positive charge makes the interior negative again — Repolarization Membrane potential returns toward rest (K⁺ out) Full entry →.
Because the K⁺ channels stay open a bit too long, K⁺ keeps leaving even after the membrane is near rest, producing a brief Hyperpolarization Membrane potential becomes more negative than rest Full entry → (undershoot) before leak channels and the pump restore −70 mV. The Na⁺/K⁺ pump slowly restores the ion gradients themselves, like recharging a battery.
Propagation: how the signal travels
Depolarization at one patch acts as the stimulus for the next: local current flows along the axon, opening voltage-gated Na⁺ channels just ahead. In unmyelinated axons this proceeds continuously; in myelinated axons the signal "jumps" between gaps called nodes of Ranvier, where the channels are concentrated — Saltatory conduction "Jumping" propagation between nodes of Ranvier Full entry →. Myelin insulates the axon, speeds conduction, and saves energy, because only the nodes need to restore ion gradients.
Refractory periods: the one-way street
- Absolute refractory period: all Na⁺ channels are inactivated, so no stimulus — however strong — can trigger another action potential. This caps the firing rate and forces one-way travel.
- Relative refractory period: some Na⁺ channels have recovered and K⁺ channels are still open, so the membrane is hyperpolarized; only a stronger-than-usual stimulus can fire a new action potential.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Action potential | Graded potential | Graded potentials are small, local, and vary with stimulus strength; action potentials are large, traveling, all-or-none |
| Depolarization | Repolarization | Depolarization moves toward positive (Na⁺ in); repolarization returns toward negative (K⁺ out) |
| Repolarization | Hyperpolarization | Repolarization returns to rest; hyperpolarization overshoots past rest |
| Absolute refractory period | Relative refractory period | Absolute: nothing can fire; relative: only a stronger-than-normal stimulus can |
| Threshold | Resting potential | Threshold (≈ −55 mV) is the trigger; resting potential (≈ −70 mV) is the baseline — a classic test trap |
Eli explains
The same idea, in plain words
Explain it like I’m 10
An action potential is like a row of dominoes on a shelf. If you flick the first domino hard enough, it tips the next, which tips the next, and the "tip" travels the whole row without stopping — each domino tips fully or not at all. A weak flick that doesn't reach the first domino does nothing. After a domino falls, you can't flick it again until it stands back up, which is why the signal only travels one way. Myelin is like putting the dominoes farther apart with a ramp between them, so the tip jumps faster.
Worked example
Imagine you touch a hot mug by accident. Receptors in your fingertip produce a small local voltage change — a graded potential. If the summed stimulus reaches threshold at the first node of the axon, an action potential fires:
- Voltage-gated Na⁺ channels open; Na⁺ floods in; the membrane depolarizes and overshoots.
- Na⁺ channels inactivate; K⁺ channels open; K⁺ exits; the membrane repolarizes, then briefly hyperpolarizes.
- Meanwhile, the depolarization spreads to the neighboring patch, which fires next — the wave travels up your arm to the spinal cord.
- Behind the wave, the membrane is in its absolute refractory period, so the signal cannot loop backward.
- The firing rate tells the CNS how intense the stimulus is: a hotter mug → higher frequency → stronger pain.
The same sequence — depolarization, repolarization, propagation, refractory period — occurs in motor neurons and cardiac muscle, which is why the concept transfers across the body.
Key takeaways
- Resting membrane potential is commonly taught as about −70 mV, maintained by the Na⁺/K⁺ pump, leak channels, and impermeable intracellular anions.
- Action potentials are all-or-none: at threshold (≈ −55 mV) the full spike fires; subthreshold stimuli produce nothing.
- Depolarization = Na⁺ enters; repolarization = K⁺ exits; hyperpolarization = K⁺ lingers.
- Na⁺ channel inactivation causes the absolute refractory period; the relative refractory period allows firing only with a stronger stimulus.
- Conduction is continuous in unmyelinated axons, saltatory in myelinated ones; myelin = speed + efficiency.
- Signal strength is coded in frequency, not spike size.
- Voltage values are textbook approximations — verify against current texts.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
What three mechanisms establish the Resting membrane potential Steady voltage at rest, commonly taught as about −70 mV Full entry →?
Show answer
The Na⁺/K⁺ ATPase pump (3 Na⁺ out, 2 K⁺ in, using ATP), leak channels (mainly K⁺), and large intracellular anions that cannot cross the membrane.
Why is an action potential described as "all-or-none"?
Show answer
Once threshold is reached, positive-feedback Na⁺ entry drives the full spike; a subthreshold stimulus produces no action potential at all — there is no "half" signal.
What ion enters during depolarization, and what ion exits during repolarization?
Show answer
Sodium (Na⁺) enters during depolarization; potassium (K⁺) exits during repolarization.
Why can an action potential not travel backward along an axon?
Show answer
The membrane behind the advancing wave is in its absolute refractory period (Na⁺ channels inactivated), so it cannot be re-excited.
How does myelination change the speed and efficiency of conduction?
Show answer
Myelin insulates the axon and concentrates voltage-gated channels at the nodes of Ranvier, so the signal propagates by saltatory "jumping" — faster, and requiring fewer ions to pump back, saving energy.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Membrane potential
- Voltage difference across the cell membrane
- Resting membrane potential
- Steady voltage at rest, commonly taught as about −70 mV
- Threshold
- The potential that must be reached to trigger an action potential
- Depolarization
- Membrane potential becomes less negative (Na⁺ in)
- Repolarization
- Membrane potential returns toward rest (K⁺ out)
- Hyperpolarization
- Membrane potential becomes more negative than rest
- Voltage-gated channel
- Channel that opens or closes with voltage changes
- Refractory period
- Window when firing is impossible or harder
- Saltatory conduction
- "Jumping" propagation between nodes of Ranvier
Sources & references
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