The Action Potential

Electricity in biology#

The nervous system doesn't transmit information via continuous current the way a copper wire does. Instead, neurons fire discrete pulses — brief, sharp spikes of electrical voltage that travel along the axon at speeds up to 120 m/s. These pulses are called action potentials, and they are the fundamental unit of neural communication.

Every sensation you feel, every muscle movement, every thought is encoded as a pattern of action potentials: which neurons fire, how often, and in what sequence. Understanding how a single action potential is generated is the foundation of neuroscience, anesthesiology, cardiology, and the design of drugs that affect the nervous system.

The resting membrane potential#

A neuron at rest maintains a voltage difference across its membrane of about −70 millivolts — the inside of the cell is more negative than the outside. This is the resting membrane potential.

How does a cell maintain this charge difference? Through two mechanisms working together:

Ion pumps: The sodium-potassium pump (Na⁺/K⁺-ATPase) actively expels 3 Na⁺ ions for every 2 K⁺ ions it imports, consuming ATP. This builds up high Na⁺ concentration outside and high K⁺ concentration inside.

Leak channels: The membrane is more permeable to K⁺ than to Na⁺ at rest. K⁺ ions drift outward down their concentration gradient, but the negative charge left behind pulls them back. The equilibrium for K⁺ occurs near −90 mV. Because Na⁺ also leaks in slightly, the true resting potential settles around −70 mV.

The threshold and what triggers a spike#

The cell is not idle at rest. It's dynamically maintaining −70 mV against constant ion drift. When excitatory inputs push the membrane potential upward past a critical threshold — around −55 mV — something qualitative changes.

Voltage-gated sodium channels open. These channels are closed at rest but open rapidly when voltage rises past threshold. Na⁺ rushes in (high outside concentration + negative inside voltage both pull it inward), further depolarizing the membrane — which opens even more Na⁺ channels. This is positive feedback, and it's nearly instantaneous.

Watch the voltage trace. The slow rise toward threshold is the summation of inputs from other neurons. When threshold is crossed, the spike is inevitable — the Na⁺ influx drives voltage from −70 mV all the way to +40 mV in under a millisecond.

The four phases#

Depolarization: Na⁺ channels open. Voltage shoots from −70 mV to +40 mV. The inside of the cell briefly becomes positive.

Repolarization: Na⁺ channels inactivate (they close even if voltage is still high). Simultaneously, voltage-gated K⁺ channels open. K⁺ flows out rapidly, pulling voltage back down.

Hyperpolarization (undershoot): K⁺ channels close slowly. They're still open when voltage returns to −70 mV, so extra K⁺ exits, pulling voltage below resting potential to about −80 mV.

Recovery: The Na⁺/K⁺ pump slowly restores ion concentrations, and the leak channels return the membrane to −70 mV. The cell is ready to fire again.

The refractory period#

During and immediately after the spike, a neuron cannot fire again — the inactivated Na⁺ channels need time to recover. This is the absolute refractory period (about 1–2 ms). Following that is a relative refractory period during hyperpolarization when only a much stronger stimulus can trigger another spike.

The refractory period has a crucial consequence: action potentials can only travel in one direction. At any point along the axon, the region behind the spike is refractory and cannot be re-excited, so the wave propagates only forward.

How information is encoded#

A single action potential is all-or-nothing — it fires at full amplitude or not at all. Try it: push the stimulus below threshold and the membrane just twitches and relaxes; push it past −55 mV and you get the full spike — and crucially, cranking the stimulus higher doesn't make the spike any bigger.

Because the spike size carries no information, the stimulus intensity isn't encoded in the spike's height (it's always the same) but in:

• Firing rate: a stronger stimulus produces a higher frequency of spikes (rate coding).
• Which neurons fire: different sensory receptors innervate different neurons (labeled-line coding).
• Temporal patterns: the precise timing of spikes relative to each other can carry information (temporal coding).

Your brain is solving an extraordinarily complex decoding problem at every moment, reconstructing a world from patterns of identical voltage spikes in billions of neurons.

From neurons to the heart#

The cardiac cells of the heart generate their own action potentials — similar in mechanism but shaped differently by different ion channel ratios. The ECG you see on a heart monitor is the summed electrical activity of billions of cardiac cells firing in coordinated waves across the heart.

Drugs like lidocaine (a local anesthetic) and certain antiarrhythmics work by blocking voltage-gated Na⁺ channels, preventing action potentials in specific tissues. Understanding the action potential is directly understanding the mechanism of drugs, diseases, and the electrophysiology that underlies both the nervous system and the heart.