What Is an Action Potential? Brain Signals Explained

An action potential is a brief, rapid electrical signal that travels along a neuron’s axon, the long fiber that carries signals away from the cell body. It is the main way neurons send information over long distances, to other neurons, to muscles, and to glands. Every movement, thought, sensation, and memory depends on action potentials.

A brief change in charge

An action potential is a temporary change in the electrical charge across a neuron’s membrane. When a neuron receives enough stimulation, it generates one, and the signal travels from the cell body down the axon to its terminals. One of its most important features is that it doesn’t fade with distance. The signal arriving at the end of the axon is as strong as the one that started.

That makes an action potential quite different from electricity running through a metal wire, a common misunderstanding. In a wire, electrons flow along the metal. In a neuron, the signal is created by sodium and potassium ions moving across the cell membrane, producing a short-lived shift in electrical charge that is renewed as it goes.

Crossing the threshold

An action potential begins only when a neuron receives enough incoming signals to reach a threshold. Think of it like a mousetrap: small nudges do nothing, but once enough pressure builds, it snaps all at once. The analogy is limited, since a neuron resets within milliseconds and can fire again, but it captures the all-at-once character of the signal. Once the threshold is reached, voltage-gated sodium channels open, sodium ions rush into the neuron, and the inside of the cell becomes briefly more positive. That is the start of the action potential.

Traveling like a wave

Once started, the action potential moves along the axon like a wave, or like a line of dominoes. As one section of the axon becomes electrically active, it triggers the next section to generate its own action potential, and the process repeats until the signal reaches the axon terminals.

Many axons are wrapped in myelin, an insulating sheath. In these myelinated axons, the signal travels much faster because it is regenerated only at small gaps in the myelin called nodes of Ranvier. This is called saltatory conduction, often described as the signal jumping from node to node.

Handing off the message

When the action potential reaches the axon terminal, it triggers the release of neurotransmitters, chemical messengers that cross the synapse and pass the signal on to the next neuron, muscle, or gland. In this way, information keeps moving through the nervous system.

Without action potentials, neurons could not send information, muscles could not contract, and the brain could not process thoughts, memories, or emotions. Nearly everything the nervous system does depends on this form of electrical signaling.

Related: What Is the Brain? Definition, Structure, and Function · What Is a Neuron? · What Is a Synapse? · What Are Neurotransmitters? · How Does the Brain Work?

References

1. Grider MH, Jessu R, Kabir R. Physiology, Action Potential. StatPearls. Updated May 8, 2023.
2. Caire MJ, Reddy V, Varacallo MA. Physiology, Synapse. StatPearls. Updated March 27, 2023.
3. Cooper GM. Signaling Molecules and Their Receptors. The Cell: A Molecular Approach. NCBI Bookshelf.