Action Potential

What is an action potential?

An action potential is a brief electrical change that travels along a neuron or another excitable cell. In a neuron, it carries a signal along the axon.

Stages of an Action Potential

In the classic neuronal sequence, voltage-gated ion channels respond when membrane voltage reaches a threshold.

1. Sodium flows into the cell, making the membrane voltage less negative: depolarization.
2. Sodium-channel inactivation and increased potassium flow then help bring the voltage back toward its resting level: repolarization.
3. The voltage may briefly dip below that level, called hyperpolarization, before settling.

Purves and colleagues’ textbook explains this sequence in more detail. Different excitable cells use different combinations of channels, so their action potentials can have different shapes.

How the signal reaches another cell

At a chemical synapse, an action potential arriving at the axon terminal can open calcium channels. Calcium entering the terminal triggers neurotransmitter release. The electrical signal has traveled along the neuron; the chemical messenger now carries the signal across the synapse.

Example: from a nerve signal to muscle contraction

When a motor-neuron action potential reaches the nerve terminal beside a skeletal muscle fiber, the neuron releases acetylcholine at the neuromuscular junction. This transmitter depolarizes the muscle membrane and can start an action potential in the muscle fiber itself. That second signal spreads across the membrane and into T-tubules, leading to calcium release from the sarcoplasmic reticulum inside the muscle. The released calcium permits the contraction machinery to work. The nerve signal, the chemical junction, and the muscle signal are distinct steps in the same sequence. OpenStax explains this sequence.