Definition
An action potential is the brief electrical impulse a neuron generates when it fires. It is the basic event of brain activity — the moment a neuron stops listening and starts speaking. When neuroscientists say a neuron is active, they mean it is producing action potentials.
The defining property is that it is all-or-nothing. A neuron does not fire a little or a lot; it either crosses its threshold and produces a full action potential, or it does not fire at all. There is no partial signal.
Why it matters
How it works
A resting neuron holds a slight voltage difference across its membrane. Incoming excitatory signals nudge that voltage upward; inhibitory signals push it down. When the voltage at the start of the axon crosses a critical threshold, ion channels snap open in a self-reinforcing cascade and an action potential sweeps down the axon.
Because each impulse is identical in size, strength of signal is encoded as firing rate — a strongly stimulated neuron fires many action potentials per second, a weakly stimulated one fires few. After each impulse the neuron briefly resets before it can fire again. Hormones and recent experience can shift the threshold, making a neuron more or less excitable — one way the body's state biases behavior.