An action potential is the quick electrical spike a neuron fires to send a message. Click anywhere on the graph (or drag along it) to zoom in on the cell’s outer skin, the membrane, at that moment.
Simplified, not to scale. Voltages and timing are typical textbook values; real neurons vary. Real membranes also have “leak” channels that are always open and help set the resting voltage, and a single action potential barely changes how much Na⁺ and K⁺ is inside the cell.