Action potential: An action potential, in cellular biology, refers to a change in a cell's membrane potential, or the electrical charge of one side of the membrane compared to the other. Action potentials are the main way neurons in the brain communicate with each other - as a neuron receives a signal from a neighboring neuron, an action potential propagates across the neuron and to the next neighboring cell. Since animal cells are surrounded by a membrane composed of a lipid bilayer, the membrane contains proteins pores that allow electrically charged ions (e.g. potassium [K+], sodium [Na+], chloride [Cl-]) to cross the membrane. When a cell receives a signal, the protein pores open allowing for the influx and efflux of charged ions which rapidly changes the electrical charge inside the cell compared to the outside. This rapid increase in electrical potential is followed by a rapid decrease in electrical potential as the cell moves back toward an equilibrated state.