In a neuron, the excitatory postsynaptic potential (EPSP) is a change in the electric polarization of the membrane that makes the neuron more likely to fire an action potential. An action potential of a neuron is a chemical reaction that is caused by the outflow of positive ions from the body of the neuron. What this actually means in simpler terms is the ability of the nerve to fire again and again if necessary rather than firing once and then going inactive. An example of this could be something like attending a Fourth of July firework show. A person tends to jump or flinch when a loud firework goes off and as the show goes on fireworks keep exploding and the watcher keeps jumping or flinching with each explosion.