In physiology, a refractory perio is a period of time during which an organ or cell is incapable of repeating a particular action, or (more precisely) the amount of time it takes for an excitable membrane to be ready for a second stimulus once it returns to its resting state following an excitation. It most commonly refers to electrically excitable muscle cells or neurons. This term is similarly used in psychological terms: it is defined as the time it takes for a neuron to repolarize so it can fire again.