 Dear students, in this module we shall discuss three major properties of action potentials. These are the refractory period, accommodation and adaptation. Every action potential is followed by a period which is called refractory period. It is the time duration during which an other action potential can either never be generated or it is difficult to fire. This is called refractory period. There are two types of refractory periods. One is the absolute refractory period and the second is relative refractory period. The absolute refractory period is the period during which no new action potential can be generated. This is because a membrane which has just fired action potential, the ion channels have not returned back to the deactivated state. They are now a bit active. It is not possible to reactivate them. While the relative refractory period is the period in which the action potentials are difficult to provoke and they require higher thresholds and stronger depolarization to initiate. This is because the absolute refractory period is deactivated. So if the more threshold is applied, that can generate another action potential. The significance of refractory period is that the interval between two action potentials cannot be shorter than absolute refractory period. The significance of this is that the only a limited number of action potentials can be generated by a nerve cell in a given time. It decreases the fatigue of the nerve fiber. It also permits propagation of discrete impulses and prevents the fusion or summation of impulses. Dear students, another property of action potentials is the property of accommodation. This is a characteristic of excitable membranes. If a neuron is continuously being stimulated by a series of sub-threshold depolarizations, there happens a decrease in the excitability of the membrane. As a result, the required threshold to initiate an action potential increases. It happens due to change in the sensitivity of the membrane channels. Those who are continuously getting stimulated by a series of thresholds become insensitive or less sensitive and then do not produce action potentials on that threshold. In addition to accommodation, the excitable membranes also have the property of adaptation. It is defined as the reduction in the frequency of action potentials in a neuron due to a sustained stimulus. If a stimulus is continuously going on and is not ending, then the membrane generates action potentials for it. Either it reduces it or stops it. This property is called adaptation. There are two types of adaptations. One is the phasic adaptation, also known as phasic response, and the other is tonic response. In phasic response, the neurons adapt rapidly and after generating one or two initial action potentials, they stop generating further action potentials. While in tonic response, the neurons adapt slowly. They keep on firing action potentials repetitively but with reducing frequency. This is a gradual decrease in producing or generation of action potentials.