<P> Refractoriness is the fundamental property of any object of autowave nature (especially excitable medium) not to respond on stimuli, if the object stays in the specific refractory state . In common sense, refractory period is the characteristic recovery time, a period of time that is associated with the motion of the image point on the left branch of the isocline u _̇ = 0 (\ displaystyle (\ dot (u)) = 0) (for more details, see also Reaction - diffusion and Parabolic partial differential equation). </P> <P> In physiology, a refractory period 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 . Absolute refractory period corresponds to depolarization and repolarization, whereas relative refractory period corresponds to hyperpolarization . </P> <P> After initiation of an action potential, the refractory period is defined two ways: The absolute refractory period coincides with nearly the entire duration of the action potential . In neurons, it is caused by the inactivation of the Na channels that originally opened to depolarize the membrane . These channels remain inactivated until the membrane hyperpolarizes . The channels then close, de-inactivate, and regain their ability to open in response to stimulus . </P> <P> The relative refractory period immediately follows the absolute . As voltage - gated potassium channels open to terminate the action potential by repolarizing the membrane, the potassium conductance of the membrane increases dramatically . K ions moving out of the cell bring the membrane potential closer to the equilibrium potential for potassium . This causes brief hyperpolarization of the membrane, that is, the membrane potential becomes transiently more negative than the normal resting potential . Until the potassium conductance returns to the resting value, a greater stimulus will be required to reach the initiation threshold for a second depolarization . The return to the equilibrium resting potential marks the end of the relative refractory period . </P>

The stage in action potential that immediately follows depolarization is known as