<Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (April 2013) (Learn how and when to remove this template message) </Td> </Tr> <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>

The absolute refractory period in cardiac muscle cells is