<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> <P> The refractory period in cardiac physiology is related to the ion currents that, in cardiac cells as in nerve cells, flow into and out of the cell freely . The flow of ions translates into a change in the voltage of the inside of the cell relative to the extracellular space . As in nerve cells, this characteristic change in voltage is referred to as an action potential . Unlike that in nerve cells, the cardiac action potential duration is closer to 100 ms (with variations depending on cell type, autonomic tone, etc .). After an action potential initiates, the cardiac cell is unable to initiate another action potential for some duration of time (which is slightly shorter than the "true" action potential duration). This period of time is referred to as the refractory period, which is 250ms in duration and helps to protect the heart . </P> <P> In the classical sense, the cardiac refractory period is separated into an absolute refractory period and a relative refractory period . During the absolute refractory period, a new action potential cannot be elicited . During the relative refractory period, a new action potential can be elicited under the correct circumstances . </P> <P> The cardiac refractory period can result in different forms of re-entry, which are a cause of tachycardia . Vortices of excitation in the myocardium (autowave vortices) are a form of re-entry . Such vortices can be a mechanism of life - threatening cardiac arrhythmias . In particular, the autowave reverberator, more commonly referred to as spiral waves or rotors, can be found within the atria and may be a cause of atrial fibrillation . </P>

When does the cardiac muscle tissue return to its resting state