Editing Cardiac action potential (section)

==Refractory period==
Cardiac cells have two [[refractory period (physiology)|refractory periods]], the first from the beginning of phase 0 until part way through phase 3; this is known as the absolute refractory period during which it is impossible for the cell to produce another action potential. This is immediately followed, until the end of phase 3, by a relative refractory period, during which a stronger-than-usual stimulus is required to produce another action potential.{{sfn|Purves et al.|2008|p= 49}}<ref name="Bullock1977p151">{{Cite book |last=Bullock |first=TH |author-link=Theodore Holmes Bullock |url=https://archive.org/details/introductiontone00theo |title=Introduction to Nervous Systems |last2=Orkand |first2=R |last3=Grinnell |first3=A |publisher=W. H. Freeman |year=1977 |isbn=978-0716700302 |location=New York |page=[https://archive.org/details/introductiontone00theo/page/151 151] |ref={{sfnref|Bullock, Orkand & Grinnell|1977}} |url-access=registration}}</ref>

These two refractory periods are caused by changes in the states of [[sodium channel|sodium]] and [[potassium channels]]. The rapid [[depolarization]] of the cell, during phase 0, causes the membrane potential to approach sodium's [[reversal potential|equilibrium potential]] (i.e. the membrane potential at which sodium is no longer drawn into or out of the cell). As the membrane potential becomes more positive, the sodium channels then close and lock, this is known as the "inactivated" state. During this state the channels cannot be opened regardless of the strength of the excitatory stimulus—this gives rise to the absolute refractory period. The relative refractory period is due to the leaking of potassium ions, which makes the membrane potential more negative (i.e. it is hyperpolarised), this resets the sodium channels; opening the inactivation gate, but still leaving the channel closed. Because some of the voltage-gated sodium ion channels have recovered and the voltage-gated potassium ion channels remain open, it is possible to initiate another action potential if the stimulus is stronger than a stimulus which can fire an action potential when the membrane is at rest.{{sfn|Sherwood|2008|p=316}}