Editing Repolarization (section)

{{Short description|Change in membrane potential}}
[[File:ActionPotential.png|thumb|A labeled diagram of an [[action potential]]. As seen above, repolarization takes place just after the peak of the action potential, when [[Potassium|K<sup>+</sup>]] ions rush out of the cell.]]
In [[neuroscience]], '''repolarization''' refers to the change in [[membrane potential]] that returns it to a negative value just after the [[depolarization]] phase of an [[action potential]] which has changed the membrane potential to a positive value. The repolarization phase usually returns the membrane potential back to the [[resting membrane potential]]. The efflux of [[potassium]] (K<sup>+</sup>) [[ion]]s results in the falling phase of an action potential. The ions pass through the [[Potassium channel#Selectivity filter|selectivity filter]] of the [[Potassium channel|K<sup>+</sup> channel]] pore.

Repolarization typically results from the movement of positively charged K<sup>+</sup> ions out of the cell. The repolarization phase of an action potential initially results in [[Hyperpolarization (biology)|hyperpolarization]], attainment of a membrane potential, termed the [[afterhyperpolarization]], that is more negative than the [[resting potential]]. Repolarization usually takes several milliseconds.<ref name="HardinBertoni2010">{{cite book | first1 = Jeff | last1 = Hardin | first2 = Gregory Paul | last2 = Bertoni | first3 = Lewis J. | last3 = Kleinsmith | name-list-style = vanc | url = https://books.google.com/books?id=3SygcQAACAAJ | title = Becker's World of the Cell | publisher = Benjamin-Cummings Publishing Company | date = December 2010 | isbn = 978-0-321-71602-6 | page = 389 }}</ref>

Repolarization is a stage of an action potential in which the cell experiences a decrease of voltage due to the efflux of potassium (K<sup>+</sup>) ions along its electrochemical gradient.  This phase occurs after the cell reaches its highest voltage from depolarization. After repolarization, the cell hyperpolarizes as it reaches resting membrane potential (−70 mV in neuron). [[Sodium]] (Na<sup>+</sup>) and potassium ions inside and outside the cell are moved by a sodium potassium pump, ensuring that electrochemical equilibrium remains unreached to allow the cell to maintain a state of resting membrane potential.<ref>{{cite book |last1=Chrysafides |first1=Steven M. |last2=Sharma |first2=Sandeep | name-list-style = vanc |title=Physiology, Resting Potential |url=https://www.ncbi.nlm.nih.gov/books/NBK538338/ |website=StatPearls |publisher=StatPearls Publishing |date=2019|pmid=30855922 }}</ref> In the graph of an action potential, the hyper-polarization section looks like a downward dip that goes lower than the line of resting membrane potential. In this afterhyperpolarization (the downward dip), the cell sits at more negative potential than rest (about −80 mV) due to the slow inactivation of voltage gated K<sup>+</sup> delayed rectifier channels, which are the primary K<sup>+</sup> channels associated with repolarization.<ref>{{cite book | vauthors = Lentz TL, Erulkar SD | date = 2018 | chapter = Nervous System | title = Encyclopædia Britannica | chapter-url = https://www.britannica.com/science/nervous-system/Active-transport-the-sodium-potassium-pump#ref606418 }}</ref> At these low voltages, all of the [[Voltage-gated potassium channel|voltage gated K<sup>+</sup> channels]] close, and the cell returns to resting potential within a few milliseconds. A cell which is experiencing repolarization is said to be in its absolute refractory period.  Other voltage gated K<sup>+</sup> channels which contribute to repolarization include A-type channels and [[Calcium-activated potassium channel|Ca<sup>2+</sup>-activated K<sup>+</sup> channels]].<ref>{{cite book |url= https://www.ncbi.nlm.nih.gov/books/NBK10799/ |title=Neuroscience |publisher=Sinauer Assoc. |year=2001 |isbn=0-87893-742-0 |veditors=Purves D, Augustine GJ, Fitzpatrick D, Katz LC, LaMantia AS, McNamara JO, Williams SM |edition=2nd | location = Sunderland, Mass }}</ref> Protein transport molecules are responsible for Na<sup>+</sup> out of the cell and K<sup>+</sup> into the cell to restore the original resting ion concentrations.<ref>{{cite web |url= https://academic.eb.com/?target=%2Flevels%2Fcollegiate%2Farticle%2Faction-potential%2F3611 | title = Action Potential |work = Britannica Academic | publisher =  Encyclopædia Britannica, Inc |access-date=2019-09-26}}</ref>