Editing Depolarization (section)

===Depolarization===
[[File:Sodium channel open closed.jpg|thumb|upright|[[Sodium channel#Voltage-gated sodium channels|Voltage-gated sodium channel]]. Open channel ''(top)'' carries an influx of Na<sup>+</sup> ions, giving rise to depolarization. As the channel becomes closed/inactivated ''(bottom)'', the depolarization ends.]]

After a cell has established a resting potential, that cell has the capacity to undergo depolarization. Depolarization is the process by which the membrane potential becomes less negative, facilitating the generation of an action potential.<ref name="Alberts">{{Citation |last1=Alberts |first1=Bruce |title=Ion Channels and the Electrical Properties of Membranes |date=2002 |work=Molecular Biology of the Cell. 4th edition |url=https://www.ncbi.nlm.nih.gov/books/NBK26910/ |access-date=2024-03-07 |publisher=Garland Science |language=en |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter}}
</ref> For this rapid change to take place within the interior of the cell, several events must occur along the plasma membrane of the cell.

While the sodium–potassium pump continues to work, the [[Sodium channel#Voltage-gated sodium channels|voltage-gated sodium]] and [[Voltage-gated calcium channel|calcium channels]]<ref>Shah, V. N., Chagot, B., & Chazin, W. J. (2006). Calcium-Dependent Regulation of Ion Channels. Calcium binding proteins, 1(4), 203–212.
</ref> that had been closed while the cell was at resting potential are opened in response to an initial change in voltage.<ref name="Alberts" /> As a change in the neuronal charge leads to the opening of voltage-gated sodium channels, this results in an influx of sodium ions down their [[electrochemical gradient]]. Sodium ions enter the cell, and they contribute a positive charge to the cell interior, causing a change in the membrane potential from negative to positive. The initial sodium ion influx triggers the opening of additional sodium channels ([[Positive feedback|positive-feedback loop]]), leading to further sodium ion transfer into the cell and sustaining the depolarization process until the positive equilibrium potential is reached.<ref>{{Citation |last1=Grider |first1=Michael H. |title=Physiology, Action Potential |date=2024 |work=StatPearls |url=http://www.ncbi.nlm.nih.gov/books/NBK538143/ |access-date=2024-03-07 |place=Treasure Island (FL) |publisher=StatPearls Publishing |pmid=30844170 |last2=Jessu |first2=Rishita |last3=Kabir |first3=Rian}}</ref>

Sodium channels possess an inherent inactivation mechanism that prompts rapid reclosure, even as the membrane remains depolarized. During this equilibrium, the sodium channels enter an inactivated state, temporarily halting the influx of sodium ions until the membrane potential becomes negatively charged again. Once the cell's interior is sufficiently positively charged, depolarization concludes, and the channels close once more.<ref name="Alberts" />