# Neurology Question

**URL:** <https://boards.straightdope.com/t/neurology-question/403361>\
**Category:** Factual Questions\
**Created:** [May 9, 2007, 5:12am UTC](https://boards.straightdope.com/t/neurology-question/403361 "2007-05-09T05:12:02Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![mswas](https://avatars.discourse-cdn.com/v4/letter/m/e99b99/32.png) [@mswas](https://boards.straightdope.com/u/mswas)\
**Post date:** [May 9, 2007, 5:12am UTC](https://boards.straightdope.com/t/neurology-question/403361/1 "2007-05-09T05:12:02Z")

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Ok, so a nerve impulse is fired if it opens its gates to allow the Na+ in and the K+ out, which move the way they do because they move down their concentration gradient. That makes sense to me.

What I don’t understand is, once they are there, and the impulse has been fired, how do they revert? How is it down their concentration gradient to go the opposite direction?

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**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\
**Post date:** [May 9, 2007, 5:16am UTC](https://boards.straightdope.com/t/neurology-question/403361/2 "2007-05-09T05:16:38Z")

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Through the action of the [sodium pump](http://en.wikipedia.org/wiki/Sodium_pump), which entails active transport through the cell membrane.

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**Author:** ![bouv](https://avatars.discourse-cdn.com/v4/letter/b/bc8723/32.png) [@bouv](https://boards.straightdope.com/u/bouv)\
**Post date:** [May 9, 2007, 5:38am UTC](https://boards.straightdope.com/t/neurology-question/403361/3 "2007-05-09T05:38:26Z")

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Yeah, there are several active transport ion channels on the cell membrane. The Na/K ATPase is probably the biggest. It uses ATP (adenosine triphosphate, a source of fuel for several cellular reactions) to move the Na back out and K back in. By moving both at the same time, the charges balance out, so it only works against a concentration gradient, not an electro-chemical one.

It’s also worth noting that calcium also plays a large roll in triggering action potentials in excitable cells. Different cells rely more on Na, some more on Ca.