Talk:Afterhyperpolarization

AHP, undershoot, and refractory period

What is the relationship between afterhyperpolarization, the undershoot phase, and the refractory period? It seems to me that people use them different ways, so the exact terminology is a bit confusing.

My impression is that AHP and the undershoot phase are equivalent. The AHP is partially responsible for the refractory period, and the AHP time period occurs within the refractory period. However, some people lump undershoot phase and refractory period together, so I don't know... Gaodifan (talk) 22:50, 20 September 2010 (UTC)

By the way, thank you for doing such a good job expanding this page from a stub!

As for your question, I noticed that issue too. The image treats the AHP as though it were the same thing as the refractory period, which it really isn't. We ought to ask Wikipedia:Graphic Lab for a revised version of the image, labeled correctly, which I'll try to do when I get around to it.

The answer to your question is that "afterhyperpolarization" and "undershoot" are, as you suspected, the same thing, with the former a more technical term. Both mean that the voltage goes more negative than baseline. But refractory period means the phase during which a second action potential cannot yet be initiated. The AHP is probably the single most important part of that (because voltage stays below threshold), but there are other mechanisms as well, such as voltage-dependent inactivation of sodium channels, that occur independently and have differing time courses. --Tryptofish (talk) 16:48, 21 September 2010 (UTC)

Not because the voltage stays below the threshold. During the absolute refractory period, there is no threshold (it is infinite), during relative refractory period it is higher than normal.128.127.38.168 (talk) 22:45, 6 November 2012 (UTC)

Most reliable sources would define threshold such that it always exists, ie is never really infinite, but is not attainable. --Tryptofish (talk) 23:08, 6 November 2012 (UTC)

This still hasn't been addressed. I propose a simple fix to change the text in the image label to something a little clearer for now, for example "Afterhyperpolarisation is one of the processes that contribute to the refactory period" rather than the current second sentence. Chris Alexander (ThirstyFerret) (talk) 14:35, 27 November 2011 (UTC)

Done. Yes, I completely forgot about this. Thanks for that excellent suggestion. --Tryptofish (talk) 17:49, 27 November 2011 (UTC)

As far as I know, the hyperpolarization only leads to the relative refractory period, during which a second action potential CAN be generated to some extent. This in contrary to the absolute refractory period, which does not have much to do with hyperpolarization, but with inactivation of voltage dependent sodium channels. — Preceding unsigned comment added by 213.200.184.19 (talk) 16:44, 6 October 2012 (UTC)

To some degree, it's a matter of terminology, but I guess it's true that a neuronal action potential can, in principle, always be generated as long as sodium channels can be activated, so that would be any time that they aren't inactivated. There's some time overlap between when the sodium channels are inactivated and the calcium-activated potassium channels that mediate the AHP are open, but it's possible for graded depolarizing currents to depolarize the membrane up to sodium channel threshold later during the AHP. But then, the AHP has been ended: you can't have an AHP when the membrane is no longer hyperpolarized! All of that being said, I don't really see a way of putting that information on this page, although perhaps it might belong on pages about refractoriness. --Tryptofish (talk) 17:19, 6 October 2012 (UTC)

As I am used to, the AHP is used to refer to the slow hyperpolarization effect (e.g. calcium sensitive K-currents) that causes spike-rate adaptation, and not to the fast undershoot (inactivation of Na-channels and activation of K-channels). It should be discussed that the slow and fast effects have different causes. This, by the way, does not distinguish the absolute from the relative refractive period. The relative refractory period already starts when the sodium gates are deinactivated enough to be able to generate a nett inward current upon (a larger than usual) stimulus. Article needs some polishing (sorry, no time) 128.127.38.168 (talk) 22:45, 6 November 2012 (UTC)

What you call the "fast undershoot" is what most reliable sources simply call the falling phase of the action potential spike. It's a decreasing depolarization, not a hyperpolarization (relative to resting potential). --Tryptofish (talk) 23:08, 6 November 2012 (UTC)