Editing Cardiac conduction system (section)

===Action potential generation===
{{main|Cardiac action potential}}
Cardiac muscle has some similarities to neurons and skeletal muscle, as well as important unique properties. Like a neuron, a given myocardial cell has a negative [[membrane potential]] when at rest. Stimulation above a threshold value induces the opening of [[voltage-gated ion channel]]s and a flood of [[cations]] into the cell. The positively charged ions entering the cell cause the [[depolarization]] characteristic of an action potential. Like skeletal muscle, depolarization causes the opening of [[voltage-gated calcium channels]]  and release of Ca<sup>2+</sup> from the [[t-tubule]]s. This influx of calcium causes [[calcium-induced calcium release]] from the [[sarcoplasmic reticulum]], and free Ca<sup>2+</sup> causes [[muscle contraction]]. After a delay, [[potassium channel]]s reopen, and the resulting flow of K<sup>+</sup> out of the cell causes [[repolarization]] to the resting state.<ref name=":0">{{Cite web|url = http://cnx.org/contents/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@7.16:127/Anatomy_&_Physiology|title = Cardiac Muscle and Electrical Activity|date = November 7, 2014|access-date = January 2, 2015|website = OpenStax CNX: Anatomy & Physiology|publisher = OpenStax CNX}}</ref><ref>{{Cite web |url=http://www.auburn.edu/academic/classes/zy/560/heart_1/sld008.htm |title=Cardiac Muscle Fibers |access-date=January 2, 2015 |website=ZY 560 Mammalian Physiology |publisher=Auburn University |url-status=dead |archive-url=https://web.archive.org/web/20050601073326/http://www.auburn.edu/academic/classes/zy/560/heart_1/sld008.htm |archive-date=June 1, 2005 }}</ref>

There are important physiological differences between nodal cells and ventricular cells; the specific differences in ion channels and mechanisms of polarization give rise to unique properties of SA node cells, most importantly the spontaneous depolarizations necessary for the SA node's pacemaker activity.