Editing Cardiac conduction system (section)

==Function==
===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.

===Requirements for effective pumping===
In order to maximize efficiency of contractions and [[cardiac output]], the conduction system of the heart has:
* Substantial [[Atrium (heart)|atrial]] to [[Ventricle (heart)|ventricular]] delay. ''This will allow the atria to completely empty their contents into the ventricles; simultaneous contraction would cause inefficient filling and backflow. The atria are electrically isolated from the ventricles, connected only via the [[AV node]] which briefly delays the signal.''
* Coordinated contraction of ventricular cells. ''The ventricles must maximize [[Systole (medicine)|systolic]] pressure to force blood through the circulation, so all the ventricular cells must work together.''
** Ventricular contraction begins at the apex of the heart, progressing upwards to eject blood into the great arteries. ''Contraction that squeezes blood towards the exit is more efficient than a simple squeeze from all directions. Although the ventricular stimulus originates from the AV node in the wall separating the atria and ventricles, the [[Bundle of His]] conducts the signal to the apex.''
** Depolarization propagates through cardiac muscle very rapidly. ''Cells of the ventricles contract nearly simultaneously.''
** The action potentials of cardiac muscle are unusually sustained. ''This prevents premature relaxation, maintaining initial contraction until the entire myocardium has had time to depolarize and contract.''
* Absence of [[tetany]]. ''After contracting, the heart must relax to fill up again. Sustained contraction of the heart without relaxation would be fatal, and this is prevented by a temporary'' '''inactivation''' ''of certain ion channels.''