An electrical impulse stimulates the muscle fibers in the heart to contract. The impulse spreads through the heart in a very organized manner, so that the atria contract first, followed by the ventricles.
The electrical impulse proceeds in the following manner:
- The electrical impulse originates at the sinoatrial (SA) node, which is located in the wall of the right atrium.
- The SA node is the heart's natural pacemaker: it regulates the heart rate.
- The impulse proceeds through the atria, stimulating them to contract.
- After the atria are stimulated to contract, the atrioventricular (AV) node slows the electrical impulse before it proceeds to the ventricles. This pause allows the ventricles to fill with blood before they contract.
- The AV node is located between the atria and the ventricles.
- After the pause, the impulse then proceeds through the ventricles, stimulating them to contract.
- The cardiac conduction system
- EKG showing electrical activity of heart
Any abnormality or disturbance in the impulse can result in a cardiac arrhythmia.
- Boullin J, Morgan JM. The development of cardiac rhythm. Heart. 2005 Jul;91(7):874-5. 
- Page RL. Clinical practice. Newly diagnosed atrial fibrillation. N Engl J Med. 2004 Dec 2;351(23):2408-16. 
- Rubart M, Zipes DP. Mechanisms of sudden cardiac death. J Clin Invest. 2005 Sep;115(9):2305-15. 
- Stieber J, Hofmann F, Ludwig A: Pacemaker channels and sinus node arrhythmia. Trends Cardiovasc Med 2004 Jan; 14(1): 23-8.