Electrical activity of the heart during a beat

The heart has its own natural pacemaker that regulates the rate of the heart. It is situated in the upper portion of the right atrium, and is a collection of special cells known as the sinus or sino-atrial node. These are able to generate a number of sparks per minute and each spark travels across a specialised electrical pathway to stimulate the muscle walls if the four chambers. This enables the heart to contract and empty in a certain pattern. The upper chambers are stimulated first, followed by aslight delay to allow the two atria to empty then the two ventricles are stimulated in the same way. www.heartsite.com