The heart has its own electrical pace maker that is in the upper part of the Right atrium(RA). It is a collection of electrical fibres that are highly specialised called the 'Sino-Atrial (SA) node.
The (SA) node creates a number of sparks per minute but this can be increased during exercise or illness and lowered during rest and when a person is very fit.
The (SA) node fires an electrical impulse which causes the Right atrium to contract. This is called A 'P' wave on an electrocardiogram (ECG). The electrical impulses then move to the ' Atrio-ventricular' node (AV) node where it pauses very briefly to allow blood to finish emptying from the Atrium into the Ventricle. This pause is called the 'PR interval' on a ECG. Next the eletrical impulses travel through the Left and Right Bundle Branches, also know as 'His fibres' and 'Purkinje fibres'. This makes them contract and blood is pumped into the Pulmonary artery and the Aorta. On an electrocardiogram this is called the 'QRS complex'. As the Ventricles recover it produces a 'ST segment' and a 'T wave' on the ECG.
Ref: Image from@www.sites.google.com/site/ehgprojectgmu/Dr. Abdulla.M @ www.heartsite.com/html/eletrical_activity.html. Last reviewed 04/04/2010. Accessed 14/04/2010.
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