During pumping action of heart, the heart muscles cause rhythmic contraction and relaxation of the heart chambers in a specific sequence. The rhythmic, sequential functioning of the cardiac chamber is maintained by sino-atrial node (SA node), atrio-ventricular node (AV node), bundle of His and Purkinje fibres.
The SA node situated in the upper, lateral wall of the right atrium is a small, flattened strip of muscle fibre that is 1.5cm x 3mm in size. The fibres of the SA node are closely associated with the muscles of auricles. SA node is capable of generating action potential that can travel throughout the auricles. The velocity of conduction is 0.3m/sec. The excitation from the SA node stimulates the AV node. The AV node in turn conducts the stimulus to bundle of His and Purkinje fibres. These myocardial fibres are found all over the wall of the ventricles. In the conduction of stimulus through the AV node and the fibrous system there is a delay in transmission.