Normal heart beats are controlled by electrical signals that start in the sino-atrial node. Sino-atrial node is located at the top of the right atrium.The heart is divided into four chambers: two atria at the top and two ventricles on the bottom.Normally, electrical impulse from the sino-atrial node propagatesat atria level, then electrical impulse reaches the AV node from where,through His beam and Purkinje network, is transmitted to the ventricles.
The sinoatrial node is the initiator of electrical impulse, though the cells of the heart could initiate themselves the electrical impulse. The sinoatrial node has the responsibility of all electrical activity of the heart. If the sinoatrial node does not work anymore, then atrioventricular node would generate the impulse and if this one stops, also, then Purkinje network of cells will generate the electrical impulse.