What structure sends the electrical impulse to stimulate the atria to contract?

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A heartbeat (heart contraction) is started by the sinoatrial node (SA node), a patch of specialized cardiac muscle cells located in the wall of the right atrium near the opening for the superior vena cava. The SA node is the heart's pacemaker. From the SA node, modified cardiac muscle cells carry electrical impulses across the myocardium of both atria, causing them to depolarize and contract together as one (shown with black arrows). While this is happening, other modified cardiac muscle cells carry electrical impulses from the SA node to the atrioventricular (AV) node, located on the right atrium's interatrial septum. It is from this node that the ventricles will be stimulated to contract. The atrioventricular (AV) bundle divides into branches that continue to carry the electrical impulses from the AV node down the interventricular septum towards the heart's apex. Purkinje fibers fan out from the ends of the AV bundle to the walls of the ventricles stimulating the cardiac muscle cells of the ventricular myocardium to depolarize and contract.

What Structure Sends the Electrical Impulse to Stimulate the Atria