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The sinoatrial node is located in the superior wall of the right atrium .
The heart has its own intrinsic regulating system called the conduction system that generates and distributes electrical impulses over the heart to stimulate cardiac muscle fibers or cells to contract. This rhythmic contraction distributes blood to tissues.
The conductive system begins at the sinoatrial node (SA node). This node (located in the superior wall of the right atrium) starts each cardiac cycle by initiating an electric impulse.
Once an impulse is initiated by the SA node the impulse spreads out over both atria causing them to contract simultaneously. At the same time it depolarizes the atrioventricular (AV) node. It is located in the lower portion of the right atrium.
From the AV node a tract of conducting fibers called the atrioventricular bundle or bundle of His runs through the cardiac muscle to the top of the interventricular septum. It then branches to form the right and left bundle branches.
The contraction of the ventricles is stimulated by the Purkinjes fibers . They emerge from the bundle branches and pass into the cells of the myocardium of the ventricles.
Diagram of the conductive system of heart :