What causes heart muscle to contract?

Cardiac muscle contraction is initiated by an electrical impulse that originates in the sinoatrial (SA) node, which is located in the right atrium of the heart. This electrical impulse spreads through the heart causing the atria to contract, which pushes blood into the ventricles. The electrical impulse then reaches the atrioventricular (AV) node, which is located between the atria and ventricles. The AV node delays the electrical impulse slightly, allowing the atria to fill with blood before the ventricles contract. The electrical impulse then travels down the bundle of His, which is a network of specialized conducting fibers that connect the AV node to the ventricles. The bundle of His divides into the left and right bundle branches, which carry the electrical impulse to the left and right ventricles, causing them to contract. The contraction of the ventricles pumps blood out of the heart and into the arteries.