These nerve-like conduits receive impulses from a modified muscle structure called the atrioventricular node. Impulses from the upper chambers of the heart are relayed by this node to large bundles of Purkinje fibers referred to as the Bundle of His. These bundles branch into smaller elements and eventually form terminal ends that burrow into left and right ventricular chamber muscles. As the impulse is passed to the ventricles, the muscles contract and pump blood. The contraction caused by the specialized fibers begins from the bottom of the ventricles and move upwards so that the blood leaves the lower chambers through the pulmonary arteries and the aorta.