Impulses reaching the AV node from the atria are delayed a little as they pass through the trunk and crura. The impulses first reach the papillary muscles and their contraction closes both atrioventricular valves. Further rapid spreading of the impulses causes simultaneous contraction of both ventricles and blood is ejected in to the pulmonary trunk and aorta. Although the heart can beat by itself each stroke is initiated at the SA node, inherent activity is influenced by the autonomic nervous system. Sympathetic fibers to the heart carry impulses that produce an increase in the rate of impulses generated at the SA node. The heart beats faster. Para sympathetic activity on the other hand slows down the rate.

There is in fact a three-tier system of rhythmic activity in the heart. First, the muscle fibers themselves have a built-in capacity for contracting rhythmically. If the SA node is destroyed the muscle still contracts, although at a slower rate. The second tier of control is the SA node which under normal circumstances dictates the frequency of contraction. Finally the autonomic nervous system influences the rate according to the particular requirements of the body for blood. One must not of course forget the direct effects of noradrenalin from the adrenal gland.