The stimuli which result in systole contraction of the cardiac muscle originate with the heart itself: automatism of the heart. The heart contains specialized muscle tissue, the impulse-conducting system, which spontaneously generates rhythmical local impulses and which conducts them to stimulate the rest of the heart muscle (myocardium) to contract. The impulse-conducting system is composed of the sinus node (the heart pacemaker), the atrioventricular node (Aschoff-Tarawa node), the bundle of His, and the Purkinje fibers.
The sinus node lies at the anterior margin of the orifice of the superior vena cava and radiates into the working muscle of the right atrium, from which the impulse reaches the atrioventricular node, which is situated in the right atrium near the orifice of the coronary sinus. The atrioventricular node extends into the trunk of the bundle of His that pierces the cardiac skeleton. At the upper margin of the muscular part of the intraventricular septum, the bundle of His divides into two branches; these run along each side of the septum beneath the endocardium to the base of the papillary muscles. The left branch fans out; the right runs mainly across the moderator band to the papillary anterior muscle.
The terminal fibers of the bundle of His, called Purkinje fibers, merge with cardiac muscle. Impulses can originate from any part of the impulse-conducting system. The spontaneous impulse frequency of the sinus node (about 70/min sinus rhythm is, however, greater than that of the atrioventricular node (about 50-60/min, AV rhythm) or that of the bundle of His (45-25/min, ventricular rhythm). As a rule, therefore, the heart works in coordinated manner determined by the sinus node (pacemaker), because the subsequent center remains silent.