SAN which is present near the opening of the superior vena cava. SA nodal cells have the highest intrinsic rhythm of spontaneous depolarization (roughly 60- 100/min) which makes them the automatic choice for the pacemaker of the heart.

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The AVN is present behind the endocardial cushions and infront of the coronary sinus. It’s important to remember that the coronary sinus is actually the attritioned left horn of the sinus venosus. AV nodal cells have the second highest intrinsic rhythm (40-60/min). This automatically makes AVN the as the pacemaker of heart in case there’s a damage to the SA nodal cells.

Bundle of HIS originates from the AV node and subsequently branches into two within interventricular septum. These two branches are the right and left bundle branches which ends up froming the HIS Purkinje system that supplies the papillary muscles and the rest of the ventricular myocardium. Papillary muscles are part of the trabeculated region of the ventricles which are derived from the primordial ventricle. Although, Purkinje cells are specialized for conduction only, they still possess an intrinsic rhythm of 35/min which gives them the property of automaticity. Hence, Purkinje system is third in line to take over as the pacemaker of the heart if anything goes wrong with both the SA and AV nodal cells.

The SA and the AV node develop from the sinus venosus. Before the sinus venosus gets incorporated into the right atrium and forms the conducting system of the heart, the primitive atrium serves as the function of the pacemaker. Atrial myocytes around the sinus venosus develop a faster intrinsic rhythm thereby naturally taking over as the pacemaker cells. This means that as the myocardial cells are developing to form atria, they develop this ability to depolarize spontaneously. This allows the primitive heart to start beating by the 22nd day and that too without a true pacemaker, hence the primitive atria starts depolarizing even before the pacemaker is formed. Since sinus venosus is at the caudal end of the heart tube and serves as the inflow region. The initial pulsations are in coherence with the direction of the blood flow i.e., from caudal to the cranial side of the developing heart tube.  Eventually as the sinus venosus is incorporated into the right atrium, the SA node develops from the sinus venosus near the entry of the superior vena cava.

The AV node also develops from the SA node near the opening of the coronary sinus. As the AV node develops, bundle of HIS also develops along with it from the sinus venosus. The bundle of HIS develops within the interventricular septum and divides into right and left bundle branches. The cells around the AV node which become consolidated into forming the Bundle of HIS exhibit the MSX-2 homeobox gene. Purkinje fibers are actually modified contractile myocytes which start to function as conducting fibers when they become connected with Bundle of HIS cells.

Another important structure is the fibrous septum which insulates the ventricles from the depolarization of the atria and vice versa. This fibrous skeleton of the heart develops from the epicardium which is the visceral pericardium of the heart. The cells of the epicardium are derived from the local mesodermal cells around the sinus venosus as well.

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