The conduction system
With today's technology, the first heart beats in an embryo can be detected already in stage 8 (ca. the 21rst - 22nd day). One finds thereby that the various cardiac regions have their own patterns of beating.
The first morphologically visible differentiation of the conduction system in human embryos is the sinu-atrial node (stage 14). It is located in the wall of the right sinus horn near its opening in the right atrium, i.e., in the sulcus terminalis.
The atrioventricular conduction system, the av-node or Aschoff-Tawara's node, becomes discernible somewhat later (stages 15-16). It lies at the dorsal circumference of the av-canal in the inner layer of the myocardium at the beginning of the dorsal av-septum. From it derives the His' bundle, which extends over the dorsal av-septum and forms the connection between the atrial and ventricular myocardium Here it divides into three subendocardial bundle branches. They extend into the cardiac apex region where a separation into fine Purkinje's fibers occurs. In situ, this conduction system forms itself from specialized myocardial cells. Once the sinu-atrial node (stages 14-16) as well as the av-node and His' bundle have been differentiated, the cardiac frequency increases rapidly and reaches 140 beats/min. The region with the highest frequency (sinus region) takes over the pacemaker function.