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 frequencies of isolated (pacemaker) cells in the individual cardiac regions decrease as one goes from the atria to the ventricles. In the atria the frequency amounts initially to ca. 62 beats/min. and in the ventricles only to 25 beats/min. Various transplantation trials with chicken embryos have shown that the beating pattern is influenced by the extracellular matrix. If atrial tissue is transplanted into the ventricle region, it adapts to the beating pattern there.
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.
the embryonic heart
- Sinu-atrial node in the sulcus terminalis
- Av-node (Aschhoff-Tawara node)
- Truncus (His' bundle)
- Right and left pedicle bundle
- Purkinje fibers
The pacemaker for normal heart rhythm is the sinus node. The excitation spreads out over the two atria and extends to the av-node, which also possesses the pacemaker capability. From here, the excitation spreads over the His' bundle and bundle branches and finally via the Purkinje fibers into the ventricular myocardium.