The constant evolution of medical knowledge and accompanying development of diagnostic and treatment possibilities for arrhythmias and conduction disturbances has reawakened interest in the structure and function of the conduction system of the human heart, especially in the region of the atrioventricular (AV) junction and within the junction itself. Of the large number of studies dealing with the AV junction few focus on the initial zones of the AV node. These were described for the first time by Tawara in 1906. Similarly, Anderson et al. distinguished two origins of the AV node, the left one running towards the basis of the mitral valve and the right one leading towards the tricuspid valve. The differences in length and scale could be the result of the adoption of different reference points.
The study was carried out on the material of 50 human hearts, of both sexes and ranging in age from 22 to 93, which were fixed in 10% formalin and 98% ethanol solution. The tissue obtained was fixed in the 10% formalin solution and, after being sunk in the paraffin, was cut into layers of about 10 μm thick. According to the age of the hearts, every 10^th or 6^th section was stained by the Masson-Goldner method. The preparations were examined under a LEICA 2000 and BIOLAR 2 microscope at magnifications of 2× to 400×.
Each of the 50 examined hearts contained the atrioventricular node and its initial parts. We observed that the initial zone of the AV node is created by an assembly of cells typical for a conduction system that can create three groups that are initially independent of each other and are always arranged around the AV nodal artery. In all the hearts examined we found at least two initial parts of the node: the superior and inferior. These two groups were present in 45 hearts (90%). In the last 5 cases (10%) there was also a middle group. No cases were found either with a single initial group or without any initial groups. In the sections examined the superior group appeared to be first in 27 hearts (54%), while in 23 cases (46%) the inferior group was first. The length of each group was measured from its first appearance to its first direct contact with the second part. The length of the superior part varied from 0.15 to 2.91 mm (mean 0.90 ± 0.6 mm), the inferior from 0.11 to 2.41 mm (mean 0.88 ± 0.6 mm) and the middle from 0.67 to 2.21 mm (mean 1.04 ± 0.7 mm). As mentioned above, in all 50 hearts there was a direct connection between the atrial muscle and the upper origin of AV node. Furthermore, in all sections (100%) the same part of the interatrial septal muscle was connected to the compact part of the node.
Additionally, in 3 cases (6%) we were able to observe direct connections between the muscle fibres running from the fasciculus limbicus inferior to the initial zone of the AV node: in 2 cases (4%) with the superior group and in 1 case (2%) with the inferior group. In 8% of the material the atrial muscle of the supra-orificial zone made direct contact with the superior initial group and the compact zone of the node and in 10% there was contact between the suborificial muscle and the inferior group and the compact part of the node. This configuration was not observed in relation to the middle and inferior groups.