Fast AV nodal pathway and structural anatomical features of the AVN
Single-cell features (which include the gene expression patterns of ion channels and connexins of single cells) and anatomical structures are equally important in providing information on conduction velocity.15Thus, the focus of our study was to clarify the conduction relationships among the CN, INE, RN and atrium. Identifying the entrance of the atria into the AVN is very important in understanding the mechanisms driving normal and abnormal AV conduction as well as some arrhythmias associated with the AVN, such as AVNRT. Fibrous tissues separate the cardiac muscle to prevent electrical conduction and provide a structural basis for the direction of conduction. Our results show a striking amount of connective tissue surrounding the CN at the level of almost the entire CN and extending to the forefront until the start of the CN, suggesting that there was no obvious direct connection between the atrium and most of the CN. The end part of the RN (the part connecting with the CN) seemed to be the only pathway connecting the atrium to the conduction tissue in the anterior septum (Figure 3). Because almost all of the retroaortic area presents a large connection with the atrial muscle, without surrounding fibrous tissues, this region is situated in an area that would more readily accept an impulse from the sinus node than the CN and INE. In a study by the M. Jackman team,16 the site of earliest atrial activation was described as being located posterior to (behind) the TT, outside the triangle of Koch, which coincides with our anatomical results. The region connecting the RN and CN was also near the start of the TT. The start of the CN was located at the apex of the Koch triangle, where the TT started on the atrial side (Figure 3). In some studies,17,18 early activation of the left side of the septum during AVNRT could be accounted for by the dispersed distribution of the RN in the atrial septum, containing the left side of the septum (Figure 2C). The descriptions of the histological characteristics and relationships of transitional tissue near the AV junction with the CN in previous studies have been confusing. The morphological features of transitional tissue cells have been described as between those of atrial cells and AV nodal cells, with low Cx43 or Nav1.5 expression.6,12 Some studies have suggested that the transitional cells above the CN may constitute the fast pathway entering the CN.6,12 In our study, we found that fibrous tissue surrounding the CN at the level of almost the entire CN seems unlikely to facilitate conduction in this direction, and the entrance of the AVN is more likely in the more forward position where the distal part of the RN is without surrounding fibrous tissue. Meanwhile, Antz et al.19 described the transitional cells crossing the TT toward the AV node. Microelectrode studies in rabbit hearts have suggested that these anterior transitional cells insert into the AV bundle relatively close to the central fibrous body.20The anatomical location of the transitional cells observed in these studies as the connection approaches the TT and inserts into the common AV bundle is similar to our description of the connection between the RN and CN. As proven by the histological labeling of serial sections in our study, these tissues actually are node-like tissues in the continuation of the RN. In conclusion, the more accurate entrance of the atria into the AVN is more likely the small end part of the RN, which could be proven by accurate patch-clamping techniques in the future.