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.