Signals are sent through gap junctions, using connexions. Bundle of His is a group of organized collagen that separates it from the AV node on histology.
The bundle of His consist of connection between the AV node and the left and right bundle branch located in the deep, dense connective tissue. Additionally, in dilated cardiomyopathy, it was found that the left nodal extension is longer. Due to the connection of the left nodal extension and the CN, a substrate is formed allowing slow conduction and potential re-entrant arrhythmias within the AV node. Comparatively the Bundle of His is composed of elongated cells. Decreased striation density also occur near the CN in comparison to the surrounding cells. Myofibrils adjacent to the SP exhibit decreased density versus closer to the CN. The SP contains small cells while the longer FP contains cells with a larger diameter. Heterogeneity within the AV node could account for dual pathways. Cell size dictates conduction velocity with the larger cells promoting faster conduction and smaller cells conducting at a slower velocity. The pacemaker cells are oval containing sparse and randomly organized myofibrils with numerous sarcoplasmic reticulum. AV node consists of numerous types of cells including rounded cells, transitional cells, Purkinje cells, and myocardial cells. The slow pathways are located in the inferior nodal extension while the fast pathway is less defined. The AV junction is found to have two conduction pathways a slow pathway (SP) and a fast pathway(FP). The lower nodal bundle extends to the rightward inferior nodal extension (INE) which spreads along with the tricuspid valve toward the coronary sinus, and the leftward nodal extension spreads from the CN along the tendon of Todaro. AV node divides into a compact node (CN) and a lower nodal bundle. The AV junction consists of the AV node and the bundle of his.
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