Anatomy of the human atrioventricular junctions revisited.

There have been suggestions made recently that our understanding of the atrioventricular junctions of the heart is less than adequate, with claims for several new findings concerning the arrangement of the ordinary working myocardium and the specialised pathways for atrioventricular conduction. In reality, these claims are grossly exaggerated. The structure and architecture of the pathways for conduction between the atrial and ventricular myocardium are exactly as described by Tawara nearly 100 years ago. The recent claims stem from a failure to assess histological findings in the light of criterions established by Monckeberg and Aschoff following a similar controversy in 1910. The atrioventricular junctions are the areas where the atrial myocardium inserts into, and is separated from, the base of the ventricular mass, apart from at the site of penetration of the specialised axis for atrioventricular conduction. There are two such junctions in the normal heart, surrounding the orifices of the mitral and tricuspid valves. The true septal area between the junctions is of very limited extent, being formed by the membranous septum. Posterior and inferior to this septal area, the atrial myocardium overlies the crest of the ventricular septum, with the atrial component being demarcated by the landmarks of the triangle of Koch. The adjacent structures, and in particular the so-called inferior pyramidal space, were accurately described by McAlpine (Heart and Coronary Arteries, 1975). Thus, again there is no need for revision of our understanding. The key to unravelling much of the alleged controversy is the recognition that, as indicated by Tawara, the atrioventricular node becomes the atrioventricular bundle at the point where the overall axis for conduction penetrates into the central fibrous body. There are also marked differences in arrangement, also described by Tawara, between the disposition of the conduction axis in man as compared to the dog.