The sinus node which is located superior vena caval - right atrial junction , is a subepicardial structure.  The node is spindle-shaped  and in adult hearts measures about 3mm in diameter at its widest part and 15mm in length.  A tapering "tail" of the node may be found extending intramyocardially towards the inferior caval orifice.

The sinus node is easily recognised by the light microscope at low magnification. It is made up of small cells grouped together in interconnecting fascicles set in a fibrous tissue matrix. The fibrous matrix becomes more prominent with increasing age. 

Trichrome stains delineate the node, because of its high content of connective tissue.

At the margins of the node is a short transitional area where nodal cells merge into atrial myocardium.

In places, discrete tongues of transitional cells are found which extend into the terminal crest.

ATRIOVENTRICULAR CONDUCTION SYSTEM: Image link

The myocardial bridge connecting the atria to the ventricles across the fibrous atrioventricular valve rings (the penetrating atrioventricular bundle) was found by His in 1893.          

Gross anatomical landmarks to the location of the atrioventricular system are important guides to sugeons who have to perform intracardiac procedures since trauma to any part of the system can produce serious consequences.

The atrioventricular node is located at the apex of an angle formed by the tendinous continuation of the Eustachian valve and the annular insertion of the tricuspid valve.

The coronary sinus completes the base of the triangular shape which is now known as "triangle of Koch".

The tendinous structure (tendon of todaro) inserts into the central fibrous body.

In the adult the atrioventricular node measures about 4mm in width and 8mm in length.  In histological section the compact part of the node is easily recognisable being composed of interconnecting fascicles of small cells, closely adherent to the central fibrous body.

In cross section the node appears like a half-oval lying against the fibrous body. A transitional zone of attenuated myocardial cells extends into the atrial myocardium.

Anteriorly , the node becomes the penetrating bundle as the conduction axis passes through the fibrous annulus.

The penetrating bundle turns to the left as it continues into the branching bundle.

While the penetrating bundle is enveloped by the fibrous annulus, the non-branching and branching bundles are encased by fibrous sheaths.

The bifurcation into left and right bundle branches marks the beginning of the branching bundle.

The right bundle branch is cord-like and frequently a continuation of the nodal-bundle axis. It turns downward and passes intra-myocardially into the substance of the septomarginal trabeculation directly beneath the medial papillary muscle complex.

It then passes subendocardially towards the right ventricular apex and crosses the ventricular cavity within the moderator band before ramifying.

The left bundle branch is morphologically different from the right bundle branch. It descends from the nodal-bundle axis as a sheet of cells within the subendocardial tissues of the aortic outflow tract.

In a case where there is clinical electrocardiographic evidence of rhythm or conduction abnormalities in life.

In general conduction defects such as, for example congenital heart block imply that there is anatomical disruption of the atrioventricular conduction system.

Serial sections are needed to demonstrate the 'break in the wiring' .

Examination of Sino-atrial node:

Examination is most easily made if the superior vena cava is divided 1cm above the sinocaval junction and a cuff of atria is removed.

The resulting tube about 2-3cm in length is opened longitudinally and the tissue is pinned out flat. The cut is made on the wall opposite to the atrial appendage, whose crest can be easily seen.

When fixed the tissue is blocked in series of longitudinal cuts. The node lies in those blocks on and lateral to the crest of the appendage.

Examination of Atrioventricular node and conduction system:

The AV node is examined by taking out a tissue block that contains the triangle of Koch and sectioning in the vertical axis (right angle to septal cusp), starting posteriorly.

 The fewer blocks that are taken and the more exactly they are cut with parallel sides the more likely it is that a good reconstruction will be possible.

As the sections are taken progressively more anteriorly - the AV node, the penetrating bundle and bifurcating bundle appear in the sections.

 

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