The myocardium can conduct an electrical impulse in any direction. The actual direction of spread will thus depend on where the activation was initiated. Conduction must occur in all directions from this point. But the predominant direction of spread will be in that direction in which the greatest mass of myocardium is available from the point of initiation.
The sino-atrial or SA node is situated in the right atrium at its junction with the superior vena cava. Normally, the SA node initiates activation of the atria, which causes a wave of contraction to pass across the atria. Following atrial contraction, the impulse is delayed at the atrioventricular (AV) node, located in the septal wall of the right atrium.
Spread of electrical activation
1) Activation of the atrial myocardium begins in the SA node. The radial spread of depolarisation then converges on the AV node, where there is a delay for about 0.1 seconds (AV nodal delay).
2) From here, His-Purkinje fibres allow rapid conduction of the electrical impulse via right and left branches, causing almost simultaneous depolarisation of both ventricles, approximately 0.2 s after the initial impulse has arisen in the SA node.
3) In humans, the ventricular depolarisation starts at the left side of the interventricular septum and moves first to the right across the mid-portion of the septum.
4) It then spreads down the septum to the apex of the heart.
5) It then returns along the ventricular walls to the AV groove, proceeding from the endocardial to the epicardial surface.
6) The last parts of the heart to be depolarised are the posterobasal portion of the left ventricle, the pulmonary conus and the uppermost portion of the interventricular septum.
The predominant direction of spread of atrial activation is to the left and somewhat downwards.
The predominant direction of spread of ventricular activation is downwards and somewhat to the left.