Rush Center for Congenital
and Structural Heart Disease




Conduction system

Primary myocardium, found in the early heart tube, give rise to the contracting myocardium (of the atria and ventricles) and the conducting myocardium (nodal and ventricular conducting tissue).  Conducting myocardial tissue is frequently referred to as being highly specialized tissue, giving the implication that they all have a homogenous function.  In reality they differ in that some are slow conducting such as nodal tissue, which in many ways resemble less developed primary myocardium, while others are fast conducting such as ventricular conduction tissue [390].

The embryological origin and formation of the sinus and AV nodal tissue is not clear, on the other hand ventricular conduction system formation is better understood.  Ventricular conduction system starts with the formation of an encircling ring of conducting myocardial tissue around the bulboventricular foramen.  The dorsal portion of the ring will become the bundle of His.  The portion of the ring covering the septum will become the left and right bundle branches.  The anterior portion of the ring is called the septal branch and it disappears during normal embryological development.  Other portions of this specialized tissue which form and later disappear are the right atrioventricular ring bundle and the retroartic branch.  The right atrioventricular ring forms due to the rightward shift of the common atrioventricular valve, which originally connects the common atrium to the primitive (left) ventricle.  This results in shift of the specialized myocardium rightwards in a ring shape around the right AV orifice, only to later on disappear.  The retroarotic branch is formed by as a result of the leftward shift of the outflow tract, causing some of the specialized conducting tissue to move and be situated behind the aorta. [390]