Development of the cardiac conduction system.

Abstract

In the formed heart, it is convention to distinguish working myocardium (the primary function of which is contraction) from the conduction system (the primary function of which is the generation and conduction of the electrical impulse). The conduction system comprises separate components with distinct functions. The SAN, which contains the leading pacemaker, generates the impulse. The impulse is subsequently conducted, via the atrial myocardium, which in this sense is part of the conduction pathway as well, toward the AVN. With a delay, the impulse is then rapidly transmitted from the AVN via the bundle branches and PPN to ensure a coordinated activation of the ventricular myocardium from apex to base. Classic reports cover the anatomy,1 pathology,1 and histology2 of the adult and developing conduction system. The myocytes of the conduction system share with those of the ordinary working myocardium four basic elements: (1) contraction, (2) autorhythmicity, (3) intercellular conduction, and (4) electromechanical coupling. In the early embryonic heart tube, an ECG, similar to an adult ECG, can be recorded, indicating the presence of sequentially activated chambers.3 Given this observation, it is as confusing to accept the presence of a conduction system because it is functionally present as it is to deny its existence because it is not morphologically recognizable. Rather, it is of paramount importance to appreciate that the arrangement of myocyte populations, with distinct contractile, conductive, and pacemaking properties, establishes the coordinated activation of the heart. Departures from these tenets have led to a confusing and fruitless search for so-called “cardiac specialized tissues” during development. The obvious key question is how this arrangement is being achieved. Early cardiac development starts with the formation of a primary heart tube from the cardiogenic mesoderm (Fig 1⇓); this topic has been reviewed recently.4 The primary heart …