'And the Beat Goes On' The cardiac conduction system: the wiring system of the heart.

Exp Physiol. 2009 Jul 10; Boyett MThe Cardiac Conduction System (CCS), consisting of the sinoatrial node, atrioventricular node and His-Purkinje system, is responsible for the initiation and coordination of the heart beat. In the last decade, our understanding of the CCS has been transformed. Immunohistochemistry used in conjunction with anatomical techniques has transformed our understanding of its anatomy: arguably, we now understand the position of the sinoatrial node (not the same as in medical textbooks) and our new understanding of the atrioventricular node anatomy means that we can compute its physiological and pathophysiological behaviour. Ion channel expression in the CCS has been shown to be fundamentally different to that in the working myocardium. Dysfunction of the CCS has previously been attributed to fibrosis, but it is now clear that remodelling of ion channels plays an important role in dysfunction during ageing, heart failure and atrial fibrillation. Differences in ion channel expression may even be responsible for the bradycardia in the athlete and differences in heart rate among different species (such as human and mouse). Recent work has highlighted less well known components of the CCS, including tricuspid, mitral and aortic rings and even a third (retroaortic) node. These additional tissues do not participate in the initiation and coordination of the heart beat and instead they are likely to be the source of various life-threatening arrhythmias. During embryological development, all parts of the CCS have been shown to develop from the primary myocardium of the linear heart tube partly under the action of the transcription factor, Tbx3.