Introduction

The intrinsic conduction system of the heart is comprised of several specialized subpopulations of cells that either spontaneously generate electrical activity (pacemaker cells), or preferentially conduct this excitation throughout the four chambers of the heart in a coordinated fashion. This tutorial will discuss details of this anatomy, as well as physiologic properties of the system. The cardiac action potential underlies signaling within the heart, and various heart cell (myocyte) populations elicit characteristic waveforms. The active sensing (or recording) of these action potentials is important in both research and clinical studies.

Although each myocyte within the heart has the capacity to conduct an electrical cardiac impulse (be excitable), there are specific myocytes that generate cardiac action potentials and/or preferentially conduct them from the atrial to the ventricular chambers. This cellular network has become known as the "conduction system" [1]. The orderly contractions of the atria and ventricles are regulated by the organized transmission of electrical impulses that pass through these modified cardiac muscle cells; these specialize cells are interposed within the contractile myocardium. More specifically, this intrinsic conduction system is thought to be comprised of the following subpopulations of cells: 1) pacemaker cells, those that spontaneously generate electrical activities; and 2) conduction fibers (in the ventricles, Purkinje fibers) those which preferentially conduct this activity throughout the heart: i.e. composed of larger diameter cell with rapid conduction velocities. Normally, following an initiating activation (or depolarization) within a pacemaker cell, this electrical excitation spreads throughout the heart in a rapid and highly coordinated fashion. This system functionally controls the timing of activities between the atrial and ventricular chambers, allowing for optimized hemodynamic performance. Interestingly, a common global architecture of this conduction system is present in mammals: but significant interspecies differences exist, primarily at the histologic level [2,3].

The History Associated with the Identification of the Conduction System

Discoveries related to the existence of the heart's intrinsic conduction system are relatively recent in medical history and now are basic to knowledge of cardiac function and anatomy. In 1845, Johannes E. von Purkinje first described the ventricular conduction system, and in 1882 Gaskell, an electrophysiologist, coined the phrase heart block. In addition, Gaskell also identified the presence of a slow ventricular activation rate to disassociation with that of the atria [4]. The first description of the mammalian sinoatrial node was reported by Sir Arthur Keith and Martin Flack in 1907, in the Journal of Anatomy and Physiology. Nevertheless it should be noted that still today, novel findings about the functionality of this node are being identified.

The elucidation of the electrical connection from the atrioventricular node through the cardiac skeleton to the ventricular portion of the conduction system, known as the bundle of His, is attributed to Wilhelm His Jr. [6]. Importantly, Tawara later verified the existence of this bundle in 1906 [7]. Due to difficulty in distinguishing the atrioventricular nodal tissue from the surrounding tissue, he also defined the beginning of the bundle of His as the point at which these specialized atrioventricular nodal cells enter the central fibrous body (which delineates the atria from the ventricles). Tawara is also credited with being the first person to clearly identify the specialized conduction tissues (modified myocytes) that span from the atrial septum to the ventricular apex, including the right and left bundle branches and Purkinje fibers.

Walter Karl Koch (1880-1962) was a distinguished German surgeon, who identified a triangular-shaped area in the right atrium of the heart that marks the relative location of the atrioventricular node (known today as Koch's triangle). Further, based on detailed anatomical and histological studies of the hearts of animals and stillborn human fetuses, Koch also observed that the last part of the heart to lose activity when the whole organ dies, is the pacemaker region (ultimum moriens). He postulated that the cardiac region near the opening of the wall of the coronary sinus was thus the true pacemaker of the heart [8,9]; note that the atrioventricular node will elicit an escape rhythm when the sinoatrial node in the right atrium fails.