Anatomy and electrophysiology of the sinoatrial node

INTRODUCTION

The sinoatrial (SA) node is normally the most rapid pacemaker. However, what we call the SA node is actually the integrated activity of pacemaker cells in a compact region of the right atrium [1,2]. These several thousand cells depolarize and produce action potentials almost synchronously. They seem to be influencing each other through cell-to-cell coupling, a process that has been called mutual entrainment [3,4]. The location of the primary pacemaker may move among groups of cells within the region of the SA node. It is estimated that only about 1 percent of the cells in the SA node act as the leading pacemaker [5].

BLOOD SUPPLY

Past studies have suggested that over 90 percent of hearts have only one arterial branch to the SA node [6], and common wisdom was that the right coronary artery supplied the SA node in about 55 percent of hearts. More recent data, however, suggest that the SA nodal artery may take one of six different routes, and two or more branches to the node may be present in about 54 percent of hearts studied morphologically [6]. This suggests that collateral blood supplies are common and perhaps explains the rarity of infarction of the SA node.

ANATOMY

The sinus node is located at the junction of the high right atrium at the junction of the crista terminalis, a thick band of atrial muscle at the border of the atrial appendage, and the superior vena cava. The sinus node is located beneath the epicardial surface of the crista terminalis; there is a layer of atrial muscle between the node and the endocardium so that it does not occupy the entire thickness of the atrial myocardium.

A characteristic feature of the sinus node is extensive connective tissue, mainly collagen and fibroblasts. In the center of the node are the characteristic "P" cells, which are the leading pacemaker cells. There is a gradual transition in cell type over several millimeters from the center in all directions to the periphery of the node. Perinodal cells, sometimes called transitional or (T) cells, transmit the electrical impulse from the SA node to the right atrium. SA nodal dysfunction may result from abnormalities in impulse generation by the P cells or in conduction across the T cells. (See "Manifestations and causes of the sick sinus syndrome" and "Sinoatrial nodal pause; arrest; and exit block".)

The conduction velocity within the sinus node is very slow compared to non-nodal atrial tissue. This is a result of poor electrical coupling arising from the relative paucity of gap junctions in the center of the node compared to the periphery [2]. These gap junctions may result in preferential conduction pathways for the propagation of the action potential from the center to the atrial muscle and might provide the structural substrate for the transitional zone, enabling the sinus node to drive the surrounding atrial muscle without being suppressed by this tissue [7].