Diagram of the electrical conduction system of the heart

Sinoatrial node

The sinoatrial (SA) node is a spindle-shaped structure composed of a fibrous tissue matrix with closely packed cells. It is 10-20 mm long, 2-3 mm wide, and thick, tending to narrow caudally toward the inferior vena cava (IVC). The SA node is located less than 1 mm from the epicardial surface, laterally in the right atrial sulcus terminalis at the junction of the anteromedial aspect of the superior vena cava (SVC) and the right atrium (RA).

The artery supplying the sinus node branches from the right coronary artery in 55-60% of hearts or the left circumflex artery in 40-45% of hearts. The artery approaches the node from a clockwise or counterclockwise direction around the SVC–RA junction. [3]

The SA node is densely innervated with postganglionic adrenergic and cholinergic nerve terminals. Neurotransmitters modulate the SA node discharge rate by stimulation of beta-adrenergic and muscarinic receptors. Both beta1 and beta2 adrenoceptors subtypes are present in the SA node. The human SA node contains a more than 3-fold greater density of beta-adrenergic and muscarinic cholinergic receptors than the adjacent atrial tissue. [4]

Internodal and intra-atrial conduction

Anatomic evidence suggests the presence of 3 intra-atrial pathways: (1) anterior internodal pathway, (2) middle internodal tract, and (3) posterior internodal tract.

The anterior internodal pathway begins at the anterior margin of the SA node and curves anteriorly around the SVC to enter the anterior interatrial band, called the Bachmann bundle (see the image below). This band continues to the left atrium (LA), with the anterior internodal pathway entering the superior margin of the AV node. The Bachmann bundle is a large muscle bundle that appears to conduct the cardiac impulse preferentially from the RA to the LA.

The middle internodal tract begins at the superior and posterior margins of the sinus node, travels behind the SVC to the crest of the interatrial septum, and descends in the interatrial septum to the superior margin of the AV node.

The posterior internodal tract starts at the posterior margin of the sinus node and travels posteriorly around the SVC and along the crista terminalis to the eustachian ridge and then into the interatrial septum above the coronary sinus, where it joins the posterior portion of the AV node. These groups of internodal tissue are best referred to as internodal atrial myocardium, not tracts, as they do not appear to be histologically discrete specialized tracts. [3, 5]

Atrioventricular node

The compact portion of the atrioventricular (AV) node is a superficial structure located just beneath the RA endocardium, anterior to the ostium of the coronary sinus, and directly above the insertion of the septal leaflet of the tricuspid valve. It is at the apex of a triangle formed by the tricuspid annulus and the tendon of Todaro, which originates in the central fibrous body and passes posteriorly through the atrial septum to continue with the eustachian valve (see the images below).

In 85-90% of human hearts, the arterial supply to the AV node is a branch from the right coronary artery that originates at the posterior intersection of the AV and interventricular grooves (crux). In the remaining 10-15% of the hearts, a branch of the left circumflex coronary artery provides the AV nodal artery. Fibers in the lower part of the AV node may exhibit automatic impulse formation. The main function of the AV node is modulation of the atrial impulse transmission to the ventricles to coordinate atrial and ventricular contractions. [3, 6]

Bundle of His

The bundle of His is a structure that connects with the distal part of the compact AV node, perforates the central fibrous body, and continues through the annulus fibrosus, where it is called the nonbranching portion as it penetrates the membranous septum. Connective tissue of the central fibrous body and membranous septum encloses the penetrating portion of the AV bundle, which may send out extensions into the central fibrous body. Proximal cells of the penetrating portion are heterogeneous and resemble those of the compact AV node; distal cells are similar to cells in the proximal bundle branches.

Branches from the anterior and posterior descending coronary arteries supply the upper muscular interventricular septum with blood, which makes the conduction system at this site more impervious to the ischemic damage, unless the ischemia is extensive. [7]

Bundle branches

The bundle branches originate at the superior margin of the muscular interventricular septum, immediately below the membranous septum, with the cells of the left bundle branch cascading downward as a continuous sheet onto the septum beneath the noncoronary aortic cusp. The right bundle branch continues intramyocardially as an unbranched extension of the AV bundle down the right side of the interventricular septum to the apex of the right ventricle and base of the anterior papillary muscle. The anatomy of the left bundle branch system may be variable and may not conform to a constant bifascicular division. However, for clinical purposes and electrocardiography (ECG), the concept of a trifascicular system remains useful (see the images below)

Terminal Purkinje fibers

The terminal Purkinje fibers connect with the ends of the bundle branches to form interweaving networks on the endocardial surface of both ventricles, which transmit the cardiac impulse almost simultaneously to the entire right and left ventricular endocardium. Purkinje fibers tend to be less concentrated at the base of the ventricle and the papillary muscle tips. They penetrate only the inner third of the endocardium. Purkinje fibers appear to be more resistant to ischemia than ordinary myocardial fibers. [3]

Innervation of the AV node, His bundle, and ventricular myocardium

The AV node and His bundle are innervated by a rich supply of cholinergic and adrenergic fibers with higher densities as compared with the ventricular myocardium. Parasympathetic nerves to the AV node region enter the heart at the junction of the IVC and the inferior aspect of the LA, adjacent to the coronary sinus ostium.

The autonomic neural input to the heart demonstrates some degree of "sidedness," with the right sympathetic and vagal nerves affecting the SA node more than the AV node and the left sympathetic and vagal nerves affecting the AV node more than the SA node. The distribution of the neural input to the SA and AV nodes is complex because of substantial overlapping innervation.

Stimulation of the right stellate ganglion produces sinus tachycardia with less effect on AV nodal conduction, whereas stimulation of the left stellate ganglion generally produces a shift in the sinus pacemaker to an ectopic site and consistently shortens AV nodal conduction time and refractoriness, but it inconsistently speeds the SA node discharge rate. However, stimulation of the right cervical vagus nerve slows the SA node discharge rate, and stimulation of the left vagus primarily prolongs AV nodal conduction time and refractoriness when sidedness is present. Neither sympathetic nor vagal stimulation affects normal conduction in the His bundle. [3, 4]

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