The conduction system of the heart or cardiac conduction system is a collection of nodes and specialized conduction cells that are responsible for the initiation of the normal heart cycle and coordinate the contractions of cardiac chambers.
Conduction system of heart consists of the following
- Sinoatrial node
- Atrioventricular node
- Atrioventricular bundle (bundle of His)
- Purkinje fibers
The conducting system of the heart provides the heart its automatic rhythmic beat to pump efficiently and in a coordinated way.
Cardiac conduction system works as follows
- SA node creates an action potential or excitation signal.
- The wave of excitation spreads across the atria and makes them contract.
- The signal is delayed at AV node [this causes desired delay in ventricular contraction]
- It is then conducted into the bundle of His, down the interventricular septum.
- The bundle of His and the Purkinje fibers spread the wave impulses along the ventricles, causing them to contract.
The conducting system of the heart does not have nervous tissue. It consists of cardiac muscle cells and conducting fibers that are specialized for initiating impulses and conducting them rapidly.
This system regulates the function so that both the atria contract together. Both ventricles also do so but atrial contraction occurs first and the ventricles follow.
Components of the Conduction System of Heart
Sinoatrial Node or SA Node
SA node is a spindle-shaped collection formed by a fibrous tissue matrix with closely packed cells, also called pacemaker cells, and is located in the upper wall of the right atrium, at the junction where the superior vena cava enters. It is located less than 1 mm from the epicardial surface.
SA node is 10-20 mm long and 2-3 mm wide. It slightly narrows caudally toward the inferior vena cava.
The cells of the SA node can spontaneously generate electrical impulses.
These impulses spread via gap junctions across both atria, resulting in atrial contraction. Atrial contraction is also called atrial systole. The contraction of atria causes blood to move from atria to the ventricles.
SA node is supplied either by branches from the right coronary artery [in about 60% people] or the left circumflex artery.
Internodal and intra-atrial conduction pathways
There are three intra-atrial pathways
- Anterior internodal pathway
- Middle internodal tract
- Posterior internodal tract
The anterior internodal pathway begins at the anterior margin of the SA node and curves anteriorly around the superior vena cava to enter the anterior interatrial band. This band is called the Bachmann bundle and is a large muscle bundle that conducts the cardiac impulse preferentially from the right atrium to the left atrium.
Bachmann bundle continues to the left atrium (LA), with the anterior internodal pathway entering the superior margin of the AV node.
The middle internodal tract begins at the superior and posterior margins of the sinus node. It goes behind the superior vena cava and the crest of the interatrial septum. From here, it descends in the interatrial septum to reach 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 superior vena cava and along the crista terminalis to the eustachian ridge and then into the interatrial septum above the coronary sinus. Here, it joins the posterior portion of the AV node.
The crista terminalis is a smooth-surfaced, thick portion of the heart muscle at the opening into the right atrial appendage. On the external aspect of the right atrium, corresponding to it is a groove called sulcus terminalis.
The Eustachian ridge or valve is also called the valve of the inferior vena cava and is a ridge of variable thickness in the right atrium.
Atrioventricular Node or AV node
The atrioventricular node is a superficial structure located within the atrioventricular septum, near the opening of the coronary sinus and anterior to it. It lies directly above the insertion of the septal leaflet of the tricuspid valve. The coronary sinus is formed by veins draining the heart.
AV node works to modulate the atrial impulse transmission to the ventricles. After the electrical impulse spreads across the atria, they converge at the atrioventricular node which acts to delay the impulses by approximately 120 ms. This gives atria enough time to fully eject blood into the ventricular contraction.
The impulse then passes from the atrioventricular node into the atrioventricular bundle.
AV node is supplied by the branch of the right coronary artery in about 85% of people. In the remaining, it is a branch of the left circumflex coronary artery.
Atrioventricular Bundle [Bundle of His]
It is a continuation of the AV node and transmits the electrical impulse from the AV node to the Purkinje fibers of the ventricles.
Bundle of His begins from the AV node and continues through the annulus fibrosus of the ventricle and penetrates the membranous septum. Proximal cells of the penetrating portion resemble AV node whereas distal cells are similar to cells in the proximal bundle branches.
The upper muscular interventricular septum is supplied by the branches from the anterior and posterior descending coronary arteries. Due to the richness of the supply, the Bundle of His is not damaged unless ischemia is extensive.
Bundle of His descends down the membranous part of the interventricular septum, before dividing into two main bundles:
- Right bundle branch– conducts the impulse to the Purkinje fibers of the right ventricle.
- Left bundle branch– conducts the impulse to the Purkinje fibers of the left ventricle.
The bundle branches originate at the superior margin of the muscular interventricular septum, immediately below the membranous septum.
The left bundle branch is a kind of continuous sheet onto the septum whereas the right bundle branch continues intramyocardially as an unbranched extension of the AV bundle.
The anatomy of the left bundle branch system may be variable.
The Purkinje fibers are terminal structures in the conduction system of the heart. These connect with the ends of the bundle branches to form interwoven networks on the endocardial surface of both ventricles. This network enables transmission of the electric impulse simultaneously to the entire right and left ventricular endocardium. Purkinje fibers penetrate only the inner third of the endocardium.
They are also more resistant to ischemia than the ordinary myocardial fibers.
They are abundant with glycogen and have extensive gap junctions.
Nerve Supply of Conduction System of Heart
The SA node is densely innervated. It is supplied by postganglionic adrenergic and cholinergic nerve terminals. It has both beta1 and beta2 adrenoceptors and muscarinic cholinergic receptors which are about 3 times than the adjacent atrial tissue.
The rate at which the SA node generates impulses is influenced by the autonomic nervous system:
- Sympathetic nervous system – increases the firing rate of the SA node, and thus increases heart rate.
- Parasympathetic nervous system – decreases the firing rate of the SA node, and thus decreases heart rate.
AV node and the bundle of His also have a rich supply of cholinergic and adrenergic fibers as compared to the ventricular myocardium.
Parasympathetic nerves to the AV node region enter the heart at the junction of the inferior vena cava and the inferior aspect of the left atrium, adjacent to the coronary sinus. The right vagus nerve primarily innervates the SA node, whereas the left vagus innervates the AV node but overlap exists.
It is seen that the right sympathetic and vagal nerves affect the SA node more than the AV node and the left sympathetic and vagal nerves affect the AV node more than the SA node. There is a substantial overlap of the innervation.
For example, stimulation of the right stellate ganglion produces sinus tachycardia but AV nodal conduction is less affected. The stimulation of left stellate ganglion consistently shortens AV nodal conduction time and refractoriness, but it has an inconsistent effect on the SA node discharge rate.
The 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.
Bundle of His is not affected by either sympathetic or vagal stimulation.