Author: Lorenzo Crumbie MBBS, BSc•Reviewer: Adrian Rad BSc (Hons)Last reviewed: October 13, 2021Reading time: 25 minutes


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The cardiac conduction system is a network of specialized cardiac muscle cells the initiate and transmit the electric impulses responsible because that the coordinated contractions of each cardiac cycle. These one-of-a-kind cells room able to create an activity potential on their own (self-excitation) and also pass that on to other adjacent cells (conduction), including cardiomyocytes.

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The components of the love conduction system can be split into those the generate activity potentials (nodal tissue) and also those that conduct lock (conducting fibers). Although all parts have actually the capacity to generate activity potentials and also thus love contractions, the sinuatrial (SA) node is the primary impulse initiator and regulator in a healthy and balanced heart. This facet makes the SA node the physiological pacemaker the the heart. Other parts sequentially receive and conduct the impulse originating native the SA node and also then happen it come myocardial cells. ~ above stimulation through the action potential, myocardial cells contract synchronously, bring about a heartbeat. The propagation of electrical impulses and also synchronous convulsion of cardiomyocytes is promoted by the visibility of intercalated discs and gap junctions.

Key facts about the cardiac conduction system
PartsNodal tissue: sinuatrial (SA) and atrioventricular (AV) nodesConducting fibers: internodal and also interatrial conduction pathways, bundle that His, bundle branches, subendocardial branches
Sinuatrial nodeContains cardiac pacemaker (P) cellsPacemaker the the heartSupplied by the SA nodal branch of the right coronary artery
Internodal conduction pathwayAnterior, middle, posterior
Interatrial conduction pathwayConducts impulses come the left atriumSupplied by the SAnodal artery
Atrioventricular nodeSecondary pacemakerSupplied by AV nodal artery
BundlesAtrioventricular (AV) bundle (of His) – oval, quadrangular, or triangularRight and also left bundlesSubendocardial branches (Purkinje fibers)
PhysiologyImpulse starts at the SA node → internodal and interatrial conduction pathways → AV node → AV bundle (of His) → bundle branches → subendocardial branchesSympathetic: boosts the price of SA node activityParasympathetic: reduce the price of SA node activity
Clinical notesSick sinus syndrome, Wolf-Parkinson-White syndrome

This short article will talk about the anatomy the the cardiac conduction system and its various parts. Obstacle of the conduction pathway and how lock manifest clinically will also be discussed.


Sinuatrial node

The sinuatrial node (SA node) is a flat, elliptical collection of committed nodal tissue through dimensions of up to 25 millimeter (mm) in length. The node is nestled in the exceptional posterolateral wall surface of the right atrium near the opening of the superior vena cava i beg your pardon is indicated by the sulcus terminalis (the junction the the venous sinus and also the appropriate atrium proper). Here it lies in the subepicardiac layer of the heart, regularly covered through a reasonably thin fat pad. 

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Heart diagram showing the atria and ventricles

Centrally, the SA node is inhabited with pale-staining cells known as cardiac pacemaker (P) cells. They are circumferentially arranged about the arterial supply of the node (the SA nodal branch the the coronary artery). Histologically, p cells contain a reasonably large, main nucleus yet a scant lot of various other organelles (likely the cause of the pale staining). Unlike the neighboring cardiomyocytes, p cells have actually very few cytoplasmic myofibrils and also no sarcotubular apparatus. The populace of p cells begins to decrease in the direction of the perimeter of the SA node, where various other transition cells become much more apparent. This slender, fusiform cell resemble a crossover between the previously mentioned P cells and also typical cardiomyocytes. These transition cells form bridges in between P cells and surrounding atrial cells. 

The SA node receives its blood supply from the sinuatrial nodal branch that the coronary artery. In about 60% of individuals, this artery is a branch that the best coronary artery (therefore emerging from the left coronary artery in the various other 40%). Over there are countless autonomic ganglion cell bordering the SA node anteriorly and posteriorly. However, nobody of this ganglia appear to end on the cardiac pacemaker cells. Instead, the p cells contain both cholinergic and adrenergic receptor to respond come the neurotransmitters exit by the neighboring autonomic ganglion cells.

Internodal conduction pathway


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The internodal conduction pathways space a part of the intra-atrial conduction network initially defined by cutting board N. James in 1963. Not just do these pathways take trip within the best atrium, but they also type direct point out of communication in between the sinuatrial and atrioventricular nodes. The internodal conduction pathway is divided into anterior, middle and posterior branches.

The anterior internodal pathway originates indigenous the anterior margin of the SA node. It continues anteriorly, coursing approximately the exceptional vena cava wherein it offers off Bachmann’s bundle. The anterior internodal band proceeds anteroinferiorly toward the atrioventricular (AV) node whereby it enters the node by way of its superior margin. 

The middle internodal pathway occurs from the posterosuperior margin the the SA node. It continues behind the remarkable vena cava toward the border that the interatrial septum. The pathway transforms caudally in the interatrial septum to enter the AV node with its exceptional margin.

Finally, the posterior internodal pathway emerges from the posterior margin the the sinus node. The takes a posterior course roughly the premium vena cava and continues throughout the crista terminalis toward the Eustachian ridge (valve of the worse vena cava). The pathway then enters the interatrial septum (above the point of the coronary sinus) whereby it start the AV node with its posterior surface. 

These conduction pathways transmit the action potential slightly much faster than the neighboring cardiomyocytes. Lock contain the Purkinje-like (myofibril-poor) cells, i m sorry ensures that the action potential arrives at the AV node at an proper time. The blood it is provided of this pathways is comparable to the blood supply of the appropriate atrium – the circumflex branch of the left coronary artery.

Interatrial conduction pathway


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The interatrial conduction pathway, also called Bachmann’s bundle, describes a preferential pathway of specialized cardiomyocytes that facilitate the conduction that impulses between the atria. The pathway branches from the anterior internodal pathway at the level that the superior vena cava. The Bachmann’s bundle the cross the interatrial groove (an external landmark the the interatrial septum) and also passes end the limbus the the fossa ovalis. A pad that fatty tissue separates the Bachmann’s bundle from the limbus.

The pathway bifurcates into right and also left branches the travel toward the right and left atrial auricles, respectively. The right branch can it is in further separated into superior and inferior arms. The superior eight originates in ~ the exterior junction of the premium vena cava and also the atrium (near the ar of the SA node). The inferior arm emerges in the vestibule the the right atrium. The left branch provides some structural assistance to the anterior atrial wall surface and continues to wrap roughly the left atrial auricle. Proximally, the superior part of the left branch overcome in front of the openings of the left pulmonary veins. The inferior part continues caudally to the vestibule of the left atrium.

From a histological perspective, the interatrial conduction pathway is a collection of parallel strands the myocardium traveling in the subepicardiac layer. The myocytes within Bachmann’s bundle are encased in thin septa do of tightly packed collagen fibrils. This uninterrupted sheath additionally forms inter-septal connections (the role of which is no yet clear). There are five established cell varieties found in ~ the interatrial pathway. This are:

Myofibril-rich cells – which room the very same as constant cardiomyocytes.Myofibril-poor cells – resemble Purkinje cells; are countless in the pathway.P cells – like those described in the sinuatrial node.Slender transitional cells – short and also narrow.Broad transitional cells – longer and more comprehensive than slim transitional cells.

The visibility of these dedicated cells facilitates rapid conduction of the action potential throughout the left atrium, minimizing the hold-up in depolarization between the atria. Bachmann’s bundle receives its blood supply from the sinuatrial nodal branch of the coronary artery.

Have you acquired into that examine groove? keep pushing if you have the momentum and also check the end these articles, quizzes and video clip for added insight right into heart anatomy, histology and also function.


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Nerves the the heart explore study unit

There is another specialized structure in the heart, comparable to the SA node described earlier, which also helps with the conduction the impulses. It is known as the atrioventricular node (AV node) and is often referred to as the secondary pacemaker of the heart. Under common circumstances, it functions as a conduit the electrical activity from the SA node come the ventricles of the heart. The is the only pathway through which the action potential can cross indigenous the atria to the ventricles; as the atrioventricular septum is make of a cartilaginous framework that is can not to conduct electric impulses. The AV node is smaller than the SA node and is located in the posteroinferior component of the interatrial septum. Specifically, the node rests in the triangle that the atrioventricular node (triangle the Koch or Koch’s triangle). This triangle is limited by the coronary sinus (basally), the septal leaflet of the tricuspid valve (inferiorly) and the tendon of inferior pyramidal space (tendon the valve of inferior vena cava or tendon of Todaro) (superiorly).

The hemi-oval-shaped node occupies the subendocardiac layer within Koch’s triangle. The base of the node also extends right into the atrial muscle. The apex the the node extend anteroinferiorly. The passes with the fibrous cardiac skeleton to type the initial component of the atrioventricular (AV) bundle (of His). The histological make-up the the AV node is fairly similar to the in the SA node. The chief differences are that there room fewer p cells and more change cells contrasted to what is observed in the SA node. 

The AV node receive arterial blood indigenous the atrioventricular nodal branch. This arises from the worse interventricular branch the the appropriate coronary artery in 80% the individuals. In the continuing to be 20% the individuals, the atrioventricular nodal branch stems from the circumflex branch of the left coronary artery. There is also a remarkable amount the autonomic ganglion cells bordering the AV node (as observed in the SA node). However, nobody of these actually type synapses with the AV node. Choose the SA node cells, the AV node cells likewise have adrenergic and also cholinergic receptor in order come respond to autonomic input.

Bundle that His

The atrioventricular (AV) bundle (of His) is the early stage segment that the AV node that penetrates through the fibrous trigone right into the membranous component of the interventricular septum. On transverse section at the level of the fibrous body, the AV bundle may show up oval, quadrangular or triangular. A unique and important function of the AV bundle is the it only allows the ‘forward’ motion of activity potentials. Therefore, the retrograde transmission of electric impulses indigenous the ventricles to the atria is not enabled in a common functioning heart. The AV bundle is supplied by the anterior and also inferior interventricular branches that the coronary arteries.

Right and left bundle branches


As the node moves from the membranous come the muscular interventricular septum, it bifurcates right into right and also left bundles.

The crus dextrum, i m sorry is Latin because that right bundle branch, increase from the AV bundle in the membranous interventricular septum. That is a round team of narrow fascicles the travels in the myocardium before moving superficially to the subendocardiac great space. It travel to the ideal side the the interventricular septum wherein it gives of branches come the ventricular walls prior to going on toward the ventricular apex. Here, that enters the septomarginal moderator band (septomarginal band) prior to reaching the anterior papillary muscles. The terminal arborization of the right branch offers the papillary muscle and also recurs to supply the rest of the ventricular wall. 

The left bundle branch or crus sinistrum (Latin) branches native the atrioventricular bundle at the start of the muscular interventricular septum. It is made up of numerous tiny fascicles that become flattened sheets. These fascicles accounting the left half of the muscular interventricular septum. The sheet move to the subendocardiac room as that travels towards the ventricular apex. Below it trifurcates right into posterior, septal and anterior divisions. The branches will go on come activate the anterior and also posterior papillary muscles, interventricular septum and also the walls of the left ventricle. 

Purkinje fibers

The right and also left majority are occupied with subendocardial branches (Purkinje fibers). These cells can be much larger than those that the neighboring heart muscles and they duty quite in different ways than the preceding cells in the AV node. Subendocardial branches are found throughout the entire length the both bundles in the subendocardiac layer. They extend toward the cardiac apex, then curve upward and also backward v the walls of the ventricles. 

The fibers have actually far much more gap junctions than the AV nodal cells and also surrounding myocytes. Together a result, they space able to transmit impulses 6 time faster than ventricular muscles and also 150 times quicker than the AV nodal fibers. The increased variety of gap junctions allow much more ions to pass from one cell to the next, hence increasing the rate of conduction. Furthermore, there room fewer myofibrils in Purkinje cells, result in little to convulsion (therefore much shorter to missing refractory periods) within this cells. Consequently, the bundles can accomplish almost instantaneous transmission that the activity potential come the rest of the ventricle once it passes through the AV node. This compensates because that the hold-up at the AV node and permits the ventricles come contract shortly after the atria. 

Note that the key branches of the atrioventricular bundle are insulated by sheaths the connective tissue. This stays clear of premature excitation of surrounding cardiac tissue. Therefore, the papillary muscles will depolarize first, complied with by the ventricular apex, then walls. The sample of depolarization additionally goes indigenous endocardium to epicardium, since the fibers space in the subendocardiac layer.

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Physiology

Now the you have actually covered the anatomy of the cardiac conduction system, ask yourself these vital questions: exactly how does the SA and AV nodes work? exactly how do the impulses travel v the heart? The straightforward answer to both concerns is an action potential

Sinuatrial node

Cardiomyocytes have the special ability to wake up themselves (self-excitation or automaticity). The initiation of the action potential is dependence on ion channels that allow passage of ions into and also out of the cells. In the case of cardiomyocytes, they have fast-acting sodium ion (Na+), slow-moving sodium-calcium ion (Na+-Ca2+) and also slow/fast potassium ion (K+) networks (among other important networks that preserve ionic equilibrium).

This automaticity is specifically enhanced in P cells. They have a lower resting membrane potential 보다 the neighboring cardiomyocytes and change cells due to the following factors:

There is a high concentration of extracellular Na+ exterior the nodal fibers.A relatively big amount that Na+ channels are currently open.There is a passive diffusion of Na+ right into the p cells in between heartbeats due to the ‘leaky’ sodium channels.The passive flow of Na+ reasons a slow climb in the membrane potential of the cell, pass it increasingly closer come the generation threshold of an action potential.

Therefore, cardiac p cells of the SA node are an ext readily depolarized than various other cardiac cells. The SA node is additionally intimately connected with the bordering heart muscle via the internodal and also interatrial conduction pathways. Consequently, the generated action potential can be rapidly transmitted to other cells. This allows the SA node to set the speed at i m sorry the love cells will depolarize and also subsequently contract, making the the pacemaker of the heart. ~ above average, the SA node deserve to fire between 60 to 100 beats per minute in ~ rest.

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Action potential graph of love muscle. Refractory period: the cabinet is depolarized (made much more positive), plateaus (little come no change), then repolarized (made much more negative). Absolute refractory period: ion networks are in the interim inactivated. Relative refractory period: part ion networks may be activated.

Atrioventricular node

Although the main function of the AV node is to facilitate i of the depolarization tide to the ventricles, it likewise has added functions. In the lack of a work SA node, the AV node has actually the capacity to take it over as the pacemaker of the heart. Recall that it likewise has ns cells that space able to develop an (albeit slower) valuation (40 to 60 beats every minute). 

The AV node is likewise responsible for slowing under the passage of the electrical impulse travel to the ventricles. This vital phenomenon allows more time for the ventricles to continue to be quiescent and fill with blood coming from the contracting atria. However how does the AV node sluggish down conduction? 

One key feature of the transition cells and P cells in the AV node is that they have fewer space junctions at the intercalated discs. Consequently, there is an ext resistance come conduction in this component of the conduction pathway 보다 there is in other areas. 

Impulse generation and conduction

Now let’s placed all that information together and also outline the cardiac conduction steps:

The SA node generates the action potential.The advertise arrives in ~ the AV node and also is slowed down to facilitate ventricular pour it until it is full (ventricular diastole).The impulse climate passes indigenous the AV node come the AV bundle.

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It is then rapidly spread through the bundle branches and subendocardial tissue leading to ventricular systole.