Conduction System of the Heart: The Electrical Pathway
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Conduction System of the Heart
The cardiac conduction system is the electrical pathway of the heart that leads to atrial and ventricular contraction.
The conduction system consists of pacemaker cells that generate spontaneous action potentials, and then deliver those impulses throughout the heart.
The cardiac conduction system comprises the following structures in order: SA node, internodal pathway and Bachmann’s bundle, AV node, bundle of His, bundle branches, and Purkinje fibers.
This lecture will walk you through the conduction pathway step-by-step using a labeled diagram of the heart.
Once you understand the conduction system of the heart, you will be able to apply it to conduction system diseases, disorders, and abnormalities (discussed in other EZmed posts).
You will be able to apply the conduction system to the different parts of an electrocardiogram (EKG/ECG) waveform as well.
As with every EZmed lecture, the material will be presented simply and concisely.
We will outline the conduction system sequence using labeled ppt images, as well as provide a summary video above.
Let’s get started!
Anatomy of the Heart
Before we discuss the cardiac conduction system, let’s briefly review the gross anatomy of the heart as the diagram below will be used throughout this lecture.
For a great step-by-step guide filled with memory tricks to remember the anatomy of the heart, check out the EZmed lecture below!
Heart Anatomy: Labeled Diagram, Structures, Function, and Blood Flow
Cardiac Chambers
The heart has 4 chambers: the right atrium, right ventricle, left atrium, and left ventricle.
The atria are positioned at the superior/upper portion of the heart, and the ventricles are located at the inferior/lower portion of the heart.
Great Vessels
The main pulmonary artery, also known as the pulmonary trunk, emerges from the right ventricle and delivers deoxygenated blood to the pulmonary circulation and lungs.
The aorta emerges from the left ventricle and delivers oxygenated blood to the rest of the body.
The superior vena cava and inferior vena cava are the main veins that deliver deoxygenated venous blood from the rest of the body back to the heart, specifically the right atrium.
The pulmonary veins are the main veins that deliver oxygenated blood from the lungs back to the heart, specifically the left atrium.
Valves
There are 4 valves in the heart: the tricuspid valve, mitral valve, pulmonic valve, and aortic valve.
The tricuspid and mitral valves are positioned between the atria and ventricles.
Specifically, the tricuspid valve is located between the right atrium and right ventricle, and the mitral valve is positioned between the left atrium and left ventricle.
The pulmonic and aortic valves are located between the ventricles and great vessels.
Specifically the pulmonic valve is positioned between the right ventricle and pulmonary trunk, and the aortic valve is located between the left ventricle and aorta.
RA = Right Atrium; RV = Right Ventricle; LA = Left Atrium; LV = Left Ventricle; TV = Tricuspid Valve; MV = Mitral Valve; PV = Pulmonic Valve; AV = Aortic Valve; SVC = Superior Vena Cava; IVC = Inferior Vena Cava; PA = Pulmonary Artery (main)
2 Types of Cardiac Muscle Cells
There are 2 main types of cardiac myocytes (muscle cells) in the myocardium:
Conducting Cells (Pacemaker Cells)
Contractile Cells (Non-Pacemaker Cells)
Pacemaker Cells
The heart has the innate ability to generate its own spontaneous action potentials without any external stimuli, a phenomenon known as automaticity.
It does this using pacemaker cells, which are specialized cardiac myocytes (muscle cells) within the myocardium that have the ability to generate spontaneous action potentials.
The pacemaker cells are located in structures that make up the electrical pathway of the heart, known as the conduction system, and they generate and transmit electrical impulses throughout the myocardium.
As the action potential travels through the conduction system and myocardium, it will lead to atrial and ventricular depolarization and contraction.
The rate at which the pacemaker cells fire is the heart rate.
The pacemaker cells do not have a true “resting phase” in their action potential cycle.
Once a pacemaker cell repolarizes, the voltage across the cell membrane slowly becomes more positive until the action potential threshold is met and rapid depolarization occurs again.
For more information about pacemaker cell action potentials, make sure to check out the EZmed lecture that makes cardiac action potentials easy!
The pacemaker cells are located within the SA node, AV node, bundle of His, right and left bundle branches, and Purkinje fibers.
These structures make up the conduction system of the heart, which will be the focus of this post.
Contractile Cells
The contractile cells are the second type of cardiac myocytes found within the myocardium.
The contractile cells make up the bulk of the myocardium (99%), and they are the cardiac myocytes (muscle cells) responsible for contraction of the heart.
They mainly rely on the above conduction system to become depolarized, which will lead to cardiac contraction and movement of blood forward.
For more information about the contractile cell action potentials, make sure to check out the cardiac action potential EZmed blog!
The myocardium has 2 main cell types: Pacemaker Cells and Contractile Cells
The pacemaker cells have the capability of generating spontaneous action potentials. They are located in the SA node, AV node, bundle of His, right and left bundle branches, and the Purkinje fibers. They make up the conduction system of the heart.
The contractile cells are the muscle cells that lead to contraction of the heart once depolarized.
SA Node
As mentioned above, the heart has the ability to generate its own spontaneous action potentials, a phenomenon known as automaticity.
In a normal functioning heart, the SA node is the primary pacemaker that produces spontaneous action potentials that will determine the heart rate.
The SA node is composed of many pacemaker cells, and it is located at the back of the right atrium near the superior vena cava entry.
The conduction system of the heart can be influenced by the sympathetic nervous system to speed up the heart rate by activating cardiac beta receptors.
Alternatively, the parasympathetic nervous system can facilitate slowing the heart rate down.
While the autonomic nervous system can influence the heart rate extrinsically, the SA node can produce spontaneous action potentials at a rate of 60-100 beats per minute intrinsically without any external stimuli.
This is known as the normal sinus rhythm.
Once an action potential is generated by the SA node, it will travel through the right atrium via the internodal pathways.
There are 3 internodal tracts: Anterior, Middle, and Posterior
The action potential will also travel from the right atrium to the left atrium via Bachmann’s bundle, a branch of the anterior internodal tract.
As the action potential travels through the atria, the atria depolarize and contract to further push blood into the ventricles during diastole.
Atrial depolarization is represented by the P wave on EKG.
For more information about how the conduction system can be applied to the different parts of an EKG, make sure to check out the EZmed blog that makes EKGs easy!
In a normal functioning heart, the SA node is the pacemaker that sets the heart rate and is the starting point of the conduction system.
The pacemaker cells within the SA node generate action potentials at 60-100 beats per minute.
The action potential travels from the SA node through the right atrium via the internodal pathway, and to the left atrium via Bachmann’s bundle.
As the action potential travels through the atria, the atria depolarize and contract.
AV Node
After the action potential travels through the atria, it will converge onto another node called the AV node.
The AV node is located at the base of the right atrium near the interventricular septum.
It is the “gatekeeper” that sends the action potential from the atria to the ventricles.
Similar to the SA node, the AV node consists of many pacemaker cells that have the ability to generate their own spontaneous action potentials as well.
The key difference, however, is that the pacemaker cells within the AV node generate their action potentials at a slower rate than the SA node.
The rate at which the AV node produces spontaneous action potentials is approximately 40-60 beats per minute.
Since the SA node produces action potentials at much faster rate than the AV node, the SA node depolarizes the pacemaker cells within the AV node before they have time to spontaneously depolarize.
For this reason, the SA node is the primary pacemaker.
If the SA node were eliminated or stopped functioning properly, then it would be up to the AV node to spontaneously depolarize the heart.
As a result, the heart rate would be approximately 40-60 beats per minute rather than the 60-100 beats per minute produced by the SA node.
The other important function of the AV node is that it slows down the conduction velocity of the action potential.
This is a critical function of the AV node because by slowing down the conduction velocity of the action potential, it gives time for the atria to contract before depolarizing and contracting the ventricles.
If there were no delay in conduction through the AV node, then the atria and ventricles would contract at the same time making it difficult for blood to flow properly.
We want the atria to contract first to push the blood into the ventricles, then the ventricles can contract to pump blood to the pulmonary and systemic circulation.
Therefore, the AV node is the transition from the end of diastole to the start of systole in the cardiac cycle.
The time between atrial depolarization (P wave) and ventricular depolarization (QRS complex) is represented by the PR segment on an EKG.
The PR segment mainly reflects the slow impulse conduction through the AV node.
The AV node is the “gatekeeper” that sends the action potential from the atria to the ventricles.
The pacemaker cells within the AV node generate action potentials at 40-60 beats per minute, and are therefore masked by the SA node (60-100 beats per minute).
The AV node slows down the conduction velocity of the action potential to allow time for the atria to contract before depolarizing the ventricles.
Bundle of His
After the action potential travels through the AV node it will enter the bundle of His, also known as the atrioventricular bundle.
The bundle of His is located in the interventricular septum.
It also comprises pacemaker cells, and they can generate their own action potentials spontaneously at a rate of 40-60 beats per minute.
The action potential exits the AV node and enters the bundle of His.
The bundle of His has pacemaker cells that can generate action potentials at 40-60 beats per minute.
Right and Left Bundle Branches
The action potential then travels from the bundle of His to the right and left bundle branches, also known as the atrioventricular bundle branches.
The right bundle branch mainly supplies the right ventricle, and the left bundle branch mainly supplies the left ventricle.
The bundle branches consist of pacemaker cells that can generate spontaneous action potentials at a rate of 20-40 beats per minute.
Again, this slow action potential rate is masked by the SA node and/or the AV node (if the SA node were not functioning properly.)
From the bundle of His, the action potential travels through the right and left bundle branches.
The right bundle branch depolarizes the right ventricle, and the left bundle branch depolarizes the left ventricle.
The pacemaker cells within the bundle branches generate action potentials at 20-40 beats per minute.
Purkinje Fibers
Lastly, the action potential travels from the right and left bundle branches to the Purkinje fibers.
The Purkinje fibers conduct the impulse throughout the right and left ventricles.
As the action potential travels through the bundle of His, the bundle branches, and the Purkinje fibers, the ventricular contractile myocytes depolarize and contract.
The heart is now in systole.
Ventricular depolarization is represented by the QRS complex on EKG.
The pacemaker cells within the Purkinje fibers have the ability to generate spontaneous action potentials at a rate of 20-40 beats per minute.
The action potential travels from the bundle branches to the Purkinje fibers, which are the terminal branches throughout the inner right and left ventricular wall.
As the action potential disperses through the ventricles, the ventricular myocytes depolarize and contract.
The pacemaker cells within the Purkinje fibers produce action potentials at 20-40 beats per minute.
Practical Application
Abnormalities within the conduction system can lead to diseases such as heart blocks, sick sinus syndrome, arrhythmias, etc which will be discussed in other EZmed posts.
Depending on the conduction abnormality, antiarrhythmics may be required.
Antiarrhythmics include sodium channel blockers, beta blockers, potassium channel blockers, and calcium channel blockers.
Conclusion
Hopefully this provided you with a clear understanding of the conduction system of the heart.
The SA node is the primary pacemaker, spontaneously depolarizing at a rate of 60-100 beats per minute.
The action potential generated by the SA node then travels through the right atrium via the internodal pathway, and to the left atrium via Bachmann’s bundle.
As the action potential travels through the atria, the atrial contractile myocytes depolarize and contract.
The action potential converges onto the AV node, located at the base of the right atrium at the interventricular septum.
The AV node is the gatekeeper that sends the action potential from the atria to the ventricles.
The AV node also slows down the conduction velocity to allow time for the atria to contract before depolarizing the ventricles.
The action potential then exits the AV node and enters the bundle of His, followed by the right and left bundle branches, and lastly through the Purkinje fibers.
As the action potential travels through this portion of the conduction system, the ventricles depolarize and contract.
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