Introduction:
The cardiac conduction system controls the heartbeat. It consists of specialized muscle cells and fibers that initiate impulses and coordinates the contractions of the heart. It also comprises a skeleton of fibrous tissue which surrounds the conduction system. Dysfunction of the cardiac conduction system can result in abnormal heart rhythms. This article gives insight into the cardiac conduction system.
What Does Cardiac Conduction System Mean?
The cardiac conduction system refers to the network of nodes, cells, and signals that control the heartbeat. Conduction denotes the pathway of electrical signals that travel through the heart to cause a heartbeat. The cardiac conduction system comprises of specialized muscle cells that are present within the myocardium (heart wall). These muscle cells transmit signals to the heart muscle to initiate contraction. These electrical signals cause the expansion and contraction of the various parts of the heart. Hence, each time the heart beats, electrical signals are transmitted through the heart. These electrical signals are essential for the heart to function normally.
What Are the Structures That Constitute the Cardiac Conduction System?
The heart is divided into the right and left sides by a muscular wall called the septum. It is divided into four chambers on the right and left sides. The two upper chambers on the right and left sides are called the atrium, and the two lower chambers on the right and left sides, are called the ventricles. The structures of the cardiac conduction system include:
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The Sinoatrial or SA Node - The SA node is a region of special cardiac muscle cells that are located in the upper back wall of the right atrium. The SA node is the pacemaker of the heart. The sinoatrial node transmits signals to initiate contractions.
- The Atrioventricular or AV Node - The AV node is present in the interatrial septum of the heart that divides the upper chamber into the left and right atrium. The atrioventricular node is smaller than the SA node. It conducts impulses between the atrium and the ventricles. The main function of this node is to slow the electrical signals from the SA node before the signals enter the ventricles. This delay allows the atrium to contract before the ventricular contraction and also allows ventricular filling.
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The Bundle of His - It is situated in the center of the heart, and it works at reducing the speed to conduct the electrical signals within the heart. The bundle branches are the division of the bundle of His that transmit the electrical signals towards the outer structure of the heart and the ventricles.
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Purkinje Fibers - The Purkinje fibers are the dense continuations of the bundle branches. These fibers transmit electrical signals rapidly to stimulate ventricular contraction. The His-Purkinje system is the collective structure and function of the bundle of His, bundle branches, and the Purkinje fibers, which form the shape of an inverted umbrella. The His-Purkinje System rapidly conducts electrical signals to the ventricles.
How Does the Cardiac Conduction System Function?
Electrical impulses arise from the SA node and induce the atria to contract. These signals then travel to the AV node situated in the interatrial septum. This node slows the impulse, and this delay gives time for ventricular filling. The electrical signals diverge and are conducted to the Purkinje fibers on each side of the heart through the left and right bundle branches of His, respectively, and to the endocardium at the apex of the heart. These impulses finally reach the ventricular epicardium resulting in ventricular contraction. These signals are generated in a rhythmic pattern resulting in coordinated contraction and relaxation of the heart.
How Are Electrical Signals Generated?
An electrocardiogram (ECG) is used to record the electrical activity of the heart.
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SA Node - P Wave: The electrical signals are spontaneously generated in the SA node. These electrical signals travel throughout the right atrium and through the Bachmann bundle (a broad band of cardiac muscle) to the left atrium and stimulate the myocardium of the atria to contract. It travels via specialized pathways, called internodal tracts, from the SA node to the AV node. The conduction of electrical signals throughout the atria is observed as the P wave on the ECG.
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AV Node and Bundle of His - PR Interval: The AV node is responsible for a vital delay in the conduction system. If this delay is not present, the atria and ventricles will contract at the same time, and blood flow from the atria to the ventricles will not be effective. The delay in the AV node is observed as the PR segment in the ECG. The distal portion of the atrioventricular node is referred to as the bundle of His. The bundle of His divides into left and right bundles in the interventricular septum. The left and right bundle branches stimulate the left ventricle and the right ventricle, respectively. The left bundle branch branches into the left anterior and poster fascicle. The left posterior fascicle transmits signals to the papillary muscles, resulting in mitral valve closure.
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Purkinje Fibers - QRS Complex: Numerous Purkinje fibers arise from the right and left bundle branches. Purkinje fibers induce contractions of individual groups of myocardial cells. The propagation of electrical signals through the Purkinje fibers or ventricular myocardium is observed as the QRS complex on the ECG.
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Ventricular Repolarization: The repolarization of the ventricles is the final step in the conduction system. It refers to the restoration of the resting state. It includes the J point, ST segment, T wave, and U wave.
What Is the Significance of the Cardiac Conduction System?
Abnormalities in the cardiac conduction system result in arrhythmias. Arrhythmia denotes the abnormal rhythms of the heartbeat. A decrease in the heart rate of 60 or fewer beats per minute is referred to as bradycardia. An increase in heart rate of more than 100 beats per minute is referred to as tachycardia.
Conclusion:
The cardiac conduction system is a vital network that initiates and coordinates the contractions of the heart muscle. Cardiac conduction is essential for the functioning of the heart. Abnormalities in this system can result in various cardiac diseases.
