Somatic nervous system
Electrical conduction system of the heart
The conducting system of the heart is a complex network of specialized cells that generates and regulates each heart beat. The components of this system generate and transmit electrical stimuli to the cells of the cardiac muscle (myocardium) which are then able to contract synchronously and produce a heartbeat.
The cells that generate an electrical impulse are called the nodal cells, which are in fact specialized and modified cardiac muscle cells. The nodal cells are organized into two clusters, or nodes, called the sinoatrial (SA) node and atrioventricular (AV) node.
The SA node is the one that generates the impulses which are then propagated to the atria of the heart and to the AV node.
The AV node further propagates the impulses through a network of specialized conducting fibers called the bundle of His. This bundle terminates with a network of fibers called the Purkinje fibers. The system of AV node and its associated fibers functions to propagate the impulses into the ventricles.
This article will discuss the anatomy and function of the cardiac conduction system.
Normal order of the electrical conduction
The normal order of the conduction pathway of the heart can be summarized as follows:
- The electrical impulse of the heart normally begins at the SA node
- The impulse is conducted to the muscle cells of the atrium, causing them to contract
- The cells of the atrium pass the impulse to the AV node
- The AV node distributes the impulse through the bundle of His and its right and left branches, that further ramify into the subendocardial network of Purkinje fibers, conveying the impulses to the ventricular walls and papillary muscles
Sinoatrial (SA) node
The SA node is located at the junction of the superior vena cava and the right atrium. It is tucked in between the epicardium and underlying myocardium of the heart. The majority of the node consists of specialized cardiac cells called the pacemaker cells or the P cells, as they are capable of initiating an electrical impulse that will be conducted throughout the heart.
The SA node is the pacemaker of the heart that initiates each heartbeat. The impulse generation in the node is regular and it is usually 70-80 impulses per minute in most of the people most of the time. This frequency is called the intrinsic rate of the SA node, or the intrinsic pacemaker rate. The impulse from the SA node spreads directly to the musculature of the left and right atria. Such type of direct spreading is called myogenic conduction. The SA node is innervated by the sympathetic and parasympathetic divisions of the autonomic nervous system. The sympathetic component accelerates the heartbeat, while the parasympathetic slows it down.
Atrioventricular (AV) node
The atrioventricular or the AV node is located in the posterior wall of the right atrium. The impulse from the SA node reaches the AV node via the myocardial cells, as well as through a few thin direct connections between the nodes called the anterior, middle and posterior internodal pathways.
The AV node then delays the further conduction of the impulse for 0.09 seconds. From the AV node, a bundle of specialized conduction fibers arises, called the bundle of His. This bundle is the only physiological electrical connection between the atria and ventricles.
Bundle of His and Purkinje fibers
The bundle of His, also called the atrioventricular (AV) bundle is a bundle of fibers that extends from the AV node and passes through the membranous part of the interventricular septum.
At the junction between the membranous and muscular parts of the septum, it divides into the left and right bundle branches. Each branch then courses down the respective side of the septum, traveling just below the endocardial layer of the ventricles.
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As electricity travels down the right bundle branch, it first stimulates the muscle of the right side of the interventricular septum, then the anterior papillary muscle and finally the wall of the right ventricle. The left bundle stimulates first the left side of the interventricular septum, then the anterior and posterior papillary muscles and ultimately the wall of the left ventricle.
The walls of the ventricles are stimulated by a network of fibers that stem from the left and right branches of the bundle of His. This network is called the subendocardial conducting network, also known as the Purkinje fibers.
The system of AV bundle branches also delays the electrical impulse for another 0.04 seconds. This totals to a delay of 0.13 seconds in the AV node and its bundle system. The delay is important because it allows the atria to contract fully before the ventricles, passing all the blood to them.
Sinus node rhythm
The sinus rhythm is the frequency of the heartbeat generated by the SA node, which is 70-80 beats per minute. However, the AV node and the Purkinje fibers also have the capability to generate the electrical impulses.
The AV node intrinsic rate is 40-60 impulses per minute, while for the Purkinje fibers it is 15-40. The fact that the intrinsic rate of the SA node is notably faster than that of the AV node and Purkinje fibers explains why the sinus node is the pacemaker of the heart, and not the other two components; When the SA node discharges, it excites both the AV node and the Purkinje fibers. But also, the next SA discharge again comes before the AV node and Purkinje fibers reach their own thresholds for excitation. Thus, the SA node is the physiological pacemaker because its intrinsic rate is faster than that of the AV node and Purkinje fibers.
The electrical conduction system of the heart: [Show/Hide answers]
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