Conduction System of the Heart

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Conduction System of the Heart...

Conduction System of The Heart:

As we know, the work of the heart is to pump blood to the lungs through pulmonary circulation and to the rest of the body through systemic circulation. This is accomplished by systematic contraction and relaxation of the cardiac muscle in the myocardium. The intercalated disks permit impulses to travel rapidly between adjacent cells so that they function together as a single electrical unit instead of as individual cells. Effective contractions of the heart depend on this characteristic, and they are coordinated by the conduction system of the heart.

An effective cycle for productive pumping of blood requires that the heart be synchronized accurately. Both atria need to contract simultaneously, followed by contractions of both ventricles. Contraction of the chambers of the heart is coordinated by specialized cardiac muscle cells that make up the conduction system.

These cells contain only a few muscle fibers and instead of contracting, they act somewhat like nerve tissue by initiating and distributing impulses throughout the myocardium to coordinate the events of the cardiac cycle.

COMPONENTS OF THE CONDUCTION SYSTEM

SINOATRIAL NODE- the first part of the conduction system is the sinoatrial node or SA node, which is located in the posterior wall of the right atrium, near the entrance of the superior vena cava.

Without any nerve stimulation, the SA node rhythmically initiates impulses 70 - 80 times per minute. Because it establishes the basic rhythm of the heartbeat, it is called the pacemaker of the heart.

The impulses from the SA node rapidly travel throughout the atrial myocardium and cause the two atria to contract simultaneously. At the same time, the impulses reach the second part of the conduction system.

ATRIOVENTRICULAR NODE- The atrioventricular node, the second part of the conduction system, is located in the floor of the right atrium, near the interatrial septum. The cells in the AV node conduct impulses more slowly than do other parts of the conduction system so there is a brief time delay as the impulses travel through the node. This allows time for the atria to finish their contraction phase before the ventricles begin contracting.

BUNDLE OF HIS

From the AV node, the impulses rapidly travel through the bundle of His or the atrioventricular bundle. This impulse travels to the right and left bundle branches. Theses bundle branches extend to the right and left sides of the interventricular septum and the apex of the heart.

These branch profusely to form the Purkinje fibers that transmit the impulses to the myocardium. The bundle of His, bundle branches and the Purkinje fibers rapidly transmit impulses throughout the ventricular myocardium so that both ventricles contract at the same time.

As the ventricles contract, blood is forced out through the SL valves into the pulmonary trunk and the ascending aorta. After the ventricles complete their contraction phase, they relax and the SA node initiates another impulse to start another cardiac cycle.

All parts of the conduction system and cardiac muscle cells are capable of initiating impulses to start a cardiac cycle. The SA nose is called the pacemaker because impulses spontaneously originate in the SA node faster than in any other part. If the SA node is unable to function, another area such as the AV node becomes the pacemaker. The resulting heat rate is slower than normal.

The components of the conduction system of the heart include the SA node, the AV node, the bundle of His, the bundle branches, and the Purkinje fibers. These coordinate the contraction and relaxation of the heart chambers.

CARDIAC CYCLE

The cardiac cycle refers to the alternating contraction and relaxation of the myocardium in the walls of the heart chambers, coordinated by the conduction system, during one heartbeat.

The two atria contract at the same time, then they relax while the two ventricles simultaneously contract.

The contraction phase of the chambers is called systole: the relaxation phase is called diastole.

At a normal heart rate, one cardiac cycle lasts for 0.8 second!

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