Cardiac Conduction System

Throughout the heart are clumps and strands of specialized cardiac muscle tissue whose fibers contain only a few myofibrils. Instead of contracting, these areas initiate and distribute impulses (cardiac impulses) throughout the myocardium. They comprise the cardiac conduction system, which coordinates the events of the cardiac cycle.

A key portion of this conduction system is the sinoatrial node (S-A node), a small, elongated mass of specialized cardiac muscle tissue just beneath the epi-cardium. It is located in the right atrium near the opening of the superior vena cava, and its fibers are continuous with those of the atrial syncytium.

The cells of the S-A node can reach threshold on their own, and their membranes contact one another. Without stimulation from nerve fibers or any other outside agents, the nodal cells initiate impulses that spread into the surrounding myocardium and stimulate the cardiac muscle fibers to contract.

S-A node activity is rhythmic. The S-A node initiates one impulse after another, seventy to eighty times a minute in an adult. Because it generates the heart's rhythmic contractions, the S-A node is often called the pacemaker (pas'mak-er). From the S-A node, bundles of atrial muscle, called internodal atrial muscle, preferentially conduct impulses along tracts to specific regions of the heart.

As a cardiac impulse travels from the S-A node into the atrial syncytium, it goes from cell to cell via gap junctions. The right and left atria contract almost simultaneously. Instead of passing directly into the ventricular syncytium, which is separated from the atrial syncytium by the fibrous skeleton of the heart, the cardiac impulse passes along fibers of the conduction system that are continuous with atrial muscle fibers. These conducting fibers lead to a mass of specialized cardiac muscle tissue called the atrioventricular node (A-V node). This node is located in the inferior portion of the septum that separates the atria (interatrial septum) and just beneath the endocardium. It provides the only normal conduction pathway between the atrial and ventricular syncytia, because the fibrous skeleton does not conduct the impulse.

The fibers that conduct the cardiac impulse into the A-V node (junctional fibers) have very small diameters, and because small fibers conduct impulses slowly, they delay transmission of the impulse. The impulse is delayed further as it moves through the A-V node, allowing time for the atria to contract completely so they empty all their blood into the ventricles prior to ventricular contraction.

Once the cardiac impulse reaches the distal side of the A-V node, it passes into a group of large fibers that make up the A-V bundle (atrioventricular bundle or bundle of His), and the impulse moves rapidly through them. The A-V bundle enters the upper part of the interventric-ular septum and divides into right and left branches that lie just beneath the endocardium. About halfway down the septum, the branches give rise to enlarged Purkinje fibers. These larger fibers carry the impulse to distant regions of the ventricular myocardium much faster than cell-to-cell conduction could. Thus, the massive ventricular myocardium contracts as a functioning unit.

The base of the aorta, which contains the aortic valves, is enlarged and protrudes somewhat into the interatrial septum close to the A-V bundle. Consequently, inflammatory conditions, such as bacterial endocarditis affecting the aortic valves (aortic valvulitis), may also affect the A-V bundle.

If a portion of the bundle is damaged, it may no longer conduct impulses normally. As a result, cardiac impulses may reach the two ventricles at different times so that they fail to contract together. This condition is called a bundle branch block.

The Purkinje fibers spread from the interventricular septum into the papillary muscles, which project inward from the ventricular walls, and then continue downward to the apex of the heart. There they curve around the tips of the ventricles and pass upward over the lateral walls of these chambers. Along the way, the Purkinje fibers give off many small branches, which become continuous with cardiac muscle fibers. These parts of the conduction system are shown in figure 15.18 and are summarized in figure 15.19.

The muscle fibers in the ventricular walls form irregular whorls. When impulses on the Purkinje fibers stimulate these muscle fibers, the ventricular walls

Interatrial septum

Sinoatrial node

S-A node

A-V node

A-V bundle

Interatrial septum

S-A node

A-V node

A-V bundle

Left bundle branch

Purkinje fibers

Interventricular septum

Figure 15.18

Left bundle branch

Purkinje fibers

Interventricular septum

Figure 15.18

The cardiac conduction system.

Atrial syncytium

Junctional fibers

Atrioventricular node

A-V bundle

Bundle branches

Purkinje fibers

Ventricular syncytium

Figure 15.19

Components of the cardiac conduction system.

contract with a twisting motion (fig. 15.20). This action squeezes blood out of the ventricular chambers and forces it into the aorta and pulmonary trunk.

Another property of the conduction system is that the Purkinje fibers transmit the impulse to the apex of the heart first. As a result, contraction begins at the apex and pushes the blood superiorly toward the aortic and pulmonary semilunar valves, rather than having the impulse begin superiorly and push blood toward the apex, as it would if the impulse traveled from cell to cell.

H What is the function of the cardiac conduction system?

^9 What kinds of tissues make up the cardiac conduction system?

^9 How is a cardiac impulse initiated?

Q How is a cardiac impulse transmitted from the right atrium to the other heart chambers?

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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