Conducting systems of the Heart

By:Dr.Norzaher
There are four basic components to the heart's conduction system

(1) sinoatrial node (SA node)

(2) inter-nodal fibre bundles

(3) atrioventricular node (AV node)

(4) atrioventricular bundle

The sinoatrial (SA) node is a small mass of specialised cardiac muscle situated in the superior aspect of the right atrium. It lies along the anterolateral margin of this chamber between the orifice of the superior vena cava and the auricle. The specialised cardiac muscle of the SA node is characterised by the property of automatic self-excitation and it initiates each beat of the heart. Therefore, the SA node is often referred to as the pacemaker of the heart.

Since the fibres of the SA node fuse with the surrounding atrial muscle fibres, the action potential generated in the nodal tissue spreads throughout both atria at a rate of approximately 0.3 meter per second and produces atrial contraction. Interspersed among the atrial muscle fibres are several inter nodal fibre bundles which conduct the action potential to the atrioventricular (AV) node with a greater velocity (approximately 1.0 meter per second) than ordinary atrial muscle. The AV node is located in the right atrium near the lower part of the interatrial septurn. Here there is a short delay (approximately 0.1 second) in transmission of the impulse to the ventricles.

This is important because it permits the atria to complete their contraction and empty their blood into the ventricles before the ventricles contract. The delay occurs within the fibres of the AV node itself as well as in special junctional fibres that connect the node with ordinary atrial fibres.

Once the action potential leaves the AV node, it enters specialised muscle fibres called Purkinje fibres. These are grouped into a mass termed the atrioventricular (AV) bundle, or the bundle of His. The Purkinje fibres are very large and conduct the action potential at about six times the velocity of ordinary cardiac muscle (i.e., 1.5 to 4.0 meters per second). Thus the Purkinje fibres permit a very rapid and simultaneous distribution of the impulse throughout the muscular walls of both ventricles.

As the AV bundle leaves the AV node, it descends in the interventricular septurn for a short distance and then divides into two large branches, the right and left bundle branches. Each of these descends along its respective side of the interventricular septum immediately beneath the endocardium and divides into smaller and smaller branches. Terminal Purkinje fibres extend beneath the endocardium and penetrate approximately one-third of the distance into the myocardium. Their endings terminate upon ordinary cardiac muscle within the ventricles, and the impulse proceeds through the ventricular muscle at about 0.3 to 0.5 meters per second. This results in a contraction of the ventricles that proceeds upward from the apex of the heart toward its base. The pathway taken by each action potential generated by the SA node is represented schematically as

The spontaneous generation of an action potential within the SA node initiates a sequence of events known as the cardiac cycle. Each cardiac cycle lasts approximately 0.8 second and spans the interval from the end of one heart contraction to the end of the subsequent heart contraction.

Ordinarily this occurs about 72 times each minute.

The cardiac cycle has two basic components:

(1) a contraction phase (systole) during which blood is ejected from the heart

(2) a relaxation phase (diastole) during which the chambers of the heart are filled with blood.

The spontaneous generation of an action potential within the SA nodal tissue represents the start of the cardiac cycle. This electrical impulse spreads throughout the atrial muscle and leads to contraction of the two atria.

As the atria contract, the AV valves remain open and additional blood is forced into the ventricles from the veins. A large amount of blood has already passed from the atria to the ventricles prior to atrial contraction.

The aortic and pulmonary (pulmonic) semilunar valves remain closed .

After the ventricles have filled (mostly by blood returning from the large veins) and the atria have contracted, the AV valves close as the ventricles begin their contraction.

Ventricular contraction forces blood through the semilunar valves into the aorta and pulmonary trunk.

Next, as the ventricles begin to relax, the aortic and pulmonic semilunar valves close, the AV valves open, and blood flows into the ventricles to begin another cycle.

While the atria are in systole, the ventricles are relaxed (in diastole). The atria relax during ventricular systole and remain in this phase even during a portion of ventricular diastole.

Blood (like any other fluid) tends to flow from a region of high pressure to one of lower pressure.

As each chamber of the heart fills with blood, the pressure increases within it. The blood moves out of the chamber, when the various one-way valves guarding those chambers permit it to do so.

For example, intra-atrial pressure increases as blood from the veins enters them, and this pressure increases further during atrial systole.

As the ventricles contract, the blood is forced in a retrograde fashion against the AV valves, which causes them to bulge inward slightly toward the atria and which also elevates atrial pressure.

In doing so, the AV valves are effectively closed and blood is prevented from regurgitating back into the atria. Near the end of ventricular systole the AV valves are still closed and since the atria are in the process of filling, this too contributes to a rise in intra-atrial pressure.

Even before the atria enter systole, the ventricles are filled with blood to approximately 70% of their capacity. When the atria do finally contract, additional blood enters the ventricles and elevates the intraventricular pressure. As the ventricles contract, blood is forced backward, closing the AV valves, and a sharp rise in ventricular pressure occurs.

Although the ventricles exist as closed chambers for a brief moment, the pressure within them soon exceeds that in the aorta and pulmonary trunk. When this happens the aortic and pulmonic semilunar valves are forced open under pressure and blood rushes out of the ventricles and is driven into these large vessels. Accompanying the opening of the semilunar valves is a rapid decline in intraventricular pressure that continues until the pressure within the ventricles becomes less than that of the atria. When this pressure differential is reached, blood within the atria pushes the AV valves open and begins to fill the ventricles once again.