The ventricular Purkinje system of the heart: rapid conduction

2021-06-02 03:45 PM

The rapid transmission of action potentials by Purkinje fibre is thought to be caused by a very high permeability of the junctional spaces in the interstitial discs between successive cells constituting the Purkinje fibre.

The human heart has a special system for self-stimulating and contracting rhythms that repeat approximately 100,000 times per day, or 3 billion times in an average human lifetime. This impressive feat is accomplished by a system that (1) generates rhythmic electrical impulses to initiate rhythmic contractions of the heart muscle and (2) conducts these pulses rapidly through the heart. When this system is functioning properly, the atria contract about a sixth of a second before the ventricles, allowing the ventricles to fill before they pump blood up to the lungs and into the main circulation. Another important aspect of the system is to allow all parts of the ventricles to contract almost simultaneously, which is most essential for the effect of pressure generation in the ventricles.

The conduction and pacing systems are vulnerable to cardiac diseases, especially myocardial ischemia resulting from decreased coronary perfusion. The result is often an arrhythmia or abnormal heart rate, and the pumping efficiency of the heart is often severely affected and even fatal.

The excitatory and conduction system of the heart controls heart contractions. This figure shows the sinus node (also known as the sinoatrial or SA node) pacing in a normal pacemaker system, following the impulse path from the sinus node to the atrioventricular (AV) node; AV node, the atrial impulse is delayed before crossing the atrioventricular septum into the ventricles; the atrioventricular bundle conducts impulses from the atria to the ventricles, and the left and right branches of the Purkinje mesh conduct impulses to all parts of the ventricles.

Figure. The sinus node and the Purkinje system of the heart also exhibit the atrioventricular (AV) node, the internodal pathway, and the ventricular bundles.

Purkinje fibre specifically leads from the AV node through the AV bundle into the ventricles. Except for the initial part of these fibres where they penetrate the AV fibre barrier, they have a functional feature that is completely opposite to those in the AV nodal fibre. They are very large fibre, even larger than normal ventricular muscle fibre, and they transmit action potentials at velocities between 1.5 and 4.0 m/s, about six times that of the cardiac muscle. normal ventricles and 150 times that of some of the AV node fibre. This velocity allows near-instant transmission of cardiac impulses throughout the rest of the ventricular muscle.

The rapid transmission of action potentials by Purkinje fibre is thought to be caused by a very high permeability of the junctional spaces in the interstitial discs between successive cells constituting the Purkinje fibre. Thus, ions are easily transferred from one cell to the next, thereby improving the transmission rate. Purkinje fibre also has very little myofibril, meaning they contract little or no during impulse conduction.

A conduction path through the AV. bundle

A distinctive feature of the AV bundle is the inability, except in abnormal states, to move action potentials backwards from the ventricles to the atria. This feature prevents the re-entry of cardiac impulses by this pathway from the ventricles to the atria, allowing only forward conduction from the atria to the ventricles.

Besides, it should be recalled that everywhere, except in the AV bundle, the atrial muscle is separated from the ventricular muscle by a continuous fibrous barrier, part of which. This barrier normally acts as an insulator to block the passage of cardiac impulses between the atrial muscle and the ventricle through any pathway other than forwarding conduction through the AV bundle. (In rare cases, an abnormal muscle bridge crosses the barrier fibres elsewhere than at the AV bundle. In such conditions, cardiac impulses can re-enter the atria from the ventricles and cause arrhythmias. severe heart).

Distribution of Purkinje fibres in the left and right ventricles

After penetrating the fibrous tissue between the atrial and ventricular muscles, the distal portions of the AV bundle descend within the interventricular septum 5 to 15 mm toward the apex of the heart. The bundle then divides into left and right bundle branches located below the endocardium on either side of the interventricular septum, respectively. Each branch disperses downward towards the apex of the ventricle, gradually dividing into smaller branches. These branches have a lateral turn around each ventricular chamber and toward the base of the heart. The ends of the Purkinje fibres penetrate approximately one-third of the extent into the myocardium and eventually become continuous with the myocardium.

The total time elapsed on average was only 0.03 s from the time the cardiac impulse entered the bundle branch in the ventricular septum until it reached the termination of the Purkinje fibres. Thus, once the cardiac impulse enters the Purkinje conduction system of the ventricles, it propagates almost instantaneously to the entire ventricular mass.