Atrioventricular node: slow conduction of impulses from the atria to the ventricles of the heart

2021-06-02 03:42 PM

The atrial conduction system is set up so that the heart impulse does not propagate from the atria to the ventricles too quickly; This slow conduction allows the atria to eject blood into the ventricles to fill the ventricles before ventricular systole.

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.

The atrial conduction system is set up so that the heart impulse does not propagate from the atria to the ventricles too quickly; This slow conduction allows the atria to eject blood into the ventricles to fill the ventricles before ventricular systole.

The AV node is proximal to the conducting fibres and is the main cause of the delay in the conduction of electrical impulses to the ventricles.

The AV node is located in the posterior wall of the right atrium immediately posterior to the tricuspid valve, and the figure shows the various parts of this node, along with connections to the atrial internodal fibres entering and exiting the AV bundle. This figure also shows the approximate interval in fractions of a second between the initial onset of the cardiac impulse in the sinus node and its subsequent arrival in the AV nodal system. Note that the impulse, after passing through the internodal pathway, reaches the AV node approximately 0.03s after its origin in the sinus node. There is then an additional 0.09s delay in the AV node itself before the impulse penetrates parts of the AV bundle, where it travels to enter the ventricles. A final 0.04s delay occurs mainly in the penetration of this AV bundle, which consists of many small bundles that pass through the fibrous tissue separating the atria from the ventricles.

Thus, the total delay time in the AV node system and the AV bundle is about 0.13s. This delay, combined with a 0.03 s delay in conduction from the sinus node to the atrioventricular node, results in a total delay of 0.16 s before the final excitatory signal arrives to contract the ventricular muscle.

Causes of slow conduction. Slow conduction in transitional, nodal and transverse AV bundle fibres is mainly due to a decrease in the number of junctional spaces between successive cells in the conduction pathway, so there is great resistance to conduction. Excited ions from one strand lead to the next. Therefore, it is easy to see why each successive cell is more slowly stimulated.