Stimulation and conduction of cardiac impulses

2021-06-02 03:51 PM

The sinus node (also called the sinoatrial node or the SA node) paces in a normal pacemaker system, following the impulse path from the sinus node to the atrioventricular (AV) node.

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.

Summary of conduction of cardiac impulses through the human heart. The numbers in the figure represent the intervals of time, in fractions of a second, between the onset of the cardiac impulse at the sinus node and its occurrence at each corresponding point in the heart. Note that the impulse propagates at a moderate rate through the atria but 0.1s slower in the AV node region before appearing in the AV bundle of the septum. Once it has entered this bundle, it spreads very rapidly through the Purkinje fibres to the entire endocardial surface of the ventricles. Then, the impulse again propagates slightly less rapidly through the ventricular muscles to the pericardial surface.

Figure. Transmission of cardiac impulses through the heart, showing the time of arrival (in fractions of a second after the onset of emergence in the sinoatrial node) in different parts of the heart. AV, atrioventricular; SA, atrial sinus.

It is important that students understand in detail the processes of cardiac impulses through the heart and the exact timing of its occurrence in each part of the heart; A thorough knowledge of this process is required to understand the electrocardiogram.