The ventricular muscle of the heart: conduction of impulses

2021-06-02 03:48 PM

The myocardium surrounds the heart in a double helix, with a fibrous septum between the layers; therefore, the cardiac impulse does not necessarily travel directly outward towards the surface of the heart.

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.

Once the impulse reaches the ends of the Purkinje fibres, it is transmitted through the ventricular muscle mass by the ventricular muscle fibres themselves. The conduction velocity is now only 0.3 - 0.5 m/s, which is 1/6 of the Purkinje fibres.

The myocardium surrounds the heart in a double helix, with a fibrous septum between the layers; therefore, the cardiac impulse does not necessarily go directly outward towards the surface of the heart but instead forms an angle towards the pericardium in the direction of the spiral. Because of this angle, conduction from the endocardial surface to the pericardial surface of the ventricles requires approximately 0.03s, approximately equal to the time required for conduction through all the ventricular portions of the ventricles. Purkinje system. Thus, the total time for the transmission of cardiac impulses from the primary to terminal bundles of ventricular muscle fibres in a normal heart is about 0.06s.