The excitation conduction system of the heart consists of glycogen-rich specialized cardiac myocytes. They focus the contraction signals generated by the excitation generation system and transmit them to the muscles of the atria and ventricles in a specific rhythm, creating an orderly sequence of systole (beating phase of the ventricles) and diastole (relaxation phase of the ventricles) that provides continuous blood circulation.
What is the excitation conduction system of the heart?
The excitation conduction system functions purely electrically via specialized cardiac muscle cells rather than nerves, so the system does not require specialized neurotransmitters. The cardiac excitation-conduction system is closely related to the excitation-generation system because it is also composed of specialized cardiac muscle cells and because parts of the excitation-conduction system themselves act as exciters in a backup process in certain situations. The overall system, excitation formation and excitation conduction, is semi-autonomous. In principle, it is autonomous, but it is also subject to the influence of the sympathetic and parasympathetic nervous systems, so that the performance of the heart can be adapted to changing demands via beat rate and blood pressure. The semi-autonomous excitation generation and conduction system can be indirectly controlled by external influences. At the same time, this means that the system can also be influenced and disturbed by certain neurotoxins via the sympathetic and parasympathetic nervous systems. The excitation conduction system of the heart begins at the sinus node, the pacemaker in the right atrium just below the superior vena cava. The electrical impulse generated by the sinus node is distributed by the excitation conduction system to the muscles of both atria so that they contract simultaneously. The impulse is then transmitted by the second pacing system, the [atrioventricular node]] (AV node) at the base of the right atrium and delivered with a delay of about 150 milliseconds to the His bundle, which is located in the septum between the atria and ventricles. The His bundle then divides into a left and two right ventricular legs, the tawara legs. The legs branch further at their ends into the Purkinje fibers, which transmit the contraction impulse directly to the muscle cells of the ventricular muscles, causing the ventricles to contract simultaneously. The excitation conduction system functions purely electrically via specialized cardiac muscle cells rather than nerves, so the system does not require specialized neurotransmitters.
Function and purpose
One of the two most important functions and tasks of the cardiac excitation conduction system is the orderly transmission of electrical impulses first to the muscle cells of the atria and then to the ventricular muscles. Normally, the electrical impulses are generated by the sinus node in the left atrium. In interaction with the excitation conduction system, the AV node and the His bundle, the normal heart beat is created, also known as sinus rhythm. Should the sinus node fail as a pacemaker or generate impulses that deviate greatly from the normal pattern, cells of the conduction system can in principle generate electrical beat impulses themselves, but these are usually not orderly and can lead to a very disorderly beating sequence of the heart, especially in the atria. The AV node can assume a regular safeguarding function as a secondary pacemaker. Its basic ordered frequency is 40 to 50 excitations per minute. The AV node automatically takes over if the impulses of the sinus node fall below the basic frequency of the AV node. If the AV node also fails as a backup, the His bundle, which is part of the excitation conduction system, steps in as a tertiary pacemaker for the ventricular muscles at a rate of 20 to 30 beats per minute. The process is also known as ventricular replacement rhythm. The excitation generation and conduction systems enable the maintenance of continuous blood flow in the body’s vascular system and rapid adaptation to changing demands created by different muscular actuations and by different sympathetic tone or stress modes.The advantages of the semi-autonomous system developed by evolution are that the sequence of the heartbeat cannot be readily influenced by ingested food or toxins, but only indirectly via the sympathetic and parasympathetic nervous plexus.
Diseases and ailments
Transmission of the electrical impulse generated by the sinoatrial node to the atrial muscles occurs over a wide area via specialized cardiac muscle cells before the impulses are reabsorbed by the AV node and delivered with a delay to the His bundle. Disturbances in the conduction of the contraction impulses occur frequently. They are manifested by extrasystoles, by an irregular heartbeat or by an increased or decreased beat frequency, as well as by an altered beat rhythm. The symptoms range from harmless to severe and immediately life-threatening. Relatively often, problems occur with the transmission of the beat impulse within the atria. The excitations then run disorderly or move in a circular pattern across the atria, which respond with disorderly rapid muscle contractions. Beat frequencies of 350 to 600 Hz may occur in this atrial fibrillation, but these are filtered by the AV node and typically only “pass” at a frequency of 100 to 160 and are transmitted to the ventricular muscles. This results in a loss of atrial contractions, which is noticeably associated with a 15 to 20 percent decrease in cardiac output and can lead to gradual overload of the ventricular muscles. Also quite frequently, cardiac arrhythmias – usually transient – are triggered by a so-called sinuatrial block (SA block). It is caused by delayed or interrupted transmission of the original sinus impulse to the muscles of the atria. Thus, it is a conduction problem even before the AV node is reached. SA block can have many different causes and can also be triggered by a disturbance in electrolyte composition or electrolyte concentration. All types of conduction disorders in the atria are grouped under the term sick sinus syndrome. A less common disease of the conduction system is Wolff-Parkinson-White syndrome, which refers to a disordered circular excitation between the atria and ventricles. It is caused by at least one additional conduction pathway between the atria and ventricles, bypassing the AV node. Because of the bypass of the AV node, the electrical impulses from the ventricles can also travel back to the atria.
