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Conduction system

1. Conduction system of heart Asra khan

2. Table of contents: Anatomy of the conduction system. Electrical changes in the heart.

3. Conducting system of heart: The heart possesses the property of autorhythmicity, which means it generates its own electrical impulses and beats independently of nervous or hormonal control. Thus, heart is not reliant on external mechanisms to initiate each heart-beat. However it is supplied with both sympathetic and parasympathetic nerve fibers, which increase and decrease the heart rate respectively. In addition, the heart responds to a number of circulating hormones, including adrenaline and thyroxine.

4. Parts of conducting system: Sinoatrial node (SA node) Atrioventricular node (AV node) Atrioventricular bundles (bundle of His) Left and right bundle branches Purkinje fibers

5. Sinoatrial node (SA node): This small mass of specialized cells lies in the right atrium wall near the superior vena cava's opening. The sinoatrial cells generate these regular impulses because they are electrically unstable. This instability leads them to discharge (depolarize) regularly, usually between 60 to 80 times a minute.

6. SA node The depolarization is followed by recovery (repolarization), but almost immediately their instability leads them to discharge again, setting the heart rate. Because the SA node discharges faster than any other part of the heart it normally sets the heart rate and is called the “pacemaker” of the heart. Firing of SA node triggers atrial contraction.

7. Atrioventricular node: The small mass of neuromuscular tissue is situated in the atrial septum wall near the atrioventricular valves. Normally, the AV node merely transmits the electrical signals from the atria to the ventricles. There is a delay here: the electrical signal takes 0.1 of a second to pass through into the ventricles. This allows the atria to finish contracting before the ventricles.

8. Atrioventricular node: The AV node also has a secondary pacemaker function and takes over this role if there is a problem with the SA node itself, or with the transmission of impulses from the atria. Its intrinsic firing rate, however, is slower than that set by SA node (40-60 beats per minute).

9. Bundle of HIS: It begins from the AV node, crosses the AV ring, and runs along the inferior portion of the membranous part of the interventricular septum where it further divides into right and left bundle branches extending inferiorly to the interventricular septum and superiorly to the lateral walls of the ventricles. Since the fibrous framework of the heart separates the muscles of the atria and ventricles, the bundle of His is the only means of conducting impulses from the atria to the ventricles.

10. Right and left bundle branches: The right bundle branch enters down the right side of the interventricular septum and becomes subendocardial on the right of the septum. The left branch descends the left side of the interventricular septum, divides into Purkinje fibers that are distributed to the septum and the left ventricle.

11. Purkinje fibers: The Purkinje fibers (subendocardial plexus of conduction cells) are a network of specialized cells. they are abundant with glycogen and have extensive gap junctions. These cells are located in the subendocardial surface of the ventricle walls and can rapidly transmit cardiac action potential from the AV bundle to the myocardium of the ventricles. This rapid contraction allows coordinated ventricular contractions (ventricular systole) and blood is moved from the right and left ventricles to pulmonary artery and aorta respectively.

12. Nerve supply to heart: The heart is influenced by sympathetic and parasympathetic (autonomic) nerves originating in the cardiovascular center in the medulla oblongata. The vagus nerve (parasympathetic) supplies mainly the SA and AV nodes and atrial muscles. Vagal stimulation reduces the rate at which impulses are produced, decreasing the force and rate of heartbeats. Sympathetic nerves supply SA and AV nodes and the myocardium of atria and ventricles, and stimulation increases the rate and force of the heartbeat.

13. Electrical changes in the heart: The body tissues and fluids conduct electricity well, so the electrical activity in the heart can be recorded on the skin’s surface using electrodes positioned on the limbs and or the chest. This recording is called an “electrocardiogram” (ECG) . ECG shows the spread of electrical signals generated by SA node as it travels through the atria, the AV node and the ventricles. The normal ECG tracing shows five waves which by convention, have been named P, Q, R, S, T.

14. P-wave: The P wave arises when the impulse from the SA node sweeps over the atria (atrial depolarization).

15. QRS complex: The QRS complex represents the very rapid spread of impulse from the AV node through the AV bundle and Purkinje fibers, including the electrical activity of the ventricular muscles (ventricular depolarization). note that the delay between the completion of the P wave and the onset of QRS complex represents the conduction of the impulse through AV node, which is much slower than elsewhere in the heart, and allows atrial contraction to finish completely before ventricular contraction starts.

16. T wave It represents the relaxation of the ventricular muscles (ventricular repolarization). Atrial repolarization occurs during ventricular contraction, and so it is not seen because of larger QRS complex.

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