1. CONDUCTING SYSTEM OF HEART; ORIGIN & SPREAD OF CARDIAC IMPULSE

2. Learning Objectives The student will be able to: (MUST KNOW) 1.Name the parts of conducting system of the heart. 2.Appreciate the importance of AV nodal delay. 3.Explain the mechanism of AV nodal delay. 4.Give the conduction velocity in different cardiac tissues. 5.Understand the propagation of electrical impulse in conducting system of heart.

3. Conducting system of heart SA node Internodal pathways AV node His bundle Bundle branches Purkinje fibers

5. SA Node It is the primary pacemaker of heart. Location - right atrium close to the opening of superior vena cava. It is about 1.5 cm long and 0.5 cm wide. Contains pacemaker (P) cells which generate impulses for excitation and contraction of muscles. The action potentials from SA node are of slow response type. The velocity of conduction of impulse in SA node is slow (0.05 m/s).

6. Internodal Pathways Three internodal pathways that connect SA node and AV node. –Anterior internodal pathway - tract of Bachman. –Middle internodal pathway - tract of Wenckebach. –Posterior internodal pathway - tract of Thorel. The velocity of conduction of impulse in internodal pathways is about 1 m/s. From SA node, a conducting tract arises and directly enters into the left atrium. This is called interatrial tract or Bachman’s bundle.

7. AV Node Location - lower part of the right atrium close to the interatrial septum and just above the atrioventricular ring. Length - 22 mm, width - 10 mm and thickness - 3 mm. In AV node, the fiber diameter is small and there are multiple sub-branches. Therefore, the rate of impulse conduction is slow in AV node (0.05 m/s). Usually, a delay of about 0.1 second occurs for the impulse to be transmitted through AV node. This is called AV nodal delay. This delay is shortened by sympathetic stimulation and lengthened by parasympathetic stimulation to the heart. The ability of the AV node to slow down the transmission of rapid impulses from SA node to the ventricle is also called decremental conduction.

8. The action potentials generated in AV node are of slow response type. Pacemaker (P) cells are also present in the AV node but normally AV node is not the pacemaker because the rate of impulse formation is lower in it than that of the SA node. The pacemaker cells of AV node are suppressed by the SA nodal impulses. However, when SA node stops producing impulses, AV node becomes the pacemaker of the heart.

10. His Bundle It arises from AV node and terminates in the Purkinje system. Location - below the AV node and passes towards the interventricular septum. Length - 1 cm, which on entering the interventricular septum divides into right bundle branch and left bundle branch. When SA node and AV node are defunct, the bundle of His generates impulses.

11. Bundle Branches His bundle divides into two bundle branches Right Bundle Branch (RBB) –RBB is longer and thinner than the left bundle branch. It exclusively supplies right ventricle. Left Bundle Branch (LBB) –LBB bifurcates into two divisions: the anterior division that supplies the anterior portion of the left ventricle, and the posterior division that supplies the posterolateral portion of the left ventricle. The bundle branches enter the ventricular walls and then branch out to form Purkinje fibers.

12. Purkinje Fibers This is a network of conducting fibers that are present in the sub- endocardial regions of right and left ventricles. The purkinje cells are the largest cells in the heart. Numerous gap junctions are present. Because of the larger diameters of the fiber and presence of gap junctions, the rate of impulse conduction is highest in the Purkinje fibers. The conduction rate is almost 4 m/s. The action potentials generated in the Purkinje fibers are of fast response type.

15. Initiation & propagation of cardiac electrical activity

16. Excitation is initiated in the SA Node Electrical activity is initiated in SA node and spreads in an orderly fashion. The wave of depolarization spreads in all directions, to right and left atria directly and to the AV node via internodal pathways. However, impulse from atrial muscle cannot directly excite the ventricular muscle because the atrioventricular ring. Therefore, transmission of electrical activity from the atria to the ventricles occurs only from AV node through the bundle of His.

17. Conduction Slows in AV Node It takes about 60 to 90msec to excite the atria. Conduction of the impulse through the AV node occurs at a much slower velocity (0.05 m/sec). Thus, the transmission of impulse is delayed in the AV node for about 0.1 s. This is called the AV nodal delay.

18. The slower conduction velocity in AV node is due to three factors: 1. Small size of the nodal cells and their branching patterns. 2. Slow upstroke of the action potential, which occurs due to slow voltage-gated Ca 2+ channels 3. Weak electrical coupling as a result of relatively few gap junctions.

19. Importance of AV Nodal Delay Physiological and clinical importance of AV nodal delay: 1. Allows Ventricular Filling: Due to AV nodal delay, the atrial depolarization completes much before the beginning of ventricular depolarization. Therefore, when atrial systole occurs, ventricle is in diastole. This helps in ventricular filling to occur. 2. Maintains low ventricular rate in AF: Rate of atrial depolarization is very high as occurs in atrial fibrillation (AF). In such conditions, all the electrical impulses from atria cannot reach the ventricles because of inherent AV nodal delay. Thus, ventricles contract at a lower rate than the atria.. 3. Effects of Drugs: Drugs such as digitalis, beta-blockers etc. reduce heart rate partly by promoting AV nodal delay. 4. Vagal Stimulation: Causes AV nodal block.

20. Conduction is Rapid in Ventricles The impulse spreads rapidly into the ventricular muscles via His- Purkinje system. The Purkinje system is composed of fibers with large diameter. The slope of depolarization is also very steep. Therefore, the conduction velocity is maximum in Purkinje fibers (4 m/s) in the heart. The wave of depolarization takes about 0.08–0.1 s to spread rapidly from the top of the septum through the Purkinje fibers to all part of the ventricles. The depolarization of the ventricle starts at the left side of the interventricular septum and then crosses to the right side through the interventricular septum, and then spreads down to the apex of the heart. Then it depolarizes the muscles in the wall of the ventricle from endocardial to the epicardial surface.

21. In ventricles, the endocardial surface depolarizes before the epicardial surface. The portions to be depolarized last are the posterobasal portion of the left ventricle, the pulmonary conus, and the upper most portion of the septum. The process of repolarization of ventricular muscle occurs almost in the reverse direction. The conduction velocity through ventricular muscle is 1 m/sec. The complete excitation of both ventricles takes about 75 msec.