Question

Describe the 5 types of autorhytmithmic cardiac cells/tissues that are involved in the cardiac conduction (nodal) system

Answer 1

Rhythmicity is the ability of a tissue to produce its own impulses regularly. It is also called autorhythmicity or self-excitation. Property of rhythmicity is present in all the tissues of heart. However, heart has a specialized excitatory structure, from which the discharge of impulses is rapid. This specialized structure is called pacemaker. From here, the impulses spread to other parts through the specialized conductive system.

Rhythmicity of Different Parts of Human Heart
1. SA node : 70 to 80/minute
2. AV node : 40 to 60/minute
3. Atrial muscle : 40 to 60/minute
4. Purkinje fibers : 35 to 40/minute
5. Ventricular muscle : 20 to 40/minute.

Components of Conductive System in Human Heart
1. AV node
2. Bundle of His
3. Right and left bundle branches
4. Purkinje fibers.
SA node is situated in right atrium, just below the opening of superior vena cava. AV node is situated in right posterior portion of intra-atrial septum. Impulses from SA node are conducted throughout right and left atria. Impulses also reach the AV node via some specialized fibers called internodal fibers.
There are three types of internodal fibers:
1. Anterior internodal fibers of Bachman
2. Middle internodal fibers of Wenckebach
3. Posterior internodal fibers of Thorel.
All these fibers from SA node converge on AV node and interdigitate with fibers of AV node. From AV node, the bundle of His arises. It divides into right and left branches, which run on either side of the interventricular septum. From each branch of bundle of His, many Purkinje fibers arise and spread all over the ventricular myocardium.

1. Sinoatrial node.
Sinoatrial (SA) node is located in the wall of right atrium, just right to the opening of superior vena cava. Its dimensions are about 15 mm length, 2 mm width and 1 mm thickness. Spontaneous rhythmical electrical impulses arise from the SA node and spread in all directions to:
Cardiac muscles of atria,
Interatrial tract to left atrium and
Internodal tracts to AV node.

2. Interatrial tract (Bachman’s bundle).
It is a band of specialised muscle fibres that run from the SA node to the left atrium. It causes simultaneous depolarization of the atria.

3. Internodal conduction pathway.
Three internodal conduction paths have been described .
Anterior internodal pathway of Bachman leaves the ante-
rior end of the SA node and passes anterior to the supe-
rior vena cava opening. It then descends on the atrial
septum and ends in the AV node.
Middle internodal pathway of Wenckebach leaves the
posterior end of the SA node and passes posterior to the
superior vena cava opening. It then descends on the atrial
septum to the end in the AV node.
Posterior internodal pathway of Thorel leaves the posterior part of the SA node and descends through the crista terminalis and the valve of inferior vena cava to the AV node.

4. Atrioventricular node.
The AV node is located just beneath the endocardium on the right side of lower part of the atrial septum, near the tricuspid valve. It is stimulated by the excitation wave that travels through the internodal tracts and the atrial myocardium. From it, the cardiac impulse is conducted to the ventricles by the AV bundle.

5. Atrioventricular bundle of His.   
The AV bundle arises from the AV node, descends through the fibrous skeleton of the heart and divides into right bundle branch for the right ventricle and the left bundle branch for the left ventricle. The branches break up and become continuous with the plexus of Purkinje fibres.

6. Purkinje fibres.
These are spread out deep to the endocardium and reach all parts of the ventricles including the bases of papillary muscles.