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Anatomy Atlases: Atlas of Microscopic Anatomy: Section 1 - Cells Atlas of Microscopic Anatomy: Section 8 - Cardiovascular System

Plate 8.150 Atrioventricular Node and Bundle

Ronald A. Bergman, Ph.D., Adel K. Afifi, M.D., Paul M. Heidger, Jr., Ph.D.
Peer Review Status: Externally Peer Reviewed

Specialized Cardiac Muscle Fibers

Plate 8.150 Atrioventricular Node and Bundle

The atrioventricular (AV) node and bundle are composed of cardiac muscle fibers specialized for impulse conduction. The AV node (node of Tawara*) is found in the subendocardium of the right atrium, close to the termination of the coronary sinus. Note the irregularly arranged branching fibers (nodal fibers) that form the AV node. Note also how the AV nodal fibers become continuous with the small unbranched fibers of the AV bundle (bundle of His). The latter originate in the AV node, continue into the interventricular septum, and divide into two trunks composed of Purkinje fibers, which pass to the right and left ventricular wall, where they become continuous with ordinary cardiac muscle fibers. Stimuli for cardiac contraction are initiated in the sinoatrial (SA) node, reach the AV node, and spread to the myocardium via the AV bundle. Injury to the bundle results in dissociation of atrial and ventricular rhythms.

Note the parasympathetic ganglion cells and the autonomic nerve fibers in the wall of the heart. The parasympathetic ganglia receive vagal fibers, which slow the heart rate while the sympathetic postganglionic fibers carry impulses that increase the heart rate. The autonomic fibers include sympathetic postganglionic and parasympathetic pre- and postganglionic fibers.

*Tawara, 1873-1952, was a Japanese physician.

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