ELECTRICAL CONDUCTION SYSTEM



Cardiac electrical impulses originate in the sinus node, a spindleshaped structure 10 to 20 mm long located near the junction of the superior vena cava and the right atrium. Even though various specialized tissues have been postulated to conduct the electrical impulse from the sinus node to the AV node, electrical transmission is probably celltocell via working atrial muscle. The AV node provides the only normal conduction pathway between the atria and the ventricles. It is situated just beneath the right atrial endocardium above the insertion of the septal leaflet of the tricuspid valve and anterior to the ostium of the coronary sinus. After conduction delay in the AV node, the electrical impulse travels to the His bundle, which descends posteriorly along the membranous interventricular septum to the top of the muscular septum. The His bundle gives rise to the right and left bundle branches. The right bundle branch is a single group of fibers that travels down the right ventricular side of the muscular interventricular septum. The left bundle branch is a larger, less discrete array of conducting fibers located on the left side of the interventricular septum. The left bundle branch may divide into two somewhat distinct pathways that travel toward the anterolateral (left anterior fascicle) and posteromedial (left posterior fascicle) papillary muscles. The left posterior fascicle is larger and more diffuse than the anterior fascicle and usually has a more reliable vascular supply than either the left anterior fascicle or the right bundle branch. The left and right bundle branches progressively divide into tiny Purkinje fibers that arborize and finally make intimate contact with ventricular muscle tissue.