A contraction of the heart’s chambers in a coordinated fashion is necessary for the heart’s normal function. The cardiac conduction system is responsible for initiating the electrical impulse and transmitting it regularly through the heart.
Sinoatrial (SA) Node
Stimulation of cardiac muscle cells is initiated in a small group of pacemaker cells located in the center of the SA node. The SA node is the natural pacemaker and sets a heart rate of 60 to 100 beats per minute. External nervous system stimulation is not necessary for SA node activity but can impact SA node activity. The SA node is existing close to the junction between the superior vena cava and the right atrium. The RCA provides the SA node in 55% of the population, and by the LCX in the remaining 45% of the community. From the SA node, the depolarization travels through the right atrial tissue via internodal pathways and the left atrial tissue via Bachmann’s Bundle. From the internodal trails, the impulse travels to the AV node.
Atrioventricular (AV) Node
The AV node slows conduction from the atria to the ventricles. Conduction is slowed to ensure that the ventricles are relaxed atrial contraction, allowing them to fill before contracting. Additionally, the AV node protects the ventricles from fast atrial rates by preventing the transmission of rapid atrial impulses.
The AV node is located in the posterior wall of the low right atrium behind the tricuspid valve and next to the origin of the coronary sinus.
The electrical impulse travels along with the His Bundle from the AV node, which divides into the bundle branches.
The AV node and the His Bundle are surrounded by tissue that is infiltrated with cells with automaticity (pacemaker cells). This is referred to as the AV Junction. The AV junction can initiate a heart rate of 40-60 beats per minute if the sinus node fails as the normal pacemaker.
The bundle branches are fiber bundles that rapidly conduct the impulse through the right and left ventricles. The right bundle branch carries the inspiration to the right ventricle. The left bundle branch divides into the left posterior fascicle and the left anterior fascicle. The left posterior fascicle carries the impulse to the posterior and inferior left ventricle. The left anterior fascicle moves the inspiration to the anterior and superior left ventricle. Blocks of the posterior fascicle of the left bundle branch are rare, as the posterior fascicle has a dual blood supply and receives oxygenated blood from the LAD and the RCA. The tissue of the posterior bundle itself is thick and dense compared to the thin band of tissue that makes up the anterior fascicle of the left bundle branch and the right bundle branch making the posterior fascicle less susceptible to injury or ischemia.
From the bundle branches, the impulse travels to the Purkinje fibers, where the depolarization is carried through the subendocardial layers of the heart. Some Purkinje cells can function as pacemaker cells and initiate a heart rate of 20 to 40 beats per minute if no impulse is received through the healthy conduction system.