Pathway through the heart: After depolarization and repolarization occur, the resulting electrical impulse travels through the heart along a pathway called the conduction system. Impulses travel out from the SA node and through the internodal tracts and Bachmann’s bundle to the AV node. From there, they travel through the bundle of His, the bundle branches, and lastly to the Purkinje fibers. Setting the pace The SA node is located in the upper right corner of the right atrium, where the superior vena cava joins the atrial tissue mass. It’s the heart’s main pacemaker, generating impulses 60 to 100 times per minute. When initiated, the impulses follow a specific path through the heart. They usually can’t flow backward because the cells can’t respond to a stimulus immediately after depolarization. Bachmann’s bundle of nerves: Impulses from the SA node next travel through Bachmann’s bundle, tracts of tissue extending from the SA node to the left atrium. Impulses are thought to be transmitted throughout the right atrium through the anterior, middle, and posterior internodal tracts. Whether those tracts actually exist, however, is unclear. Impulse transmission through the right and left atria occurs so rapidly that the atria contract almost simultaneously. AV: The slow node: The AV node, located in the inferior right atrium near the ostium of the coronary sinus, is responsible for delaying the impulses that reach it. Although the nodal tissue itself has no pacemaker cells, the tissue surrounding it (called junctional tissue) contains pacemaker cells that can fire at a rate of 40 to 60 times per minute. The AV node’s main function is to delay impulses by 0.04 second to keep the ventricles from contracting too quickly. This delay allows the ventricles to complete their filling phase as the atria contract. It also allows the cardiac muscle to stretch to its fullest for peak cardiac output. Branch splitting: The bundle of His, a tract of tissue extending into the ventricles next to the interventricular septum, resumes the rapid conduction of the impulse through the ventricles. The bundle eventually divides into the right and left bundle branches. The right bundle branch extends down the right side of the interventricular septum and through the right ventricle. The left bundle branch extends down the left side of the interventricular septum and through the left ventricle. The left bundle branch then splits into two branches, or fasciculi: the left anterior fasciculus, which extends through the anterior portion of the left ventricle, and the left posterior fasciculus, which runs through the lateral and posterior portions of the left ventricle. Impulses travel much faster down the left bundle branch (which feeds the larger, thicker- walled left ventricle) than the right bundle branch (which feeds the smaller, thinner- walled right ventricle). The difference in the conduction speed allows both ventricles to contract simultaneously. The entire network of specialized nervous tissue that extends through the ventricles is known as the His-Purkinje system. Those perky Purkinje fibers: Purkinje fibers extend from the bundle branches into the endocardium, deep into the myocardial tissue. These fibers conduct impulses rapidly through the muscle to assist in its depolarization and contraction. Purkinje fibers can also serve as a pacemaker and are able to discharge impulses at a rate of 20 to 40 times per minute, some times even more slowly. Purkinje fibers usually aren’t activated as a pacemaker unless conduction through the bundle of His becomes blocked or a higher pacemaker (SA or AV node) doesn’t generate an impulse.........!!
Posted on: Wed, 11 Sep 2013 04:19:24 +0000