Cardiac muscle tissue is only found in the heart, specifically the myocardium. Highly coordinated contractions of cardiac muscle pump blood into the vessels of the circulatory system such as the aorta and pulmonary artery. Cardiac muscle is striated and organized into sarcomeres, possessing the same organization of myofilaments as skeletal muscle. However, cardiac muscle fibers are shorter than skeletal muscle fibers and usually contain only one nucleus, which is located in the central region of the cell. Cardiac muscle fibers also contain many mitochondria. This is necessary as large amounts of ATP are required by the cells and this is produced primarily through aerobic metabolism.
Cardiac muscle fibers cells also are extensively branched and are connected to one another at their ends by intercalated discs. An intercalated disc allows the cardiac muscle cells to contract in a coordinated fashion so that the heart can work as a pump. Intercalated discs are part of the sarcolemma and contain two structures important in cardiac muscle contraction: gap junctions and desmosomes. Gap junction form channels between adjacent cardiac muscle fibers that allow the depolarizing action potential to move between one cardiac muscle cell to the next. It allows the quick transmission of action potentials and the coordinated contraction of the entire heart. The remainder of the intercalated disc is composed of desmosomes that were discussed in the epithelium chapter. Desmosomes anchor the ends of cardiac muscle fibers together so the cells do not pull apart during individual fiber contraction.
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