Cardiac muscles, as is evident from their name, make up the muscular portion of the heart. While almost all cardiac muscle is confined to the heart, some of these cells extend for a short distance into cardiac vessels before tapering off completely. The heart muscle is also called the myocardium. The heart muscle is responsible for more than two billion beats in a lifetime. The myocardium has some properties similar to skeletal muscle tissue, but it is also unique. Like skeletal muscles, myocardium is striated; however, the cardiac muscle fibers are smaller and shorter than skeletal muscle fibers averaging 5-15 micrometers in diameter and 20-30 micrometers in length. In addition, cardiac muscles align lengthwise more than side-by-side compared to skeletal muscle fibers. The microscopic structure of cardiac muscle is also unique in that these cells are branched such that they can simultaneously communicate with multiple cardiac muscle fibers.
Cardiac muscle cells are surrounded by an endomysium like the skeletal muscle cells. But innervation of autonomic nerves to the heart do not form any special junction like that found in skeletal muscle. Instead, the branching structure and extensive interconnectedness of cardiac muscle fibers allows for stimulation of the heart to spread into neighboring myocardial cells; this does not require the individual fibers to be stimulated. Although external nervous stimuli can enhance or diminish cardiac muscle contraction, heart muscles can also contract spontaneously making them myogenic. Like skeletal muscle cells, cardiac muscle fibers can increase in size with physical conditioning, but they rarely increase in number.