Cardiac muscle tissue is found exclusively in the heart. Compared to skeletal muscles, cardiac muscle cells are small and usually mononucleated. They are typically branched and composed of regularly arranged sarcomeres in the myofibrils, giving them a striated appearance. At the boundaries of these sarcomeres, the short but broad transverse or T-tubules arise from the sarcolemma and closely associate with the sarcoplasmic reticulum. To meet the high energy demand of these cells, they have large and numerous mitochondria, along with vast reserves of glycogen and lipids. Adjacent cardiac muscle cells are connected by interlocking regions of thickened sarcolemma called intercalated discs. These regions contain gap junctions, which control the passage of ions and help propagate electrical signals during muscle contraction. The intercalated discs also have an abundance of desmosomes that anchor the cells to each other and prevent cell separation during contraction. In this way, the cardiac cells can coordinate to produce the rhythmic contractions of the heart.