Cardiac muscle has the same types and arrangement of contractile filaments as skeletal muscle. Therefore, cardiac muscle cells and the fibers they form exhibit cross-striations evident in routine histologic sections. In addition, cardiac muscle fibers exhibit densely staining cross-bands, called intercalated discs, that cross the fibers in a linear fashion or frequently in a way that resembles the risers of a stairway (Fig. 11.14 and Plate 24, page 346). The intercalated discs represent highly specialized attachment sites between adjacent cells. This linear cell-to-cell attachment of the cardiac muscle cells results in “fibers” of variable length. Thus, unlike skeletal and visceral striated muscle fibers that represent multinucleated single cells, cardiac muscle fibers consist of numerous cylindrical cells arranged end to end. Furthermore, some cardiac muscle cells in a fiber may join with two or more cells through intercalated discs, thus creating a branched fiber.
terminal cisterna of sarcoplasmic reticulum
FIGURE 11.15 • Diagram of the organization of cardiac muscle fiber. The T tubules of cardiac muscle are much larger than the T tubules  of skeletal muscle and carry an investment of external lamina material into the cell. They also differ in that they are located at the level of the Z disc. The portion of the sarcoplasmic reticulum adjacent to the T tubule is not in the form of an expanded cisterna but rather is organized as an anastomosing network.
Structure of Cardiac Muscle
The cardiac muscle nucleus lies in the center of the cell.
The central location of the nucleus in cardiac muscle cells is one feature that helps distinguish them from multinucleated skeletal muscle fibers, whose nuclei lie immediately under the plasma membrane. The transmission electron microscope (TEM) reveals that the myofibrils of cardiac muscle separate to pass around the nucleus, thus outlining a biconical juxtanuclear region in which the cell organelles are concentrated. This region is rich in mitochondria and contains the Golgi apparatus, lipofuscin pigment granules, and glycogen. In the atria of the heart, atrial granules measuring 0.3 to 0.4 m in diameter are also concentrated in the juxtanuclear cytoplasm. These granules contain two polypeptide hormones: atrial natriuretic factor (ANF) [L. natrium, sodium] and brain natriuretic factor (BNF). Both hormones are diuretics, affecting urinary excretion of sodium. They inhibit renin secretion by the kidney and aldosterone secretion by the adrenal gland. They also inhibit contractions of vascular smooth muscle. In congestive heart failure, levels of circulating BNF increase.
Numerous large mitochondria and glycogen stores are adjacent to each myofibril.
In addition to the juxtanuclear mitochondria, cardiac muscle cells are characterized by large mitochondria that are densely packed between the myofibrils. These large mitochondria often extend the full length of a sarcomere and contain numerous, closely packed cristae (Fig. 11.15). Concentrations of glycogen granules are also located between the myofibrils. Thus, the structures that store energy (glycogen granules) and the structures that release and recapture energy (mitochondria) are located adjacent to the structures (myofibrils) that use the energy to drive contraction.
The intercalated discs represent junctions between  cardiac muscle cells.