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 ¡xm 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 (see pages 622 and 671). 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. 10.14). 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 disks represent junctions between cardiac muscle cells
As previously noted, the intercalated disk represents the attachment site between cardiac muscle cells. In the light microscope, the disk appears as a densely staining linear structure that is oriented transversely to the muscle fiber. Often it consists of short segments arranged in a step-like fashion (Fig. 10.15). When the site of the intercalated disk is examined with the TEM, the densely staining structure seen in the light microscope can be attributed to the presence of a transverse component that
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