Like all cells, muscle cells depend on the energy source contained in the high-energy phosphate bonds of ATP and phosphocreatine. The energy stored in these high-energy phosphate bonds comes from the metabolism of fatty acids and glucose. Glucose is the primary metabolic sub-strate in actively contracting muscle. It is derived from the general circulation as well as from the breakdown of glyco-gen, which is normally stored in the muscle fiber cyto-plasm. As much as 1% of the dry weight of skeletal and cardiac muscle may be glycogen. In rapidly contracting muscles, such as the leg muscles in running or the extraocular muscles, most of the energy for contraction is supplied by anaerobic glycolysis of stored glycogen. The buildup of intermediary metabolites from this pathway, particularly lactic acid, can produce an oxygen deficit that causes ischemic pain (cramps) in cases of extreme muscular exertion. Most of the energy used by muscle recovering from contraction or by resting muscle is derived from oxidative phosphorylation. This process closely follows the -oxidation of fatty acids in mitochondria that liberates two carbon fragments. The oxygen needed for oxidative phosphorylation and other terminal metabolic reactions is derived from hemoglobin in circulating erythrocytes and from oxygen bound to myoglobin stored in the mus-cle cells.
Myosin II, a 510-kilodalton protein, is composed of two polypeptide heavy chains (222 kilodaltons each) and four light chains. Light chains are of two types (essential light chains [18 kilodaltons] and regulatory light chains [22 kilodaltons]), and one molecule of each type is present in association with each myosin head. The phosphorylation by myosin light chain kinase of the regulatory light chain initiates contraction in smooth muscles. Each heavy chain has a small globular head that projects at an approximately right angle at one end of the long rod-shaped molecule. This globular head has two specific binding sites, one for ATP and one for actin. It also demonstrates ATPase and motor activity. Myosin molecules in striated muscles aggregate tail to tail to form bipolar thick myosin filaments; the rod-shaped segments overlap so that the globular heads project from the thick filament. The “bare” zone in the middle of the filament (i.e., the portion of the filament that does not have globular projections) is the H band. The projecting globular heads of the myosin molecules form cross-bridges between the thick and thin filaments on either side of the H band (see Fig. 11.6).
Accessory proteins maintain precise alignment of thin and thick filaments.
To maintain efficiency and speed of muscle contraction, both thin and thick filaments in each myofibril must be aligned precisely and kept at an optimal distance from one another. Proteins known as accessory proteins are essential in regulating the spacing, attachment, and alignment of the myofilaments. These structural protein components of skeletal muscle fibrils constitute less than 25% of the total protein of the muscle fiber. They include the following (see also Fig. 11.7):  Titin, a large (2,500-kilodalton) protein, forms an elastic lattice that anchors thick filaments in the Z lines. Two springlike portions of the protein adjacent to the thin  filaments help stabilize the centering of the myosincontaining thick filament, preventing excessive stretching of the sarcomere.
 -Actinin, a short, bipolar, rod-shaped, 190-kilodalton actin-binding protein, bundles thin filaments into parallel arrays and anchors them at the Z line.
 Nebulin, an elongated, inelastic, 600-kilodalton protein, is attached to the Z lines and runs parallel to the thin filaments. It helps -actinin anchor thin filaments to Z lines and is thought to regulate the length of thin filaments during muscle development.