Skeletal muscle is the effector of all motor commands from the CNS. The degree of movement, or force, depends on the magnitude of neural excitation of skeletal muscle. Striated muscle is composed of bundles of myofibers, which are the multinucleated cells of muscle. Muscles are composed of two types of myofibers that are distinguished by their energy metabolism. Type I fibers correspond to the slow-contracting fibers that rely primarily on aerobic oxidative metabolism and are involved in more sustained contractions such as maintenance of posture. Type IIA fibers are fast-contracting but are also capable of sustained activity. These fibers are both aerobic and anaerobic; although oxidative enzymes are present, their activity is slightly less than that of type I fibers. Type IIB fibers are characterized by anaerobic or glycolytic metabolism and correspond to the fast-contracting fibers that are recruited for short periods of vigorous exercise. [4 The differences in the predominant metabolic enzymes used by these two types of muscle fibers can be readily seen by histochemical stains specific for the enzymes of either type.
Each myofiber contains many bundles of slender filaments called myofibrils ( Fig 15-1 ). Under the electron microscope myofibrils are seen to contain dark bands alternating with light bands bisected by a narrow dark line called the Z line, which gives a regular striated appearance to the muscle. The segment from one Z line to another is called a sarcomere; this is the unit of contraction because it contains the contractile proteins responsible for transforming chemical energy into mechanical force. Each sarcomere consists of thick filaments, primarily composed of the protein myosin, and thin filaments, which contain actin. Actin interacts with myosin to produce contraction of the muscle. The thin filaments also contain regulatory proteins including tropomyosin and troponin. [5
Two other ultrastructural elements are important in the process of contraction [4 (. Fig 15-2 ). The transverse tubule (T-tubule) is a specialized invagination of the plasma membrane of the myofiber, the sarcolemma. The T-tubule is responsible for propagation of the action potential into the
Figure 15-1 Histological and molecular structure of skeletal muscle. E represents a longitudinal section of a sarcomere showing the arrangement of the myofilaments actin (thin filaments) and myosin with projecting bridges (thick filaments). F, G, H, I are cross sections through the sarcomere showing the arrangements of the thin and thick filaments at the sites indica1(From Bloom W, and Fawcett DW: A Textbook of Histology. Philadelphia, W. B. Saunders, 1968.)
interior of the myofiber in response to depolarization of the sarcolemma. The other important structure involved in contraction is the sarcoplasmic reticulum, which is a set of tubules and cisterns surrounding the myofibrils. The sarcoplasmic reticulum stores calcium, which is released or taken up again in the process of excitation-contraction coupling and relaxation. In response to neurally initiated depolarization of the sarcolemma, calcium is released from the sarcoplasmic reticulum. The sudden increase in calcium concentration changes the conformation of the regulatory proteins on the thin filament, allowing interaction between myosin and actin to form cross-bridges and contraction. Contraction is terminated by the reuptake of calcium into the sarcoplasmic reticulum. This active transport of calcium requires energy. If energy is not available to reduce the calcium accumulation in sarcoplasm, the actin and myosin cross-bridges persist and relaxation does not occur. When this occurs after death it is known as rigor mortis.
A variety of metabolic disorders of muscle are possible if defects occur at any step in this orderly sequence of excitation-contraction coupling and relaxation. Disturbances in sodium channels in the sarcolemma membrane result in episodic weakness, giving rise to the disorder known as periodic paralysis (see Chapter^.). Muscle weakness can result from a shortage of fuel when defective glycolytic enzymes cannot convert glycogen to glucose ( glycogen storage diseases) or when the machinery that converts glucose to energy is defective (mitochondrial disorders) (see Chapter 31 ). Defects in the uptake of calcium into the sarcoplasmic reticulum disrupt relaxation and also result in weakness (myotonia, myotonic dystrophy) (see Chapt§L..3.6 ).
All of these defects in the contraction and relaxation of muscle are genetic in origin. Recently, work in molecular genetics has shown that the muscular dystrophies, in particular Duchenne's and Becker's dystrophies, result from a defective gene that codes for a major structural protein in the sarcolemmal membrane known as dystrophin (see Chapter.36 ).[e' Immunohistochemical studies show that dystrophin is completely absent in Duchenne's dystrophy, whereas in Becker's dystrophy the dystrophin protein is either smaller than normal or is present in low levels.
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