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Included under this title are two sizable groups of muscle diseases: one is an assemblage of congenital deformities that involve muscle and the other is a unique class of congenital myopathies. Insofar as all of the disorders comprising these categories develop in utero, i.e., are congenital, it may be helpful by way of introduction to summarize briefly the main facts about the natural development and aging of muscle. These diseases are of particular importance in pediatric neurology, for most of them attract notice at an early age.

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The Development and Aging of Muscle

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The accepted view of the embryogenesis of muscle is that muscle fibers form by fusion of myoblasts soon after the latter differentiate from somatic mesodermal cells. Muscle connective tissue derives from the somatopleural mesoderm. After fusion of the myoblasts, a series of cellular events including the sequential activation of myogenic transcription factors leads to myofibril formation. The newly formed fibers are thin, centrally nucleated tubes (appropriately called myotubes) in which myofilaments begin to be produced from polyribosomes. As myofilaments become organized into myofibrils, the nuclei of the muscle fiber are displaced peripherally to a subsarcolemmal position. Once the nuclei assume a peripheral position, the myofiber is fully formed. The detailed mechanisms whereby myoblasts seek one another, the manner in which each of a series of fused nuclei contribute to the myotube, the formation of actin and myosin fibrils, Z-discs, and the differentiation of a small residue of satellite cells on the surface of the fibers are reviewed by Rubenstein and Kelly.

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The mechanisms that determine the number and arrangement of fibers in each muscle are not as well understood. Presumably the myoblasts themselves possess the genetic information that controls the program of development, but within any given species there are wide individual variations that account for obvious differences in the size of muscles and their power of contraction.

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The number of fibers assigned to each muscle is probably attained by birth, and growth of muscle thereafter depends mainly on the enlargement of fibers. Although the nervous system and musculature develop independently, muscle fibers continue to grow after birth only when they are active and under the influence of nerve. Measurements of muscle fiber diameters from birth to old age show the growth curve ascending rapidly in the early postnatal years and less rapidly in adolescence, reaching a peak during the third decade. After puberty, growth of muscle is less in females than in males, and such differences are greater in the arm, shoulder, and pelvic muscles than in those of the leg; growth in ocular muscles is about equal in the two sexes. At all ages, disuse of muscle decreases fiber size by as much as 30 percent, and overuse increases the size by about the same amount (work hypertrophy). Normally, type 1 (oxidative enzyme-rich) fibers are slightly smaller than type 2 (phosphorylative enzyme-rich) fibers; the numerical proportions of the ...

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