TWO MUSCLE FIBER POPULATIONS are repeatedly demonstrated in human skeletal muscle by histochemical staining techniques. The 2 populations are termed type I and type II'*? and bear a reciprocal relationship to each other for a number of given enzyme reactions (table 1 ) . The type I fibers stain intensely with most mitochondria1 oxidative enzyme reactions and lightly with several myofibrillary and sarcoplasmic enzyme reactions. The type I1 fibers show the converse reactions. The authors contend that fiber typing by histochemical staining characteristics constitutes an immutable signature of normal muscle fibers. Assuming this premise to be true, the present study is designed to inquire into any relationship that may exist between the muscle fiber alterations in infantile spinal muscular atrophy and histochemical fiber type. The clinical and pathological features of infantile spinal muscular atrophy (WerdnigHoffmann disease) are well known and have been recently reviewed by Byers and Banker.3 The age of onset may vary from before birth to the second year of life. Weakness tends to begin in the proximal musculature and then progresses distally. Death usually occurs before age 5, although some patients may linger for many years. The disorder is transmitted as an autosomal recessive trait. The pathologic findings of the central nervous system are characteristic. Degenerative changes are notecl in the large motor cells of
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