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A histochemical study of muscle spindles and their relationship to extrafusal fiber types in the rat

1969; Wiley; Volume: 125; Issue: 1 Linguagem: Inglês

10.1002/aja.1001250103

1553-0795

Herbert Yellin,

Muscle activation and electromyography studies

Abstract Hindlimb muscles of the rat were studied in serial section with enzyme histochemical methods. Muscle fibers, both extrafusal and intrafusal, were compared as to their relative succinic dehydrogenase and phosphorylase activity, indicative of oxidative and glycolytic metabolism respectively. The superficial fascicles of a number of white (fast‐twitch) muscles are composed almost exclusively of glycolytic (type A) muscle fibers, whereas the deep fascicles contain a mixture of oxidative (type B) and oxidative‐glycolytic (type C) fibers. Muscle spindles are found only among the deep fascicles of these fast muscles. Neither the soleus, a red (slow‐twitch) muscle comprised mainly of type B fibers, nor the middle lumbrical muscles, with a fairly uniform distribution of several fiber types, exhibit so specific a spindle disposition. Thus a close spatial relationship exists between spindles and certain types of muscle fibers, presumbly representing specific types of motor units. Within the spindles, the nuclear‐bag intrafusal fibers are generally of types A and B, and of a third variety characterized by both low oxidative and low glycolytic activity. The latter fiber‐type is not commonly observed in the extrafusal musculature. Nuclear‐chain intrafusal fibers resemble type C fibers. Speed of contraction and enzymes of energy metabolism of extrafusal fibers are known to be neurally regulated. The several types of histochemically distinguishable intrafusal fibers probably reflect differences in motor innervation and possibly differing contraction‐kinetics.