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ARTHUR HESS; The Structure of Vertebrate Slow and Twitch Muscle Fibers. Invest. Ophthalmol. Vis. Sci. 1967;6(3):217-228.
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© ARVO (1962-2015); The Authors (2016-present)
Slow muscle fibers, which undergo a contracture after a nerve stimulus rather than a twitch, have been shown in various animals to have, in contrast to twitch fibers, the following morphological characteristics: multiple nerve terminals, absence of a regularly occurring transverse tubular T system, reduced amounts of sarcoplasmic reticulum, irregular disposition and size of fibrils, zig-zag Z line, no M line, and no postjunctional sarcolemmal folds under the nerve terminals. The slow fibers of the frog iliofibularis muscle, the chicken anterior latissimus dorsi muscle, and the cat superior oblique muscle have been studied and compared with the twitch fibers in these animals to determine if they have any or all of the above characteristics. All slow muscle fibers have multiple nerve terminals, no well-organized T system (only aberrant T system elements occur), reduced amounts of sarcoplasmic reticulum, irregular disposition and size of fibrils, and a zig-zag Z line. The frog slow muscle fiber does not have an M line; that of the chicken and cat does. Virtually no postjunctional sarcolemmal folds occur under the nerve terminals of slow fibers. However, even in the twitch fibers of the chicken and cat, postfunctional folds are irregular, infrequent, and inconspicuous.
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