. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 578 Comparative Animal Physiology consist of separate fibers, and analysis of the ultimate contractile elements of muscle is currently being pushed from microscopic to molecular dimen- sions. Cross striations per se have no direct importance in contraction; non-striated muscles can contract; extraction of myosin, the principal con- tractile protein, leaves the striations intact.^^^ Yet cross-striated muscles are in general faster than non-striated, and the material of the striations may have important metabolic
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 578 Comparative Animal Physiology consist of separate fibers, and analysis of the ultimate contractile elements of muscle is currently being pushed from microscopic to molecular dimen- sions. Cross striations per se have no direct importance in contraction; non-striated muscles can contract; extraction of myosin, the principal con- tractile protein, leaves the striations intact.^^^ Yet cross-striated muscles are in general faster than non-striated, and the material of the striations may have important metabolic effects on the contractile proteins. An elec- tron micrograph of a myofibril (Fig. 213) shows that it consists of myofila- ments which pass continuously through the A and I bands and which do not fold in contraction.^^^ These myofilaments sometimes show regular spacings 400 a apart, but these spacings are much less regular and sharp than are the 640 X spacings of collagen (Fig. 213). y 3c. Fig. 213. Electron micrograph of a myofibril of frog sartorius muscle—note filaments in the fibril showing striations. Collagen fibrils cross the muscle fibril. Stained with phos- photungstic acid. Scale 1 micron. From Hall, Jakus, and ;" Chemical extracts of the contractile protein of striated muscle can be divided into two parts, actin and myosin, which combine to form the con- tractile protein, actomyosin. Discussion of this important field of biophysics is beyond the scope of this book, but molecular threads have been obtained which may be the same as the contractile units of muscle. Figure 214, A shows fibrous actin prepared by polymerization of globular actin and align- ment of the threads gives the appearance of striae. Fibrous actin threads (Fig. 214, B) strikingly resemble threads obtained by maceration of muscle in a Waring blendor (Fig. 214, C). Molluscan smooth muscle yields, in addition to "myosin," another protein, paramyosin, with much longer x-ray
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