Muscle fiber type grouping

Figure 1.3 (a-c) Established reinnervation, indicated by muscle fiber type-grouping, in three adult males with chronic dysimmune peripheral neuropathy. Darkly-stained type-2 fibers and Ughtly-stained type-1 fibers are type-grouped, in contrast to what normally would be a rather even intermixture of fiber types (not illustrated). The successfully "foreign reirmervated" fibers among the type-grouped type-1 fibers have retained, or re-achieved, their normal diameter. 

This is manifest by muscle fiber type-grouping (detailed above) [4, 9, 16-18, 42, 43]. 

Paucity of one of the muscle-fiber types this is usually an aspect of a type-grouping phenomenon (a hypothetical, much less likely alternative is that it might be reflecting a neuropathic or myopathic preferential loss of one type of lower motor neuron or type of one muscle-fiber). 

Figure 15. Grouping of muscle fibers of uniform histochemical type indicating reinnervation in a patient with SMA.

It is now generally accepted, however, that a classification into two fiber types is an oversimplification. Stein and Padykula (S21.) distinguished a third, intermediate fiber type in rat muscles, and the existence of such a type is now recognized by many workers. Other classifications have been proposed thus Romanul (R4), on the basis of phosphorylase activity, divided fibers into eight types which were grouped into three main classes. 

A. Chemical energy is the type that bonds molecules together, so it is what is released when a bond in ATP is broken to remove a phosphate group. Mechanical energy is the type needed to move objects, such as muscle fibers in a contraction. 

Whole muscles are composed of groups of muscle fibers, which vary from 1 to 400 mm in length and from 10 to 60 tm in diameter. Muscle fibers, in turn, are composed of groups of myofibrils (Fig. 6.2b), and each myofibril is a series of sarcomeres added end to end (Fig. 6.2c). The sarcomere is both the structural and functional unit of skeletal muscle. During contraction, the sarcomeres are shortened to about 70 percent of their uncontracted, resting length. Electron microscopy and biochemical analysis have shown that each sarcomere contains two types of filaments thick filaments, composed of myosin, and thin filaments, containing actin (Fig. 6.2d). Near the center of the sarcomere, thin filaments overlap with thick filaments to form the AI zone (Fig. 6.2c). 

Relative paudty of one type of muscle fiber in a patient s biopsy can be caused, hypothetically, by (a) preferential impairment of the corresponding type of LMNs or Schwann cells (b) if both LMN types are equally abnormal, preferentially more successful sprouting and reinnervation ability of the opposite type of LMNs (c) if there are large groups of both types of muscle fibers in a chronic reinnervation sitnation, biopsy sampling could produce a non-representative impression of paudty or (d) preferential myopathic loss of that type of muscle fiber. (We use the term fiber-type paucity and not fiber-type predominance because it is more likely that the muscle fibers that are too few reflect the abnormal status.)... 

Table 2. Area of muscle fiber, muscle weight and type 1 collagen content, intramuscular fat content, adipose cell size and number in Longissimus dorsi muscles at the body weight of 115 kg in Control group and in Low group. ...

The most abundant microfilaments are composed of fibrous actin (F-actin Fig. 7-10). The thin filaments of F-actin are also one of the two major components of the contractile fibers of skeletal muscle. There is actually a group of closely related actins encoded by a multigene family. At least four vertebrate actins are specific to various types of muscle, while two (P- and y-actins) are cytosolic.298 299 Actins are present in all animal cells and also in fungi and plants as part of the cytoskeleton. The microfilaments can associate to... 

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