Muscle fiber slow-twitch

Distinguish among the three types of muscle fibers slow-twitch oxidative, fast-twitch oxidative, and fast-twitch glycolytic... 

Tissue-Specific Expression. In adult rodents, PPAR.a is expressed in liver, kidney, intestine, heart, skeletal muscle, retina, adrenal gland, and pancreas. In adult human, PPARa is expressed in the liver, heart, kidney, large intestine, skeletal muscle (mostly slow-twitch oxidative type I fibers), and in cells of atherosclerotic lesions (endothelial cells, smooth muscle cells, and monocytes/macrophages). Therefore, regardless of... 

Comment A common association of type-2 fiber atrophy (discussed in Chapter 1) is an aspect of a lower-motor-neuron abnormality resulting in functional "dysinnervation," as may be part of the pathogenic mechanism in this patient. Note that routine diagnostic EMG cannot quantify the size of the type-2 muscle-fiber fast-twitch units because they are activated only by vigorous contraction, whereas slow-twitch type-1 motor-units are activated by... 

EMG did not detect recent denervation, and motor-unit action potentials were of normal size (note that routine EMG does not allow quantitation of the size of the later-activated type-2-fiher fast-twitch motor units, but ordy of the earher-activated type-1-fiber slow-twitch motor units) [9], Nerve conductions were normal. Muscle biopsy histochemistry showed moderate-prominent type-2 fiber atrophy, slight recent denervation, and shght- moderate established reinnervation. 

Skeletal muscle contains three types of fiber fast-twitch oxidative glycolytic (type 2A), fast-twitch glycolytic (type 2B), and slow-rwitch oxidative fibers (type 1). The proportion of each fiber type varies in different muscles. Different fiber types contain different isoforms of myosin, although there is no evidence that their mitochondria differ qualitatively. It has been reported that there are differences between subsarcolemmal mitochondria and those deeper in the same fiber but this has been questioned (see Sherratt et al., 1988 for references). 

Histopathological examination shows the typical corelike lesions in a high proportion of muscle fibers in older patients this may amount to 100%. Most typically the cores are large and centrally-placed, but multiple cores may occur in the same fiber cross section. Most older patients show a striking predominance of type 1 (slow twitch oxidative) fibers and virtually all fibers with cores are type 1. Sometimes younger family members have more normal proportions of type 1 and type 2 fibers but, again, the cores are confined to the type 1 fibers. It is well established that muscle fiber types can interconvert due to altered physiological demands, and it is likely that fibers with cores convert to a basically slow twitch-oxidative metabolism to compensate for the fact that up to 50% of their cross sectional area may be devoid of mitochondria. 

Hypothyroid myopathy occurs in about 30% of patients with hypothyroidism irrespective of its cause. Muscle pain, cramps, and stiffness may be seen, and are often exacerbated by cold weather. Pseudomyotonic features of delayed muscle contraction and relaxation are common. Myoedema (the mounding phenomenon) is due to the painless, electrically silent contracture produced on direct percussion. Muscle biopsy often shows a predominance of type 1 (slow-twitch) fibers, again analogous to that seen in experimental hypothyroidism (Figure 22). Muscle hypertrophy with weakness and slowness of movement occurs in the Debre-Semelaigne syndrome seen in severely hypothyroid children, and Hoffman s syndrome is a similar condition seen in adults with hypothyroidism, but is also accompanied by painful spasms. 

Figure22. (a) Euthyroid(normal)ratmuscleshowingmixtureoftype1 (slow-twitch) and type 2 (fast-twitch) fibers, (b) Hypothyroid rat muscle showing uniformly type 1 (slow-twitch) histochemical profile myofibrillar ATPase after alkaline preincubation.

SKELETAL MUSCLE CONTAINS SLOW (RED) FAST (WHITE) TWITCH FIBERS... 

Different types of fibers have been detected in skeletal muscle. One classification subdivides them into type I (slow twitch), type IIA (fast twitch-oxidative), and type IIB (fast twitch-glycolytic). For the sake of simphcity, we shall consider only two types type I (slow twitch, ox-... 

Skeletal muscle contains different types of fibers primarily suited to anaerobic (fast twitch fibers) or aerobic (slow twitch fibers) conditions. 

Fast-twitch muscle fibers develop tension two to three times faster than slow-twitch muscle fibers because of more rapid splitting of ATP by myosin ATPase. This enables the myosin crossbridges to cycle more rapidly Another factor influencing the speed of contraction involves the rate of removal of calcium from the cytoplasm. Muscle fibers remove Ca++ ions by pumping them back into the sarcoplasmic reticulum. Fast-twitch muscle fibers remove Ca++ ions more rapidly than slow-twitch muscle fibers, resulting in quicker twitches that are useful in fast precise movements. The contractions generated in slow-twitch muscle fibers may last up to 10 times longer than those of fast-twitch muscle fibers therefore, these twitches are useful in sustained, more powerful movements. 

Finally, slow-twitch muscle fibers have a small diameter. This facilitates the diffusion of oxygen through the fiber to the mitochondria where it is utilized. Taken together, each of these characteristics enhances the ability of these fibers to utilize oxygen. Therefore, in slow-twitch oxidative muscle... 

Fast-twitch muscle fibers have a high capacity for anaerobic glycolysis but are quick to fatigue. They are involved primarily in short-term, high-intensity exercise. Slow-twitch muscle fibers in arm and leg muscles are well vascularized and primarily oxidative. They are used during prolonged, low-to-moderate intensity exercise and resist fatigue. Slow-twitch fibers and the number of their mitochondria increase dramatically in trained endurance athletes. 

There is probably no biological phenomenon that has excited more interest among biochemists than the movement caused by the contractile fibers of muscles. Unlike the motion of bacterial flagella, the movement of muscle is directly dependent on the hydrolysis of ATP as its source of energy. Several types of muscle exist within our bodies. Striated (striped) skeletal muscles act under voluntary control. Closely related are the involuntary striated heart muscles, while smooth involuntary muscles constitute a third type. Further distinctions are made between fast-twitch and slow-twitch fibers. Fast-twitch fibers have short isometric contraction times, high maximal velocities for shortening, and high rates of ATP hydrolysis. 

Haimoto H, Kato K. 1987. SlOOaO (alpha alpha) protein, a calcium-binding protein, is localized in the slow-twitch muscle fiber. J Neurochem 48(3) 917-923. 

Similarly, measurements of blood flows to sections of muscle, which are primarily composed of a single fiber type, exhibit large differences consistent with the expected demands of oxygen supply based on mitochondrial content (32,33). Thus, mammalian skeletal muscle is typically comprised of three biochemically and functionally distinct fiber types slow-twitch red, fast-twitch red and fast-twitch white. These fiber types are also commonly referred to as Type I, Type Ila, and Type 11b, respectively (7 ). 

In summary, all mammals possess a large fraction of hlgh-oxl-datlve muscle. The ordered pattern of motor unit recruitment Involves these high oxidative muscle fibers before the low-oxldatlve fibers, as exercise Intensity progresses from mild, to moderate, to severe. This progression favors an enhanced exercise performance at submaxlmal exercise Intensities, since the slow and fast—twitch red fibers are capable of repeated contractions for long periods of time. 

Figure 2. The Influence of exercise intensity (treadmill running) on muscle cytochrome c content in the rat. Red vastus = fast-twitch red fiber section Soleus = slow-twitch red fiber section White vastus = fast-twitch white fiber section. "Reproduced with permission from Ref. 44. Copyright 1982, American Physiological Society. "...

Phosphorylation by ADP, catalyzed by adenylate kinase - this enzyme is especially important in the rapid synthesis and turnover of thiamin triphosphate in slow-twitch white muscle fibers. 

Racay P, Gregory P, Schwaller B. Parvalbmnin deficiency in fast-twitch muscles leads to increased slow-twitch type mitochondria, but does not affect the expression of fiber specific proteins. FEBS J. 2006 273 96-108. 

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