Muscle fiber types slow-twitch

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 ). 

Importantly, the precise electrical stimulus protocol chosen also impacts the training effect on the muscle. In particular, the frequency of stimulus delivered likely determines the types of muscle fibers activated. Slow-twitch fibers normally have a firing frequency of -10 Hz, whereas fast-twitch fibers have a firing frequency of 30 Hz (97). A TCEMS stimulus frequency up to 10 Hz would, therefore, preferentially activate slow-twitch fibers and may selectively improve resistance to fatigue (101), whereas frequency greater than 30 Hz may activate both types of fibers or may selectively recruit fast-twitch fibers and... 

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. 

Skeletal muscle cells can be subdivided into type I and type IIfibers. Type I fibers are slow-twitch fibers that use primarily oxidative metabolism for energy, whereas the type 11 fibers (fast-twitch) use glycolysis as their primary energy-generating pathway. 

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... 

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). 

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.

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. 

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

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. 

Mammalian skeletal muscle can be separated Into two distinct fiber populations, based on relative contraction characteristics, and are referred to as slow-twltch (Type I) or fast-twitch (Type II) fibers. The slow-twltch fiber type exhibits a relatively low shortening velocity (27), a low rate of tension development (27). a low myosin ATPase activity (28) and a low rate of calcium sequestration by the sarcoplasmic reticulum (29). The converse Is true for the fast-twitch fibers. Since contraction velocity highly correlates with myosin ATPase activity (30), It Is possible to easily Identify,... 

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. 

Anabolic steroids decrease catabolism and increase skeletal muscle protein synthesis. Whether this results in muscular hypertrophy or hyperplasia, or a combination of these, is unclear and probably depends upon the muscle studied. Different muscle types contain different cytosolic receptor numbers and, therefore, the response to anabolic steroids varies. Anabolic steroids initiate an increase in RNA polymerase activity and the synthesis of either structural or contractile proteins. In some muscles, anabolic steroids may increase the ratio of fast twitch to slow twitch fibers (Nimmo et al 1982, Snow et al 1982). Increased activity of enzymes involved in energy metabolism may also occur. However, the total glycogen content may remain unchanged (Hyyppa et al 1997). The effects are most profound in females and castrated males (Snow 1993). 

The total tissue content of P-ARs was greater in the soleus, a muscle consisting almost entirely of slow-twitch (type I) fibers than in superficial white vastus lateralis, a muscle composed of greater than 95% fast-twitch (type lib) fibers (73). 

Skeletal muscle (type I, slow twitch, or red fibers) 50,000 <1 3 97... 

Ila (fast-twitch oxidative) and type I (slow-twitch oxidative) muscle fibers have greater oxidative capacity and are less dependent on the cycle than type Ilb (fast-twitch glycolytic) fibers. Thus, gradual exercise programs that lead to production of a greater proportion of type Ila and type I fibers might improve exercise tolerance in AMP deaminase deficiency. 

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