White twitch fibers

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

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 ). 

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. 

In fast white fibers, glycolysis catabolizes glucose. The relative lack of mitochondria in these fibers causes the white appearance. The rapid breakdown of glucose by anaerobic metabolism means that ATP is made rapidly. These muscles are used in rapid, short-duration movement and exhibit a fast twitch when electrically stimulated. The flight muscles of birds are of this type—remember that you find the white meat of a chicken on the breast. 

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. "...

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 11, fast twitch, or white fibers) 50,000 <1 1 99... 

Although the human has no muscles that consist entirely of this fiber type, many animals do. Examples are white abdominal muscles of fish and the pectoral muscle of game birds (turkey white meat). These muscles contract rapidly and vigorously (the fast twitch refers to the time to peak tension), but only for short periods. Thus, they are used for activities such as flight in birds and sprinting and weight-lifting in humans. 

EXAMPLE 13.21 Type II muscle fibers are subdivided into type Ila and type Ilb. Type Ila can use both aerobic and anaerobic metabolism to produce ATP whereas type lib fibers use only anaerobic metabolism. In contrast to type I muscle fibers, type lib fibers contract rapidly after stimulation by a nerve impulse. They have evolved for short-lived, powerful contractions by the possession of characteristically active myosin ATPases and a dense packing of contractile filaments. So much of the cytoplasmic space is taken up with filaments that little exists for mitochondria. Similarly, these fibers are associated with a relatively poor blood supply. Type lib fibers are also known as white and fast-twitch muscle fibers and are adapted for short-lived but powerful contractions. The relative paucity of mitochondria and the poor blood supply impose obvious constraints on the generation of ATP during exercise. 

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