Muscle contraction fast twitch

Dudley, G. A. and Terjung, R. L. (1985) Influence of acidosis on AMP deaminase activity in contracting fast-twitch muscle. 

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... 

The cells of the latter three types contain only a single nucleus and are called myocytes. The cells of skeletal muscle are long and multinucleate and are referred to as muscle fibers. At the microscopic level, skeletal muscle and cardiac muscle display alternating light and dark bands, and for this reason are often referred to as striated muscles. The different types of muscle cells vary widely in structure, size, and function. In addition, the times required for contractions and relaxations by various muscle types vary considerably. The fastest responses (on the order of milliseconds) are observed for fast-twitch skeletal... 

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. 

Parvalbumin is another protein in search of a function. It contains three HLH motifs (Mr 1 IK), but only the second and third are functional Ca " -binding sites. These are high-affinity Ca /Mg sites, and both are filled with Ca in the known crystal structures (references in Table I). In fast twitch muscle, where most parvalbumins are found, the protein is postulated to act as a Ca " buffer (Haich a/., 1979 Gi s etal., 1982). As Ca is released from troponin C after muscle contraction, the Ca may be bound by parvalbumin to prevent reinitiation of contraction. In resting cells parvalbumin likely binds Mg ", rather than Ca (Haiech etal., 1979). 

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. 

In muscle cells, the contraction is induced by Ca2+ release from the sarcoplasmic reticulum, as a result of membrane depolarization and activation of RyRl receptors located at the surface of the SR. The subsequent transport of cytoplasmic Ca2+ back into the lumen of the sarcoplasmic reticulum restores low resting calcium levels and allows muscle relaxation. In fast-twitch skeletal muscle fibers, Ca2+ uptake is mediated by the sarco(endo)plasmic reticulum Ca2+ ATPase SERCA1 which represents more than 99% of SERCA isoforms in these muscle fibers. 

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,... 

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. 

Muscle fibers can be classified as either fast-twitch or slow-twitch. The slow-twitch fibers, or type 1 fibers (also called slow-oxidative), contain large amounts of mitochondria and myoglobin (giving them a red color), utilize respiration and oxidative phosphorylation for energy, and are relatively resistant to fatigue. Compared with fast-twitch fibers, their glycogen content is low. The slow-twitch fibers develop force slowly but maintain contractions longer than fast-twitch muscle. 

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. 

The function of muscle is to generate a force on contraction, either to maintain or change the position of a joint. There are two main types of muscle fibres Type 1 (slow twitch), which allow sustained low energy contraction, and Type 2 (fast twitch), which allow rapid forceful contractions. Muscles are comprised of varying proportions of both types of fibres. Postural muscles are composed mainly of Type 1 fibres, with a predominantly linear arrangement of fibres, whereas non-postural muscles... 

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