Skeletal Muscle

Three types of muscle are known to exist in man and animals: skeletal muscle, cardiac muscle, and smooth muscle (9:34). Skeletal musclewhich connects to bones and jointsis typically activated by motoneurons (5:121). These muscles allow for locomotion and the performance of work (13:205). In addition, they account for over a quarter of the body's weight and the major part of its energy expenditure (5:121).

A typical skeletal muscle is composed of many muscle fibers, or skeletal muscle cells. These cells are elongate and multinuclear (5:121). Moreover, the contractile material within them shows crossstriations (5:121). Hence, skeletal muscle is one form of striated muscle (5:121).

Skeletal muscle fibers are formed by the fusion of elongated uninucleate cells called myoblasts (5:121). Mature fibers may be as long as the muscle that they comprise (5:121). Their lengths thus range up to several centimeters (12:3). Their diameters may be anywhere from 10 to 100 microns (5:121). The cell's nuclei tend to occur around the fi

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ay in one sarcomere is linked to that in the adjacent sarcomere by a crosslinking structure termed the Zband, or Zline (12:6). Actin filaments are polar structures and the Zline serves as a structure which produces the required polarity reversal of the two actin arrays in successive sarcomeres (12:6). Hence, the Zline is the structure from which the actin molecules emanate (12:6). This allows actin filaments at both ends of a sarcomere to have the appropriate polarity for interaction with the two oppositely polarized ends of the Aband (12:6). Approximately 90 percent of muscle fiber mass can be attributed to sarcomeres (9:5). Running transversely across the fibers are the transverse, or Ttubules (9:5). Ttubules are about 400 microns in diameter (9:56). They occur at the AI junction (the outer boundaries of the Aband) of each sarcomere (9:7). Often they comprise the central tubular element of a structure known as a "triad (5:144)." A triad is made up of a transverse tubule and sarcoplasmic reticulum (SR) (9:7). In skeletal muscle cells the SR tends to be a complex, anastomosing network of intracellular channels, 200 to 400 microns in diameter (9:7). Two SR terminal cisternae generally attach to a Ttubu
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Approximate Word count = 2789
Approximate Pages = 11 (250 words per page)

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