Chapter 1 THE MUSCLE FIBRE

This chapter discusses the structure of muscle fiber. Voluntary” muscles are composed of two types of muscle fibers. By far the most common are those fibers that make up nearly the entire bulk of the muscle and are described as extrafusal. This term distinguishes them from the much smaller muscle fibers that are found inside the muscle spindles. Because of their location, these smaller muscle fibers are referred to as intrafusal. At birth, human fibers are slender, measuring about 10–20 μm in diameter. Throughout childhood and particularly during the growth spurt in puberty, the sizes increase and eventually attain adult dimensions with most diameters in the 40–80 μm range. The membrane of the muscle fiber is termed the sarcolemma and is some 75 A thick. Just outside the sarcolemma and clearly visible in electronmicrographs is the basement membrane which is composed of proteins and polysaccharides. Each muscle fiber contains numerous nuclei which are dispersed along the inner surface of the sarcolemma, particularly in the region of the motor end-plate. The most obvious structures within the muscle fiber are the myofibrils, which are the units responsible for contraction and relaxation of the fiber.