Skeletal muscle may be defined as voluntary striated muscle that is usually attached to one or more bones. A typical skeletal muscle cell is about 100 ^m in diameter and 3 cm long; some are as thick as 500 ^m and as long as 30 cm. Because of their extraordinary length, skeletal muscle cells are usually called muscle fibers or myofibers. A skeletal muscle fiber exhibits alternating light and dark transverse bands, or striations, that reflect the overlapping arrangement of the internal contractile proteins (fig. 11.1). Skeletal muscle is called voluntary because it is usually subject to conscious control. The other types of muscle are involuntary (not usually under conscious control), and they are never attached to bones.
Recall from chapter 10 that a skeletal muscle is composed not only of muscular tissue, but also of fibrous connective tissue: the endomysium that surrounds each muscle fiber, the perimysium that bundles muscle fibers together into fascicles, and the epimysium that encloses the entire muscle. These connective tissues are continuous with the collagen fibers of tendons and those, in turn,
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with the collagen of the bone matrix. Thus, when a muscle fiber contracts, it pulls on these collagen fibers and moves a bone.
Collagen is not excitable or contractile, but it is somewhat extensible and elastic. It stretches slightly under tension and recoils when released. Because of this elasticity and because the connective tissue components are connected to each other in a linear series, the connective tissues are called the series-elastic components of a muscle. Their elasticity helps to return muscles to their resting lengths when contraction ceases. Elastic recoil of the tendons adds significantly to the power output and efficiency of the muscles.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.