Skeletal muscles are the engines of the body. They account for over a quarter of its weight and the major part of its energy expenditure. They are attached to the bones of the skeleton and so serve to produce movements or exert forces. Hence they are central to such activities as voluntary movement, maintenance of posture, breathing, eating, directing the gaze and producing gestures and facial expressions. Skeletal muscles are activated by motoneurons, as we have seen in previous chapters. Their cells are elongate and multi-nuclear and the contractile material within them shows cross-striations. Hence skeletal muscle is a form of striated muscle. In contrast, cardiac and smooth muscles have cells with single nuclei, and smooth muscles are not striated; we shall examine their properties in Chapters 12 and 13.

Anatomy

Skeletal muscle fibres are multi-nucleate cells formed by fusion of elongated uni-nucleate cells called myoblasts whose respective nuclei become arranged around the edge of the fibre. Mature fibres may be as long as the muscle (Figure 9.1) of which they form part, and 10 to 100 µm in diameter. Bundles of muscle fibres are surrounded by a further sheet of connective tissue, the perimysium, and the whole muscle is contained within an outer sheet of tough connective tissue, the epimysium. These connective tissue sheets are continuous with the insertions and tendons which serve to attach the muscles to the skeleton. An excellent blood supply provides a network of blood capillaries between individual fibres.