Skeletal muscles are composed of individual muscle fibers that contract when stimulated by a motor neuron. Each motor neuron branches to innervate a number of muscle fibers, and all of these fibers contract when their motor neuron is activated. Activation of varying numbers of motor neurons, and thus varying numbers of muscle fibers, results in gradations in the strength of contraction of the whole muscle.
Skeletal muscles are usually attached to bone on each end by tough connective tissue tendons. When a muscle contracts, it shortens, and this places tension on its tendons and attached bones. The muscle tension causes movement of the bones at a joint, where one of the attached bones generally moves more than the other. The more movable bony attachment of the muscle, known as its insertion, is pulled toward its less movable attachment known as its origin. A variety of skeletal movements are possible, depending on the type of joint involved and the attachments of the muscles (table 12.1). When flexor muscles contract, for example, they decrease the angle of a joint. Contraction of extensor muscles increases the angle of their attached bones at the joint. The prime mover of any skeletal movement is called the agonist muscle; in flexion, for example, the flexor is the agonist muscle. Flexors and extensors that act on the same joint to produce opposite actions are antagonistic muscles.
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