Concepts of Human Anatomy & Physiology   5/e   Van De Graaff/Fox
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I. Structure and Actions of Skeletal Muscles

Concept: Skeletal muscles are composed of individual muscle fibers that contract when stimulated by nerve impulses through motor neurons. Each motor neuron branches to innervate a number of muscle fibers. Activation of varying numbers of motor neurons results in gradations in the strength of muscle contraction.

II. Development of Skeletal Muscles

III. Mechanisms of Contraction

Concept: The A bands within each muscle fiber are composed of thick filaments, and the I bands contain thin filaments. Movement of cross bridges that extend from the thick to the thin filaments causes sliding of the filaments, and thus muscle tension and shortening. The activity of the cross bridges is regulated by the availability of Ca2+, which is increased by electrical stimulation of the muscle fiber through a nerve impulse. Electrical stimulation produces contraction of the muscle through the binding of Ca2+ to regulatory proteins within the thin filaments.

IV. Energy Requirements of Skeletal Muscles

Concept: Skeletal muscles generate ATP through aerobic and anaerobic respiration and through the use of phosphate groups donated by creatine phosphate. The aerobic and anaerobic abilities of skeletal muscle fibers differ according to muscle fiber type. Slow-twitch (type I) fibers are adapted for aerobic respiration; fast-twitch (type II) fibers are adapted for anaerobic respiration.

V. Cardiac and Smooth Muscle

Concept: Cardiac muscle, like skeletal muscle, is striated and contains sarcomeres that shorten by sliding of thin and thick filaments. But while skeletal muscle requires a nerve stimulation to contract, cardiac muscle can produce impulses and contract spontaneously. Smooth muscle lacks sarcomeres, but they do contain actin and myosin that produce contractions in response to a unique regulatory mechanism.

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