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SKELETAL MUSCLE - STRUCTURE AND FUNCTION Muscle cells (fibers) are the only cells in the body that have the property of contractility, which allows them to shorten and develop tension. There are three types of muscle tissue: 1. Skeletal muscle - attaches to and moves the skeleton. The contractile molecules are very organized giving skeletal muscle a striated pattern, hence the name, “striated muscle”. It is under voluntary control. Skeletal muscle comprises about 36% of the total body weight in women and 42% in men. 75 percent of skeletal muscle is water, 20% is protein, and the remainder consists of inorganic salts, pigments, fats, and carbohydrates. 2. Smooth muscle - found in the walls of hollow organs and blood vessels. The contractile molecules are not aligned in a set pattern like skeletal muscle, hence the name smooth muscle. This muscle is under involuntary control. 3. Cardiac muscle - the contractile tissue of the heart wall. This muscle has characteristics of both smooth muscle and skeletal muscle. The contractile molecules are organized very much like skeletal muscle. Like smooth muscle, it is also under involuntary control. I. Overview of Skeletal Muscle A. Criteria Used To Name Muscles 1. Shape - deltoid (triangular), trapezius, rhomboid, latissimus (wide) 2. Action - various muscle names include the terms flexor, extensor, adductor, or pronator. 3. Location - tibialis anterior, intercostals, pectoralis major 4. Divisions - triceps brachii, quadriceps femoris 5. Size relationships - gluteus maximus, gluteus medius, gluteus minimus. Several names include the terms "brevis" (short), and "longus" (long). 6. Direction of fibers - transversus (across), rectus (straight)
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B. Skeletal Muscle Shapes Fasciculi may run parallel to long axis of muscle (great range of motion, little strength) or insert diagonally into a tendon running the length of the muscle (small range of motion, greater strength). 1. Unipennate - all fasciculi insert on one side of a tendon - semimembranosus 2. Bipennate - fasciculi insert on both sides of tendon - rectus femoris 3. Multipennate - convergence of several tendons - deltoid 4. Longitudinal (strap) - fasciculi run parallel to the long axis of the muscle - sartorius, rectus abdominus 5. Radiate - fibers fan out from a single attachment - pectoralis major C. Muscle Actions Most movements require the cooperative action of several muscles functioning as a group. 1. Prime mover - a muscle whose contraction is primarily responsible for a particular movement. Most movements are the result of the contraction of more than one muscle, and frequently a single muscle contributes to the production of two or more movements. 2. Antagonist - muscles that oppose one another upon contraction - biceps and triceps. Antagonists are located on opposite sides of a joint. 3. Fixators/stabilizers - muscles that immobilize a bone or joint near the origin of the prime mover so that the prime mover can act more efficiently.
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