In the limbs the extensive amount of fascia separates the muscles into isolated

In the limbs the extensive amount of fascia separates

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In the limbs, the extensive amount of fascia separates the muscles into isolated sections or compartments. Muscle Attachment Muscles form attachments to other structures in three ways: the tendon attaches the muscle to the bone, muscles attach directly (without a tendon) to a bone or to soft tissue, and a flat sheetlike fascia called aponeurosis connects muscle to muscle or muscle to bone. Structures and Function of a Single Muscle Fiber The muscle cell is an elongated muscle fiber. The muscle fiber has more than one nucleus and is surrounded by a cell membrane called a sarcolemma. At several points, the cell membrane penetrates deep into the interior of the muscle fiber, forming transverse tubules (T tubules). Within the muscle fiber is a specialized endoplasmic reticulum called the sarcoplasmic reticulum (SR) . Each muscle fiber is composed of long cylindrical structures called myofibrils. Each myofibril is made up of a series of contractile units called sarcomeres . Each sarcomere extends from Z line to Z line and is formed by a unique arrangement of contractile proteins, referred to as thin and thick filaments . The thin filaments extend toward the center of the sarcomere from the Z lines. The thin filament is composed of two proteins called actin and the troponin–tropomyosin complex. The actin contains binding sites for the myosin . The thicker myosin filaments sit between the thin filaments. Extending from the thick myosin filaments are structures called myosin heads. Certain muscle fibers contain a reddish-brown protein called myoglobin . The myoglobin stores oxygen in the muscle and gradually releases it when the muscle starts to work. Fibers that contain myoglobin are red because of the iron pigment How Muscles Contract The sliding filament mechanism describes how muscle contracts. Muscles can only pull, not push! To pull, muscles contract. When muscles contract, they shorten. Muscles shorten because the
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sarcomere length shortens, and the sarcomere length shortens because the thin and thick filaments slide past each other. The following statements explain how the sarcomere shortens. When the contractile apparatus is stimulated, the sarcomere is flooded with calcium. This enables the myosin heads to make contact with special sites on the actin, forming temporary connections called cross- bridges. Once the cross-bridges are formed, the myosin heads rotate, pulling the actin toward the center of the sarcomere. The rotation of the myosin heads causes the thin filaments to slide past the thick filaments. (Sarcomere length shortens.) Muscle relaxation occurs when the cross-bridges are broken, and the thin and thick filaments return to their original positions. The role of calcium and adenosine triphosate Calcium and adenosine triphosphate (ATP) play important roles in the contraction and relaxation of muscle. Calcium is necessary for the formation of the actin-myosin connections called cross-bridges.
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  • Fall '19
  • muscle weakness, muscle atrophy, functions of muscles

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