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VOCABULARY Skeleton the bones of a human or an animal considered as a whole, together forming the framework of the body. Axial skeleton the skeleton of the head and trunk. Appendicular skeleton the part of the skeleton that includes the pectoral girdle and the pelvic girdle and the upper and lower limbs Periosteum the normal investment of bone, consisting of a dense, fibrous outer layer, to which muscles attach, and a more delicate, inner layer capable of forming bone. Compact bone the compact noncancellous portion of bone that consists largely of concentric lamellar osteons and interstitial lamellae. Cancellous bone light, porous bone enclosing numerous large spaces that give a honeycombed or spongy appearance. The bone matrix, or framework, is organized into a threedimensional latticework of bony processes, called trabeculae, arranged along lines of stress. The spaces between are often filled with marrow Bone marrow the soft, fatty, vascular tissue that fills most bone cavities and is the source of red blood cells and many white blood cells. Skeletal muscle a usually voluntary muscle made up of elongated, multinucleated, transversely striated muscle fibers, having principally bony attachments. Also called striated muscle. Antagonistic pairs a pair of muscles that work against each other Muscle fiber one of the structural cells of a muscle. Myofibril a contractile fibril of skeletal muscle, composed mainly of actin and myosin. Myofilament a threadlike filament of actin or myosin that is a component of a myofibril. Sarcomere any of the segments of myofibril in striated muscle fibers. Z line Any of the dark thin protein bands to which actin filaments are attached in a striated muscle fiber and that mark the boundaries of adjoining contractile units. I band a pale band across a striated muscle fiber that consists of actin, is much less birefringent topolarized light than the A bands, is situated between two A bands, and is bisected by a narrow dark-staining Z line called also isotropic band A band one of the cross striations in striated muscle that contain myosin filaments and appear dark under the light microscope and light inpolarized light H zone a narrow and less dense zone of myosin filaments bisecting the A band in striated muscle Sliding-filament model the process by which muscles contract Tropomyosin any of a group of muscle proteins that bind to molecules of actin and troponin to regulate the interaction of actin and myosin. Troponin complex a group of proteins that control the position of tropomyosin on thin filaments Actin a globulin that is present in muscle plasma and that in connection with myosin plays an important role in muscle contraction. Myosin the principal contractile protein of muscle Sarcoplasmic reticulum a system of membrane-bound tubules that surrounds muscle fibrils, releasing calcium ions during contraction and absorbing them during relaxation. Motor neuron a nerve cell that conducts impulses to a muscle, gland, or other effector. Motor unit a motor neuron and the muscle fibers innervated by its axon. Action potential the change in electrical potential that occurs between the inside and outside of a nerve or muscle fiber when it is stimulated, serving to transmit nerve signals. Tetanus a state of sustained contraction of a muscle during which the muscle does not relax to its initial length or tension, induced by a rapid succession of stimuli. Cardiac muscle a specialized form of striated muscle occurring in the hearts of vertebrates. Smooth muscle involuntary muscle tissue in the walls of viscera and blood vessels, consisting of nonstriated, spindleshaped cells.