skeletal muscle fibers are large, c

skeletal muscle fibers are large, cylindrical multinucleated cells with a diameter between 10 and 100 and a length of up to 40 cm.Each fiber is surrounded by a plasma membrane,the sarcolemma. the fiber contains a number of segments, called sarcomeres, arranged end to end along the length of the fiber.the sarcomere is the basic contractile unit of the muscle fiber.itcontains thick and thin fibers of myosin and actin arranged in a regular fashion to produce the characteristic cross-striations of the fiber in an alternate light and dark pattern.enzyme-filled mitochondria and ribosomes for protein synthesis exist within the fluid matrix of the fiber.
forming a network around the sarcomere is a closed system of tubules, the sarcoplasmic reticulum,in which calcium ions are retained.nerve impulses cause the liberation of calcium ions from the sarcoplasmic reticulum into the sarcomere,and cross bridges are established between the myosin and actin filaments.the formation of these crossbriddes causes a sliding movement to occur between the two sets of filaments,with subsequent shortening of the sarcomeres and therefore of the muscle fiber.the calcium ions then retuen into the sarcoplasmic reticulum,the cross bridges are broken,and the muscle reverts to its original length.