Each muscle fiber is invested by a thin layer of connective tissue called the basal lamina, or basement membrane. It is now believed that the basement membrane contains molecules important to the development and differentiation of the neuromuscular apparatus. Satellite cells, enclosed between the basement membrane and the sarcolemma, are believed to derive from undifferentiated myoblasts and are considered the skeletal muscle stem cell niche, capable of fusing with damaged muscle fibers in a regenerative process.
A muscle fiber exerts force by contracting. The microscopic structure of the muscle fiber gives a great deal of information about the way it functions. The contractile apparatus of each muscle fiber is subdivided into myofibrils, which are longitudinally oriented bundles of thick and thin filaments. The thick and thin filaments provide the mechanical force of contraction by sliding past one another. A myofibril measures about 1 µm in diameter and extends the entire length of the fiber. The thin filaments of the myofibril are anchored at one end to a meshlike lattice structure made up largely of protein and oriented at right angles to the filaments. Seen from the side, this lattice appears narrow and dense. The resulting image in a longitudinal section observed on light microscopy is called the Z band (Zwischenscheibe). Z bands occur at very regular intervals along the length of the myofibril. The stretch of myofibril between two adjacent Z bands is called a sarcomere, which can be considered the unit of contractile action. Thus, myofibrils are made up of many sarcomeres linked end to end. The thick filaments are disposed in the center of the sarcomere. Because they strongly rotate polarized light, the thick filaments are responsible for the appearance of the anisotropic bands, or A bands, on longitudinal section.
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