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Surrounding each muscle fiber (cell) is a delicate external lamina composed primarily of reticular fibers. This external lamina is produced by the muscle fiber and, when observed through the light microscope, cannot be distinguished from the cell membrane of the muscle fiber, the sarcolemma.
There are two types: MYOSIN-thick filaments ACTIN-thin filaments. Structure of a SARCOMERE (functional unit of a muscle). Actin and Myosin filaments are arranged in an overlapping pattern of light (I bands) and dark (A bands). In the middle of each I band is a line called a Z line.The section of a myofibril from one Z-line to the next Z-line is the SARCOMERE.. Muscle and Muscle Fiber Structure: A muscle is composed of many muscle fibers (muscle fiber = muscle cell). The individual muscles are separated from each other and held in place by a covering called the FASCIA.