The different mysial tissues intertwine with the collagen of a tendon, while the tendon itself fuses with the periosteum that surrounds the bone. The tension created by the contraction of muscle fibers gets transferred through the mysial tissues, to the tendon, and finally, to the bone for the movement of the skeletal structures. The mysia may also fuse with a tendon-like sheet called the aponeurosis that connects to certain organs and can fuse with fascia, the connective tissue between skin and bony tissue.
SKELETAL MUSCLE FIBERS These muscle fibers are long and cylindrical. They can be as long as 30 cm in length and are built embryologically from myoblasts that fuse with hundreds of myoblasts to form multinucleated skeletal muscle fibers. The multiple nuclei allow for multiple gene copies and a great deal of proteins and enzymes being produced—all of these necessary for muscle contraction. There are special terms used to describe muscle cells that start with the term “sarco,”which means flesh. The plasma membrane of muscle fibers is called the sarcolemma; the cytoplasm is called the “sarcoplasm,” and the smooth endoplasmic reticulum is called the “sarcoplasmic reticulum” or SR. The SR releases and retrieves, stores, and releases the necessary calcium ions for the contraction of muscle fibers. Skeletal muscle is divided into sarcomeres, which is a packet of actin and myosin, along with their regulatory proteins (troponin and tropomyosin). The sarcomere is the functional unit of the muscle fiber; it is bundled in the muscle cell’s myofibril that runs the entire length of the fiber, attaching to the sarcolemma at the end of the muscle fiber. One muscle fiber contains hundreds to thousands of myofibrils, and each myofibril consists of thousands of sarcomeres. Sarcomeres do not run the length of the muscle fiber but are attached to Z-discs that anchor the actin myofilaments. Figure 51 illustrates what a sarcomere looks like:
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