5.Epithelial tissue- found in capillaries that feed skeletal muscle. B.CELL LEVEL – structures of one muscle fiber seen with light microscope: 1.Nuclei- multiple, peripheral location; 2.Sarcolemma – plasma membrane which can be stimulated to depolarize; 3.Myofibrils- elongated organelles that contract. The banding pattern of myofibrils is responsible for the striations of the muscle fiber; 4.Sarcomere- contractile unit of muscle. Myofibrils are long ‘chains’ of sarcomeres, which are the repeating unit of the banding pattern. a)Z lines (membranes)- at each end of the sarcomere; b)A bands- darker areas located at the middle of the sarcomere; c)I bands – lighter areas at the ends; Z line splits each I band;
C.CELL ULTRASTRUCTURE LEVEL- structures seen with ELECTRON microscope. 1.Myofilaments- inner threads of protein which make up myofibril; a) Thick filaments – Myosin protein in an A band; b) Thin filaments – include Actin protein in I band attached to Z membranes ; *thin filaments also project into A bands between thick filaments; *thin filaments are pulled toward center of A band by myosin heads during contraction; 2. T (transverse) tubules- carry stimulation deep into muscle fiber; 3. Sarcoplasmic reticulum – stores &releases calcium ions to trigger contraction of sarcomere; 4. Mitochondria- supply ATP to power the contraction. FUNCTIONS: * produces movement; *maintains posture; *stabilizes joints; *generates heat. Muscle -> fascicles -> muscle fibers -> myofibrils -> myofilaments -> actin or myosin
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