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Excitation Contraction Coupling |
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| Muscle fiber anatomy/structure |
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• Epimysium; connective tissue covering the muscle.
• Muscle fibers are grouped into fascicles, covered by perimysium. • Endomysium; connective tissue covering each individual muscle fiber. • Sarcolemma; thin surface membrane of the muscle fiber. • Sarcoplasm ; semi-fluid plasm and intracellular content of the muscle fiber. • Myosin and Actin are myo-fibrils, arranged in parallel within the sarcoplasm, responsible for muscle fiber contraction. • Myosin molecules are thick filaments. • Thin filaments made of actin molecules bound to troponin and tropomyosin. • A sarcomere is a sub-unit within the muscle fiber, capable of contraction. It consists of strands of actin and myosin arranged in parallel. • Transverse tubule (T-tubule). Transverse orientation with relation to muscle fiber; structurally internal to the muscle fiber cell, but contains extracellular fluid. Muscle action potentials travel through T tubules to into the muscle. • Longitudinal tubule ( Sarcoplamic reticulum); expand to form bulbous Terminale Cisternae at the junction with T-tubules. Longitudinal tubules do not contain sarcoplasm or extra-cellular fluid. Two Terminale Cisternaes and one interposed T-Tubule form a triad in longitudinal sections of muscle. |
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| Excitation-contraction coupling refers to the link between the electrical impulse nerve event and the mechanical component of muscle fiber twitch. |
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• Nerve impulse at the motor end plate spreads over the outer surface of
the muscle fiber, through the T-tubules, coming into contact with Terminal
Cisternae and then the Longitudinal tubules.
• Ca++ is released from the Longitudinal tubules (Sarcoplasmic Reticulum) into the sarcoplasm. |
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• Ca++ binds to troponin within the sarcoplasm, resulting in bridge
formation between actin and myosin filaments.
• Cross bridge formation causes actin fibers to slide relative to the thicker myosin strand. • Calcium is sequestering into the sarcoplasmic reticulum (in the presence of adenosine-triphosphate) breaking the existing bridge, relaxing the contraction. |
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