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Skeletal Muscle A. Structure of Muscle Fiber and Myofilaments 1

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Skeletal Muscle A. Structure of Muscle Fiber and Myofilaments 1 BIO 378 Lecture 7 - Skeletal Muscle A. Structure of Muscle Fiber and Myofilaments 1. attached to Bones by Tendons (connective tissue) 2. muscle is made up of muscle fibers 3. each muscle fiber is composed of myofibrils (myofilaments) 4. There are two types of myofilaments a. Thick myofilaments are made of MYOSIN - have myosin heads which have ATP binding sites b. Thin myofilaments are made of three proteins 1). Actin - have binding sites for Myosin heads 2). Troponin 3). Tropomyosin 5. These myofibrils (myofilaments) have distinct banding patterns a. A-band: Dark, dense portion where thick and thin filaments overlap b. H-zone: Central area where there are only thick filaments c. I-band: Light area between A-band and Z-line where there are only thin filaments d. Z-line: Dark line separating sarcomeres 6. Skeletal muscles have a Sarcoplasmic Reticulum (SR) a. similar to ER of other cells b. SR is c. stores Calcium Ions d. The SR has calcium gates that pump calcium (via active transport) into the SR from the cytoplasm of the muscle cell. B. In a relaxed muscle 1. Calcium gates are closed therefore calcium stays in the SR 2. there is a very high concentration of calcium in the SR and very low concentration of Ca in the cytoplasm 3. the binding sites for the myosin heads on the actin filaments are covered by Tropomyosin C. Muscle contraction 1. A nervous impulse depolarizes the SR and causes calcium gates to open 2. Calcium ions diffuse rapidly out of the SR into the cytoplasm 3. The calcium binds to a binding site on the Troponin molecules 4. When a calcium ion fills this site, it causes a change in the shape & position of the Troponin. When the Troponin shifts, if pulls the Tropomyosin to which it is attached 5. Movement of the Tropomyosin permits the Myosin head to contact the Actin 6. Contact of Myosin head with Actin causes the ATP molecules to break down to ADP - THIS RELEASES ENERGY 7. The release of energy causes the myosin head to swivel forward . During the swivel, the Myosin head is firmly attached to Actin. When the head swivels, it pulls the Actin and the entire thing myofilament forward. Many Myosin heads are swivelling simultaneously...their collective efforts are enought to pull the entire thin myofilament 8. At the end of the swivel the ATP has completely broken down to ADP. a. Thus, the Myosin head ATP sites are empty and the Myosin head cannot maintain its attachment to Actin. b. Therefore the Myosin head swivels back into the starting position c. however, lots more ATP are starting to fill the ATP binding sites because 9. By the time the Myosin head has swivelled back, the ATP binding sites have been refilled. As a result the Myosin head can once again bind firmly to the Actin 10. As long as Ca is present (attached to Troponin) steps 7-9 will continue (see Actin Myosin Crossbridge 3D Animation) 11. When the nervous impulse stops then the calcium gates close. 12. Ca is now transported back into the SR. Thus the Ca ions leave the binding sites on the Troponin molecules. Without Ca, Troponin returns to its original shape and position as does the attached Tropomyosin. This breaks contact between the Myosin head and Actin, and therefore, the muscle stops contracting (relaxes). !! As ATP concentration in a muscle declines, the Myosin heads can no longer bind to Actin and swivel efficiently. This decline in ATP levels in a muscle causes !! So during contraction 1. Z lines come closer 2. Width of A bands decreases 3. Width of H zones decreases 4. There is no change in the width of the I bands Useful links: Human Physiology: Muscle Muscles Back to BIO 378 Syllabus .
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