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Skeletal Muscle

Skeletal Muscle. ~40-45% of BW 400+ in body Responsible for all movement and support. Composition. 75% water 20% protein 5% other: fats, CHO, high energy phosphates, urea, lactic acid, enzymes, Na+, K+, Cl-, and minerals: Ca2+, Mg, and phosphorous. Striated in Apperance.

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Skeletal Muscle

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  1. Skeletal Muscle • ~40-45% of BW • 400+ in body • Responsible for all movement and support

  2. Composition • 75% water • 20% protein • 5% other: fats, CHO, high energy phosphates, urea, lactic acid, enzymes, Na+, K+, Cl-, and minerals: Ca2+, Mg, and phosphorous

  3. Striated in Apperance • Structure is fascia and connective tissue sheaths which separate individual muscles and hold muscles in place • Under fascia: epimysium, CT surrounding muscle • Under epimysium: fascicles, bundles of muscle fibers enclosed by perimysium • There are ~150 fibers/fasciculus

  4. Fasciulus • Bundle of muscle cells (fibers) surrounded by perimysium • Each fiber in fasciulus is surrounded by endomsysium • Under endomysium is sarcolemma, cell membrane • Connective tissue surrounds muscle and forms a network that extends throughout the muscle

  5. Sarcoplasm • Cytoplasm of muscle cell • Under sarcolemma • Contains contractile proteins, enzymes, fat, and glycogen particles, nuclei, and specialized cellular organelles

  6. Proteins • Greatest amounts: • Myosin • Actin • Tropomysin • Also, large amounts of myglobin

  7. Muscle cells • Multi-nucleated • Striated appearance

  8. Sarcoplasmic Reticulum (SR) • Embedded in sarcoplasm • Network of channels and vessicles • Lies in parallel to the myofibrils • Lateral end: sac-like cistern which stores Ca2+

  9. Transverse Tubules (T-Tubules) • Perpendicular to myofibril • T-tubule and two terminal cisternae are the region of the Z-line • Called a triad • Two triads/sarcomere • One sarcomere: Z line – Z line • T-tubules open to the outside of each muscle fiber • Functioning as a network, spreading the AP from outer to inner portion of muscle fiber

  10. Muscle Blood supply • With exercise, increase in blood flow to muscle

  11. Myosin molecule • composed of 6 polypeptide chains • 2 heavy • 4 light • One myosin filament is made up of 200 or more myosin molecules • Exactly 1.6 micrometers in length

  12. Tails are in center • Heads coming out from the center (0.2 micrometers) • hinges at two points • arm separates from the filament • where the head attaches to the arm • filament is twisted, cross-bridges are displaced from previous set by 120° • Insures that cross-bridges extend in all directions from the filament

  13. Actin • Primary protein in thin filament • Forms the backbone of filament • Approximately 3000 actin/myofibril

  14. Troponin and Tropomyosin • Two other proteins in filament

  15. Two shapes of actin 1. Globular (G-actin) polymerizes and unfolds into 2. Fibrous (F-actin) double stranded in helix, complete revolution every 70 nanometers • 13 G-actin molecules in each revolution of the helical strand • Each G-actin has an ADP molecule attached to it

  16. ADP are purported to be the active sites on the actin for cross-bridge interaction • Active sites are staggered, one site on total filament every 2.7 nanometers • each actin is 1 micrometer long

  17. Filament bases • Inserted into Z discs, other end protrudes into sarcomere • In spaces between myosin molecules

  18. Z line • Actin filaments extending from either side • Into neighboring sarcomeres • Passes from myofibril to myofibril • Attaching the myofilaments to each other across the muscle fiber

  19. Sarcomere • Basic contractile unit, from Z line to Z line

  20. Tropomyosin • Associated with the actin filament • Loosely connected to the F-actin strands • Wrap themselves spirally around the sides of the F-actin helix

  21. At rest, tropomyosin molecules lie on top of the actin active sites, inhibiting interaction between actin and myosin • each tropomyosin covers about seven active sites

  22. Troponin • Protein attached near one end of each tropomyosin molecule Complex of 3 protein subunits 1. Troponin I (TnI) strong affinity for actin 2. Troponin T (TnT) strong affinity for tropomyosin 3. Troponin C (TnC) strong affinity for calcium

  23. Troponin complex may attach the tropomyosin to the actin • Each subunit plays a role in the contractile process

  24. Sarcomere Ultrastructure alternating light and dark bands along the length of the muscle fiber give it its characteristic striated appearance Lighter area: I band Darker zone: A band

  25. I band (isotropic): when light passes through this band, its velocity is the same • A band (anisotropic): light does not scatter equally • Z line bisects the I band and adheres to the sarcolemma, adding stability to the sarcomere • Z line has alpha actin (maintains spacing of actin) and desmin (connects z lines of different myofibrils together)

  26. Position of the actin and myosin results in an overlap of the filaments in the sarcomere • Center of the A band is the H zone, a lighter area due to the absence of actin filaments in this region • Central part of the H zone is bisected by the M line, which delineates the sarcomere’s center • M line is protein structures that support the arrangement of the myosin filaments • also has myomesin which provides an anchor for titin (elastic filament • Helps maintain centering between Z lines) and M-CK, provides ATP from CP

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