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Composition and Structure of Skeletal Muscle

Skeletal muscle, constituting around 40-45% of body weight, plays a vital role in movement and support. This muscle type is predominantly made of water and protein, with essential components like fats, enzymes, and minerals. The intricate composition includes myosin, actin, and tropomyosin, shaping the striated appearance. The muscle cells exhibit multi-nucleation and contain the sarcoplasmic reticulum essential for muscle function. Exploring the sarcomere's ultrastructure reveals the alternating light and dark bands determining the muscle's striation, emphasizing the critical overlap of actin and myosin filaments.

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Composition and Structure of 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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