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8 muscle i sr2002 2013 al

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8 muscle i sr2002 2013 al

  1. 1. Skeletal Muscle I SR2002 October 24, 2013 Dr. Arimantas Lionikas
  2. 2. Muscle I Plan • Muscles and connective tissue • Structure and components of muscle fibre • Muscle contraction Reading list: 1. Enoka R. Neuromechanics of human movement. 2008. Publishers: Human Kinetics, p. 205-213; 2. MacIntosh, B.R., Gardiner, P.F. McComas, A.J. Skeletal muscle, 2nd edition. 2006. Publishers: Human Kinetics, p. 151-160.
  3. 3. Muscles in numbers • Humans have ~660 skeletal muscles • Average muscle contains ~100,000 fibres • Diameter of muscle fibres is 10 -100 µm • Muscle fibre length is 1-400 mm • Note: Ratio of fibre length / muscle length is 0.2 – 0.6 (Muscle fibres do not go from end to end of muscles)
  4. 4. Muscles and connective tissue • • • • • • Skeletal muscles are connected to bones by tendons Muscles are enclosed by epimysium (connective tissue) Groups of muscle fibres form muscle fascicles surrounded by perimisium Muscle fibres are enlosed by endomysium Connective tissue maintains integrity of skeletal muscles and is important in transmission of force. Connective tissue makes around 6% of total dry muscle weight. How are muscle fibres connected to endomysium? Via focal adhesions, consisting of a number cytoskeletal anchor proteins.
  5. 5. Muscle shape and arrangement of fibres
  6. 6. Contractile machinery of the muscle • Muscle fibres (∅10100 μm) are covered by sarcolemma (≈7.5 nm thick) • T-tubules are invaginations of sarcolemma • Sarcoplasmic reticulum is a network of tubes surrounding myofibrils • Sarcoplasm is fluid enclosed within the fibre • Myofibrils (∅ ~1 μm) are packed bundles of myofilaments running along the fibre
  7. 7. Essentials of muscle contraction • Myofibril contains two myofilaments, known as think and thin filaments. Each filament is composed of several proteins. – Thick filaments consist of myosin and myosinbinding proteins: C protein, H protein, M protein, myomesin – Thin filament is composed of actin, tropomyosin and troponin complex (TNT, TN-I, TN-C • Myofilaments within myofibril are arranged in a series of repeating units, the sarcomere, which is basic contractile unit of muscle.
  8. 8. Sarcomere • • • One end of thin filaments projects into the sarcomere while another connects the Z line Thin filaments contain troponin and tropomyosin proteins that participate in blocking and unblocking of thin filaments. Because of this function troponin and tropomyosin are referred to as regulatory proteins. Thick filaments (myosin) are in the centre of sarcomere and overlap thin filaments from both sides Myosin head
  9. 9. A Closer Look: Myosin ATPase Myosin light chains
  10. 10. Cytoskeletal proteins of the sarcomere What holds actin and myosin in place? Prado et al. J. Gen. Physiol. 2008:126:461-480 Titin acts as a molecular spring permitting return of stretched sarcomeres to “optimal” length.
  11. 11. Muscle contraction ≈ 3.0 • Myosin heads drag thin filaments from both ends towards each other • The distance between Z lines shortens (sarcomere shortens) • Shortening of sarcomeres in series add up. • Will variation in the number of sarcomeres affect contraction speed of a fibre? ≈ 2.0
  12. 12. Force-length relationship (sarcomere)
  13. 13. Cross-bridge cycle • Cross bridges circulate between different states: 1) No binding; 2) Weak binding; 3) Power stroke; 4) Strong binding; 5) Rigor • Velocity of muscle shortening is determined by the rate of transition between these states 5 4 1 3 2
  14. 14. Sarcolema ECC T-tubule SR Ca2+ DHP Ca2+ATPase RYR Relaxation Myosin AP Myosin binding site Myosin Contraction
  15. 15. Muscle contraction: in vivo & in vitro Electrodes Force Dynamometer strap 5 mm
  16. 16. Force–velocity relationship Muscle fibre Ecc. Con. • Ecc is eccentric (muscle lengthens) • Con is concentric (muscle shortens) • Max speed of shortening increases with a decrease in external load (force) • Muscle force is greatest in lengthening (eccentric) contractions • Muscle cross-bridges are stretched in eccentric contractions
  17. 17. Muscle I Summary • Muscles contain significant amounts of connective tissue • Muscle fibres are muscle cells which contain myofibrils with contractile elements • Sarcomere is the basic contractile unit. Myosin filaments in the middle of the sarcomere pull actin filaments from both sides • Muscle contraction is based on conformational changes in the shape an orientation of myosin heads due to ATP hydrolysis • Speed of contraction decreases with an increase in resistance • Cytoskeletal proteins provide a frame for actin and myosin filaments

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