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Skeletal Muscle - 1

Skeletal Muscle - 1

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Skeletal Muscle - 1

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  1. Skeletal Muscle - 1

  2. Muscle Types

  3. Gross Anatomy

  4. Structure and Function • Skeletal muscle represents the largest tissue mass in the body (40-45% body weight) • Composite structure • Muscle cells • Nerves • Blood vessels • Extra cellular connective tissue • Aponeurosis • Tendon (with interdigitating junctions) • Basic unit • Muscle fiber (myofiber) • Cytoplasm of myofiber is sarcoplasm

  5. Muscle tendon junction

  6. Muscles and contraction • Contract when stimulated by muscle-nerve pulses at motor unit of peripheral nervous system • Produce body movement, bones serve a levers, joints serve as fulcrum • Muscles stabilize joints • Pull only and do not push, arranged in opposition • Agonist and antagonist muscles balance force • Move eyes • Produce facial expression • Chewing • Etc.

  7. Body movement frommuscle lever systems • Third class lever has muscle force between fulcrum (joint) and load (limb and limb loading) • 3rd class is common lever system in body with 1st class as antagonist • Amplifies limb motion for relatively small muscle contraction • Requires high muscle loads relative to limb loading

  8. Pennation • Amplifies muscle strength in limited anatomical space. • Limits length of contraction W Herzog, Muscle Mechanics

  9. Structural Hierarchy

  10. Structural Hierarchy 2

  11. Structural Hierarchy • Fascicles • Bundles of muscle fibers • Confined in sheath (perimysium) • Fibers (10-60 m) • Up to 30 cm long • Myofibril (~1m) • Made up of contactile myofilaments • Functional units show striations (sarcomeres) • Sarcomeres (2.5 m length) • Actin (5 nm dia) and myosin (12 nm dia)

  12. Muscle • Skeletal muscle consists of thousands of muscle fibers, the cellular units of muscle. • Fibers are densely packed elongated multi-nucleated cells

  13. Muscle Fiber Each muscle fiber is made up of thousands of myofibrils

  14. Myofibril / Sarcomere • Myofibrils contain filaments of actin and myosin. • Filaments form an ordered array and make up • sarcomeres, the functional units of muscle.

  15. Sarcomere

  16. Sarcomere filament interactions

  17. Myofilament Structure

  18. Molecular basis of muscle contraction • Sliding filament mechanism • Thin filaments (actin) slide toward center of sarcomere (A band) pulling their respective Z lines together (shortening the sarcomere) • Filaments do not change length (effectively) • Filaments are pulled forward in ratcheting action of thick filament (myosin) cross-bridges • Cross-bridges • Myosin has globular head that makes up cross-bridge • Actin has binding sites for globular myosin cross-bridge • Tropomyosin obstructs binding sites • Troponin holds tropomyosin in place Animation of this process at:

  19. Actin myosin contraction

  20. Actin myosin contraction

  21. Sarcoplasmic reticulum YC Fung, Biomech, 1993

  22. Cross-bridge cycle • To form a cross-bridge: • Ca2+ is released from long tubules of sarcoplasmic reticulum • Ca2+ binds to troponin • Allows topomyosin thread to reconfigure • Exposes binding site • # of sites determined by concentration of Ca2+ • Cross-bridges bend to pull actin filament inward • When maximum range of bending reached, bridge connection is broken • Globular head returns to oblique angle • Connection to new binding site can be established • Numerous cycles are required for complete shortening

  23. Cells and formation of myofibers J Huard et al., JBJS, 2002

  24. Histology

  25. Innervation J Huard et al., JBJS, 2002

  26. Innervation • Motor unit (MU) consists of all fibers innervated by one single motor nerve fiber • Small precise muscles 2-3 muscle fibers/MU • Large muscles, up to 1000 muscle fibers/MU Feedback via muscle spindles to sense tension in the sensory peripheral nervous system

  27. Contractions J Huard et al., JBJS, 2002

  28. Contraction cycle Action potential (AP) at neuromuscular junction Muscles can not push, they may only only CONTRACT (pull)A muscle contraction is called a muscle TWITCH

  29. Muscle contraction • To increase strength of contraction • Recruit more motor units • Increase stimulation frequency (wave summation) • Efficiency of muscle contraction • 20-25% of metabolic energy becomes mechanical work • 75-80% becomes heat • Isotonic contractions – same force • Isometric contractions – “same” length • Eccentric contractions – lengthening • Concentric contractions – shortening

  30. Length-tension relationship (sarcomeres) • Optimum overlap • Few available binding sites • No available binding sites • Fewer binding sites due to overlap • Not continuous F-L curve • Isometric forces at max stimulation • at various lengths W Herzog, Muscle Mechanics

  31. Anatomy of leg muscles Grey’s Anatomy

  32. Muscle types • Two main types of fibers • Differ in the mechanism they use to produce ATP • Amount of each type varies from muscle to muscle and from person to person • Red ("slow-twitch") fibers have more mitochondria, store oxygen in myoglobin, rely on aerobic metabolism, have a greater capillary to volume ratio and are associated with endurance; these produce ATP more slowly. Marathon runners tend to have more red fibers, generally through a combination of genetics and training. • White ("fast-twitch") fibers have fewer mitochondria, are capable of more powerful (but shorter) contractions, metabolize ATP more quickly, have a lower capillary to volume ratio, and are more likely to accumulate lactic acid. Weightlifters and sprinters tend to have more white fibers.

  33. ATP Production Strategies • Aerobic – ATP produced by breakdown of precursors in the presence of O2 • High efficiency pathway but cannot proceed without O2 • Anaerobic – Anaerobic respiration (glycolysis) produces ATP w/o O2 • Less efficient than Aerobic respiration • Produces the undesirable Lactic Acid, which produces muscle ache after strenuous exercise

  34. Fast twitch fibers • Fast fibers come in three varieties, types IIa, IIx and IIb. • Type IIa is very common fiber in humans • Type IIx fibers (used to be called, confusingly, type IIB) are our fastest fibers. • Type IIb fibers predominate in the fast muscle of small mammals that have to accelerate their limbs very fast against little load.

  35. Muscle phenotype comparison

  36. Muscle diseases and pathologies • Blunt injury • Tears • Muscle pulls • Usually damage at muscle-tendon junction or muscle-aponeurosis junction • Myasthenia gravis • Autoimmune disease which involves neuromuscular junction characterized by impaired neural impulse transmission. • Duchenne’s muscular dystrophy • Most common MD is deficiency of dystrophin, an integral plasma membrane protein that links various structural proteins to membrane. Associated with degeneration of skeletal muscle • Myotonic dystrophy • Genetic muscle disease associated with extreme muscle wasting • Myositis • Inflammatory muscle diseases (infectious and immune) • Poliomyelitis • Infectious disease causing muscle weakness • Amyotrophic lateral sclerosis • Neurological disease that attacks neurons for controlling voluntary muscles • Cerebral palsy • Neurological disorders that appear in infancy and permanently affect muscle coordination and body movement