Muscle Physiology I Skeletal Muscle

1. Muscle Physiology ISkeletal Muscle PHIS 206 Roland Pittman September 17, 2014 Reading: Chap 8, pp 195-200

2. Muscle • Comprises largest group of tissues in body • 3 types of muscle • Skeletal muscle: Make up muscular system • Cardiac muscle: Found only in the heart • Smooth muscle: Appears throughout the body systems as components of hollow organs and tubes • Classified as: Striated/unstriated and voluntary/involuntary

3. Unstriated muscle Striated muscle Skeletal muscle Cardiac muscle Smooth muscle Voluntary muscle Involuntary muscle Fig. 8-1, p. 196

4. Muscle • Controlled muscle contraction allows • Purposeful movement of the whole body or parts of the body • Manipulation of external objects • Propulsion of contents through various hollow internal organs • Emptying of contents of certain organs to external environment

5. Structure of Skeletal Muscle • Muscle consists of a number of muscle fibers lying parallel to one another and held together by connective tissue • Single skeletal muscle cell is known as a muscle fiber • Multinucleated • Large, elongated, and cylindrically shaped • Fibers usually extend entire length of muscle

6. Structure of Skeletal Muscle • Myofibrils • Contractile elements of muscle fiber • Regular arrangement of filaments • Thick filaments – myosin (protein) • Thin filaments – actin (protein) • Viewed microscopically myofibril displays alternating dark (the A bands) and light bands (the I bands) giving appearance of striations

8. Muscle Tendon Muscle fiber (a single muscle cell) Connective tissue (a) Relationship of a whole muscle and a muscle fiber Fig. 8-2a, p. 197

9. Musclefiber Dark A band Light I band Myofibril (b) Relationship of a muscle fiber and a myofibril Fig. 8-2b, p. 197

10. M line Z line A band I band Portion of myofibril H zone Thick filament Thin filament A band I band Sarcomere H zone Z line M line Cross bridges (c) Cytoskeletal components of a myofibril Fig. 8-2c, p. 197

11. Myosin head Actin Myosin tail Troponin Tropomyosin Thick filament Thin filament (d) Protein components of thick and thin filaments Fig. 8-2d, p. 197

12. Z line Z line M line Sarcomere A band I band H zone (a) Electron micrograph of a myofibril Fig. 8-3a, p. 198

13. (b) Low-power light micrograph of skeletal muscle fibers Fig. 8-3b, p. 198

14. Structure of Skeletal Muscle • Sarcomere • Functional unit of skeletal muscle • Found between 2 Z lines • Regions of sarcomere • A band:Made up of thick filaments along with portions of thin filaments that overlap on both ends of thick filaments • H zone:Lighter area within middle of A band where thin filaments do not reach • M line:Extends vertically down middle of A band within center of H zone • I band:Consists of remaining portion of thin filaments that do not project into A band

15. Structure of Skeletal Muscle • Titin • Giant, highly elastic protein (largest in body) • Extends in both directions from M line along length of thick filament to Z lines at opposite ends of sarcomere • 2 important roles: • Helps stabilize position of thick filaments in relation to thin filaments • Greatly augments muscle’s elasticity by acting like a spring

16. Myosin • Component of thick filament • Protein molecule consisting of two identical subunits shaped somewhat like a golf club • Tail ends are intertwined around each other • Globular heads project out at one end

17. Myosin • Tails oriented toward center of filament and globular heads protrude outward at regular intervals • Heads form cross bridges between thick and thin filaments • Cross bridge has 2 important sites critical to contractile process • An actin-binding site • A myosin ATPase (ATP-splitting) site

18. Actin-binding site Myosin ATPase site Heads Tail 100 nm (a) Myosin molecule Cross bridges Myosin molecules (b) Thick filament Fig. 8-4, p. 198

19. Actin • Primary structural component of thin filaments • Spherical in shape • Thin filament also has 2 other proteins • Tropomyosin • Troponin • Each actin molecule has special binding site for attachment with myosin cross bridge • Binding results in contraction of muscle fiber

20. Binding site for attachment with myosin cross bridge Actin molecules Actin helix Tropomyosin Troponin Thin filament Fig. 8-5, p. 199

21. Actin and Myosin • Actin and myosin are often called contractile proteins. • Neither actually contracts • Actin and myosin are not unique to muscle cells, but are more abundant and more highly organized in muscle cells

22. Tropomyosin and Troponin • Often called regulatory proteins • Tropomyosin • Thread-like molecules that lie end to end alongside groove of actin spiral • In this position, covers actin binding sites blocking interaction of actin and myosin that leads to muscle contraction • Troponin • Made of 3 polypeptide units • One binds to tropomyosin • One binds to actin • One can bind with Ca2+

23. Tropomyosin and Troponin • Troponin • When not bound to Ca2+ • Troponin stabilizes tropomyosin in blocking position over actin’s cross-bridge binding sites • When Ca2+ binds to troponin • Tropomyosin moves away from blocking position • With tropomyosin out of the way, actin and myosin bind, interact at cross-bridges • Muscle contraction results

25. Tropomyosin Troponin Actin Actin-binding site Myosin cross-bridge binding sites Myosin cross bridge (a) Relaxed Fig. 8-6a, p. 200

26. (b) Excited Fig. 8-6b, p. 200