Structural Support and Movement

1. Structural Support and Movement Chapter 36 Part 2

2. 36.7 How Does Skeletal Muscle Contract? • Myofibrils (bundles of contractile filaments) run the length of the muscle fiber • Myofibrils are divided into bands (striations) that define units of contraction (sarcomeres) • Z-bands attach sarcomeres to each other • Sarcomeres contain two types of filaments • Thin, globular protein filaments (actin) • Thick, motor protein filaments (myosin)

3. Fine Structure of Skeletal Muscle

4. Fig. 36-17a, p. 628

5. one bundle of many muscle fibers in parallel inside the sheath outer sheath of one skeletal muscle one myofibril in one fiber Fig. 36-17a, p. 628

6. Fig. 36-17b, p. 628

7. one myofibril inside fiber b Skeletal muscle fiber, longitudinal section. All bands of its myofibrils line up in rows and give the fiber a striped appearance. sarcomere sarcomere Z band Z band H zone Z band Fig. 36-17b, p. 628

8. Fig. 36-17c, p. 628

9. Z band H zone Z band c Sarcomeres. Many thick and thin filaments overlap in an A band. Only thick filaments extend across the H zone. Only thin filaments extend across I bands to the Z bands. Different proteins organize and stabilize the array. I band I band A band Fig. 36-17c, p. 628

10. Fig. 36-17 (d-e), p. 628

11. part of a thin filament one actin molecule d Arrangement of actin molecules in the thin filaments part of a myosin molecule part of a thick filament e Arrangement of myosin molecules in the thick filaments Fig. 36-17 (d-e), p. 628

12. Animation: Structure of skeletal muscle

13. The Sliding Filament Model • Sliding filament model • Interactions among protein filaments within a muscle fiber’s individual contractile units (sarcomeres) bring about muscle contraction • A sarcomere shortens when actin filaments are pulled toward the center of the sarcomere by ATP-fueled interactions with myosin filaments

14. The Sliding Filament Model

15. Fig. 36-18a, p. 629

16. A Relative positions of actin and myosin filaments inside a sarcomere between contractions actin myosin actin Z band Z band Fig. 36-18a, p. 629

17. Fig. 36-18b, p. 629

18. B Relative positions of actin and myosin filaments in the same sarcomere, contracted Fig. 36-18b, p. 629

19. Fig. 36-18c, p. 629

20. myosin head one of many myosin-binding sites on actin C Myosin in a muscle at rest. Earlier, all myosin heads were energized by binding ATP, which they hydrolyzed to ADP and inorganic phosphate. Fig. 36-18c, p. 629

21. Fig. 36-18d, p. 629

22. cross-bridge cross-bridge D A rise in the local concentration of calcium exposes binding sites for myosin on actin filaments, so cross-bridges form. Fig. 36-18d, p. 629

23. Fig. 36-18e, p. 629

24. E Binding makes each myosin head tilt toward the sarcomere’s center and slide the bound actin along with it. ADP and phosphate are released as the myosin heads drag the actin filaments inward, which pulls the Z bands closer. Fig. 36-18e, p. 629

25. Fig. 36-18f, p. 629

26. F New ATP binds to myosin heads, which detach from actin. ATP is hydrolyzed, which returns myosin heads to their original positions. Fig. 36-18f, p. 629

27. A Relative positions of actin and myosin filaments inside a sarcomere between contractions myosin actin actin Z band Z band B Relative positions of actin and myosin filaments in the same sarcomere, contracted myosin head C Myosin in a muscle at rest. Earlier, all myosin heads were energized by binding ATP, which they hydrolyzed to ADP and inorganic phosphate. one of many myosin-binding sites on actin D A rise in the local concentration of calcium exposes binding sites for myosin on actin filaments, so cross-bridges form. cross-bridge cross-bridge E Binding makes each myosin head tilt toward the sarcomere’s center and slide the bound actin along with it. ADP and phosphate are released as the myosin heads drag the actin filaments inward, which pulls the Z bands closer. F New ATP binds to myosin heads, which detach from actin. ATP is hydrolyzed, which returns myosin heads to their original positions. Stepped Art Fig. 36-18, p. 629

28. Animation: Sliding filament model

29. 36.8 From Signal to Response: A Closer Look at Contraction • Like neurons, muscle cells are excitable • Skeletal muscle contracts in response to a signal from a motor neuron • Release of ACh at a neuromuscular junction causes an action potential in the muscle cell

30. Nervous Control of Contraction • Action potentials travel along muscle plasma membrane, down T tubules, to the sarcoplasmic reticulum (a smooth endoplasmic reticulum) • Action potentials open voltage-gated channels in sarcoplasmic reticulum, triggering calcium release that allows contraction in myofibrils

31. Nervous Control of Contraction

32. Fig. 36-19a, p. 630

33. motor neuron A A signal travels along the axon of a motor neuron, from the spinal cord to a skeletal muscle. section from spinal cord Fig. 36-19a, p. 630

34. Fig. 36-19b, p. 630

35. neuromuscular junction B The signal is transferred from the motor neuron to the muscle at neuromuscular junctions. Here, ACh released by the neuron’s axon terminals diffuses into the muscle fiber and causes action potentials. section from skeletal muscle Fig. 36-19b, p. 630

36. Fig. 36-19c, p. 630

37. sarcoplasmic reticulum T tubule one myofibril in muscle fiber C Action potentials propagate along a muscle fiber’s plasma membrane down to T tubules, then to the sarcoplasmic reticulum, which releases calcium ions. The ions promote interactions of myosin and actin that result in contraction. muscle fiber’s plasma membrane Fig. 36-19c, p. 630

38. Animation: Nervous system and muscle contraction

39. The Roles of Troponin and Tropomyosin • Two proteins regulate bonding of actin to myosin • Tropomyosin prevents actin from binding to myosin • Troponin has calcium binding sites • Calcium binds to troponin, which pulls tropomyosin away from myosin-binding sites on actin • Cross-bridges form, sarcomeres shorten, and muscle contracts

40. Interactions of Actin, Tropomyosin, and Troponin

41. A Actin (tan) with troponin (teal) and tropomyosin (green) in a thin filament of muscle at rest. myosin-binding site blocked by tropomyosin B View of a section through the filament shown above. C Some calcium ions (orange) released by the sarcoplasmic reticulum bind to troponin. D Troponin changes shape and pulls tropomyosin away from the myosin-binding site. myosin head E The myosin head binds to the now-exposed binding site. F A cross-bridge forms between actin and myosin. Fig. 36-20, p. 631

42. Animation: Troponin and tropomyosin

43. 36.9 Energy for Contraction • Multiple metabolic pathways can supply the ATP required for muscle contraction • Muscles use any stored ATP, then transfer phosphate from creatine phosphate to ADP to form ATP • With ongoing exercise, aerobic respiration and lactic acid fermentation supply ATP

44. Three Metabolic Pathways Supply ATP

45. pathway 1 ADP + Pi dephosphorylation of creatine phosphate creatine pathway 2 pathway 3 aerobic respiration lactate fermentation glucose from bloodstream and from glycogen breakdown in cells oxygen Fig. 36-21, p. 631

46. pathway 1 ADP + Pi dephosphorylation of creatine phosphate creatine pathway 2 pathway 3 aerobic respiration lactate fermentation glucose from bloodstream and from glycogen breakdown in cells oxygen Stepped Art Fig. 36-21, p. 631

47. Animation: Energy sources for contraction

48. 36.10 Properties of Whole Muscles • Motor unit • One motor neuron and all of the muscle fibers its axons synapse with • Muscle twitch • Contraction produced by brief stimulation of a motor unit • Tetanus • A sustained contraction caused by repeated stimulation of a motor unit in a short interval

49. Muscle Twitch and Tetanus

50. relaxation starts A A single, brief stimulus causes a twitch, a rapid contraction followed by immediate relaxation. Force contraction stimulus B Repeated stimuli over a short time have an additive effect; they increase the force of contraction. Force six stimulations per second C Sustained stimulation causes tetanus, a sustained contraction with several times the force of a twitch. twitch tetanic contraction Force repeated stimulation Time Fig. 36-22, p. 632