1. Albia Dugger Miami Dade College Cecie Starr Christine Evers Lisa Starr www.cengage.com/biology/starr Chapter 32 Structural Support and Movement (Sections 32.5 - 32.7)

2. 32.5 How Skeletal Muscle Contracts A skeletal muscle consists of bundles of muscle fibers covered by an outer sheath Bones of a human in motion move when skeletal muscles attached to them shorten The internal organization of a skeletal muscle promotes a strong, directional contraction A muscle shortens when muscle fibers, and the contractile units inside the fibers, shorten

3. Motion and Skeletal Muscles

4. Fig. 32.11a, p. 528 outer sheath of one skeletal muscle one bundle of many muscle fibers in parallel inside the sheath Motion and Skeletal Muscles

5. Structure of Skeletal Muscle Many myofibrils run the length of a muscle fiber skeletal muscle fiber Multinucleated skeletal muscle cell myofibrils Threadlike, cross-banded skeletal muscle components that consist of sarcomeres arranged end to end

6. Structure of Skeletal Muscle (cont.) Each myofibril has light-to-dark cross-bands which are units of muscle contraction (sarcomeres) The sarcomere has parallel arrays of thin and thick filaments sarcomere Contractile unit of skeletal and cardiac muscle

7. Structure of Skeletal Muscle (cont.) Each thin filament consists of two chains of a globular protein (actin) Thicker filaments consist of myosin actin Protein that is the main component of thin filaments of muscle fibers myosin Protein in thick filaments of muscle fibers

8. Sarcomere Structure

9. Fig. 32.11b, p. 528 B one myofibril, made up of sarcomeres arranged end to end sarcomere Z line Sarcomere Structure

10. Fig. 32.11c, p. 528 C one sarcomere, with parallel actin and myosin filaments actinmyosin Z line actin Z line Sarcomere Structure

11. Animation 6.7: Structure of skeletal muscle To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

12. Sliding-Filament Model The sliding-filament model describes how ATP-driven sliding of actin filaments past myosin filaments shortens the sarcomere Shortening of all sarcomeres in all myofibrils of all muscle fibers brings about muscle contraction sliding-filament model How interactions among actin and myosin filaments shorten a sarcomere and bring about muscle contraction

13. Steps in the Sliding-Filament Model 1.In a muscle at rest, actin and myosin filaments lie next to one another, but do not interact 2.Myosin heads in the thick filaments are activated by a phosphate-group transfer from ATP 3.Release of calcium from intracellular storage allows myosin heads to bind to sites on adjacent actin filaments, forming cross-bridges

14. Steps in the Sliding-Filament Model 4.A myosin head releases bound ADP and phosphate as it tilts toward the sarcomere center with actin filaments attached 5.New ATP binds to myosin heads, causing them to release actin and return to their original orientation 6.Myosin heads repeatedly binding to and pulling on adjacent actin filaments cause the sarcomere to contract

15. The Sliding-Filament Model

16. Fig. 32.12.1, p. 529 The Sliding-Filament Model

17. Fig. 32.12.1, p. 529 Sarcomere between contractions myosinactin 1 The Sliding-Filament Model

18. Fig. 32.12.2,3, p. 529 The Sliding-Filament Model

19. Fig. 32.12.2,3, p. 529 one of many myosin-binding sites on actin cross-bridge myosin head with bound ADP and P cross-bridge 2 3 The Sliding-Filament Model

20. Fig. 32.12.4, p. 529 The Sliding-Filament Model

21. Fig. 32.12.4, p. 529 ADP and P released 4 The Sliding-Filament Model

22. Fig. 32.12.5,6, p. 529 The Sliding-Filament Model

23. Fig. 32.12.5,6, p. 529 cross-bridge broken Same sarcomere, contracted cross-bridge broken 5 6 The Sliding-Filament Model

24. ANIMATION: Sliding filament model To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

25. Animation: Structure of a Sarcomere

26. Animation: Muscle Contraction Overview

27. 32.6 From Signal to Response Release of ACh at a neuromuscular junction causes an action potential which is propagated along the plasma membrane of the muscle cell, and along T tubules to the sarcoplasmic reticulum Calcium ions released by this organelle allow actin and myosin heads to interact so muscle contraction occurs sarcoplasmic reticulum Specialized endoplasmic reticulum in muscle cells Stores and releases calcium ions

28. Nervous Control of Contraction (1) A signal travels along the axon of a motor neuron from the spinal cord to a skeletal muscle

29. Fig. 32.13a, p. 530 section from spinal cord motor neuron Nervous Control of Contraction (1)

30. Animation 6.9: Nervous system and muscle contraction To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

31. Nervous Control of Contraction (2) Signal transfers from motor neuron to muscle at neuromuscular junctions ACh from axon terminals diffuses into muscle fiber and causes action potentials

32. Fig. 32.13b, p. 530 section from skeletal muscle neuromuscular junction Nervous Control of Contraction (2)

33. Nervous Control of Contraction (3) Action potentials propagate along the plasma membrane to T tubules, to sarcoplasmic reticulum, which releases calcium ions Ions promote interactions of myosin and actin

34. Fig. 32.13c, p. 530 muscle fiber’s plasma membrane one myofibril in muscle fiber sarcoplasmic reticulum T tubule Nervous Control of Contraction (3)

35. Fig. 32.13, p. 530 section from spinal cord motor neuron A signal travels along the axon of a motor neuron, from the spinal cord to a skeletal muscle. 1 Stepped Art section from skeletal muscle neuromuscular junction 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. 2 muscle fiber’s plasma membrane one myofibril in muscle fiber sarcoplasmic reticulum T tubule Action potentials propagate along a muscle fiber’s plasma membrane down toT tubules, then to the sarcoplasmic reticulum, which releases calciumions. The ions promote interactions of myosin and actin that result in contraction. 3 Nervous Control of Contraction

36. Motor Units and Muscle Tension A motor neuron and all of the muscle fibers it synapses with constitute one motor unit Brief stimulation of a motor unit causes a muscle twitch Repeated stimulation causes a sustained contraction that generates more force (muscle tension), depending on the number of muscle fibers that contract

37. Key Terms motor unit One motor neuron and the muscle fibers it controls muscle twitch Brief muscle contraction muscle tension Force exerted by a contracting muscle

38. Muscle Tension

39. Fig. 32.14b, p. 530 A Brief stimulation causes a twitch. stimulus contraction relaxation starts Force Muscle Tension

40. Fig. 32.14b, p. 530 B Repeated stimulation within a short interval causes a sustained contraction with greater force. Time repeated stimulation sustained contraction twitch Force Muscle Tension

41. Animation: Types of contractions

42. Energy for Contraction Muscle fibers produce ATP needed for contraction by three pathways: dephosphorylation of creatine phosphate, aerobic respiration, and lactate fermentation ATP The first energy source a muscle uses, but muscle has a limited amount of ATP

43. Three Pathways of ATP Production Muscle has a large store of creatine phosphate, which can transfer a phosphate to ADP and form ATP – fuels muscle contraction until other pathways increase ATP output Aerobic respiration (oxygen-requiring) yields most ATP used by a muscle during prolonged, moderate activity Lactate fermentation produces less ATP than aerobic respiration, but operates even when oxygen level is low

44. Three Pathways of ATP Production

45. Fig. 32.15, p. 531 dephosphorylation of creatine phosphate ADP + Pi oxygen glucose from bloodstream and from glycogen breakdown in cells creatine lactate fermentation aerobic respiration 1 23 Three Pathways of ATP Production

46. Animation: Energy sources for contraction To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERECLICK HERE

47. Types of Muscle Fibers Muscles have a mix of red and white fibers, which differ in the way they produce ATP Muscle fibers can also be subdivided into fast fibers or slow fibers based on the ATPase activity of their myosin Fast fibers split ATP more efficiently and contract more quickly than slow fibers when stimulated

48. Red Fibers and White Fibers Red fibers make ATP mainly by aerobic respiration Have many mitochondria and oxygen-storing myoglobin Can be either fast or slow White fibers make ATP mainly by lactate fermentation Have no myoglobin and few mitochondria All white fibers are fast fibers

49. Key Concepts How Skeletal Muscle Contracts A muscle fiber contains many myofibrils, each divided crosswise into sarcomeres Sarcomeres contain parallel filaments of the proteins actin and myosin Muscle contracts when ATP-driven interactions between these proteins shortens sarcomeres

50. BBC Video: How Muscles Hold Tension

51. 32.7 Muscles and Health In humans, all muscle fibers form before birth Exercise can’t add muscle fibers, but it can increase muscle strength and endurance Muscle function can be impaired by genetic disorders, infectious disease, and some toxins

52. Effects of Exercise Aerobic exercise, which is low in intensity and long in duration, makes muscles more resistant to fatigue Strength training (intense, short-duration exercise such as weight lifting) stimulates formation of more actin and myosin, as well more enzymes of glycolysis

53. Aerobic Exercise Aerobic exercise increases the number of mitochondria in muscles, which increases endurance

54. Strength Training Strength training involves two types of muscle contractions: Isotonically contracting muscles shorten and move some load, as when you lift an object Isometrically contracting muscles tense but do not shorten, as when you try to lift an object but fail because its weight exceeds the muscle’s capacity

55. Isotonic and Isometric Contraction

56. Fig. 32.17, p. 532 A Isotonic contraction. Muscle tension is greater than the opposing force and the muscle shortens, as when you lift a light weight. B Isometric contraction. Muscle tension is less than the opposing force and the muscle remains at the same length, rather than shortening. Isotonic and Isometric Contraction

57. Fig. 32.17a, p. 532 A Isotonic contraction. Muscle tension is greater than the opposing force and the muscle shortens, as when you lift a light weight. Isotonic and Isometric Contraction

58. Fig. 32.17b, p. 532 B Isometric contraction. Muscle tension is less than the opposing force and the muscle remains at the same length, rather than shortening. Isotonic and Isometric Contraction

59. Muscles and Aging As people age, muscles shrink: Number of muscle fibers declines Fibers grow more slowly in response to exercise Muscle injuries take longer to heal Exercise can be helpful at any age: Strength training slows loss of muscle tissue Aerobic exercise improves circulation, helps lift depression, and improves brain functions

60. Muscular Dystrophy Muscular dystrophies are a class of genetic disorders that cause muscles to break down In Duchenne muscular dystrophy, the affected gene encodes, a protein (dystrophin) in muscle fibers’ plasma membranes Mutated dystrophin allows foreign material to enter a muscle fiber, which causes the fiber to break down, resulting in death by respiratory failure

61. Effects of Muscular Dystrophy

62. Motor Neuron Disorders Muscular weakness or paralysis occurs when motor neurons cannot signal muscles to contract Poliovirus infects and kills motor neurons, causing death or paralysis Amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease) also kills motor neurons, causing death by respiratory failure

63. Botulism and Tetanus Some bacteria make deadly toxins that disrupt the flow of signals from nerves to muscles Botulinum toxin prevents motor neurons from releasing acetylcholine (ACh) – muscles can’t contract without this neurotransmitter Tetanus toxin in the spinal cord blocks release of neurotransmitters that inhibit motor neurons – muscles stiffen and cannot be released from contraction

64. Tetanus

65. Key Concepts Factors Affecting Contraction Muscle fibers in a muscle are organized into motor units that contract in response to signals from a motor neuron Diseases or disorders can interfere with muscle function Exercise improves muscle strength and endurance

66. Muscles and Myostatin (revisited) Drugs that inhibit myostatin production or prevent myostatin activity may help to slow the muscle loss that results from muscular dystrophy, ALS, and even normal aging