fisologia de musculo breve resuman

1. Chapter 6: Contraction of Skeletal Muscle

2. Anatomy of Skeletal Muscle Gross organization: Figure 6-1; Guyton & Hall

4. multinucleated

6. surrounded by the sarcoplasm Cellular organelles - lie between myofibrils (mitochondria, sarcoplasmic reticulum etc.) Figure 6-1; Guyton & Hall

7. Molecular Organization Figure 6-1; Guyton & Hall

8. sarcomere A band H zone I band M line Z disc The Sarcomere thick filament (myosin) thin filament (actin) titin (filamentous structural protein)

9. “Sliding Filament” Mechanism Contraction results from the sliding action of interdigitating actin and myosin filaments RELAXED: CONTRACTED:

13. composed of polymerized G-actin

15. T - binds tropomyosin

17. four light chains (MW 20,000)

18. “head” region - site of ATPase activityFigure 6-5; Guyton & Hall

19. Mechanism of Muscle Contraction Theory: Binding of Ca2+ to troponin results in a conformational change in tropomyosin that “uncovers” the active sites on the actin molecule, allowing for myosin to bind.

20. “Walk-Along” Theory Figure 6-7; Guyton & Hall

21. ADP ATP hydrolysis Pi “Walk-Along” Theory 1. Myosin head attached to actin POWER STROKE 4. Myosin head attaches to new site (ADP bound) 2. Myosin head releases (ATP bound) 3. Myosin head “cocked” (ADP and Pi bound)

24. Length-Tension Relation for Skeletal Muscle Normal operating range Active tension cannot be measured directly What can be measured? (1) passive tension - tension required to extend a resting muscle (2) total tension - active tension and passive combined Active is calculated from 1 & 2 (AT = TT – PT) Note that active tension falls away linearly with increasing length 100 total tension 50 Tension (%age max contraction) active tension passive tension 1.0 2.0 0 Length (proportion of resting length)

25. 100 50 0 Total tension 0 1x 2x Length (proportion of resting length) Zero tension stimulus 1 2 2 3 3 1 Passive tension Length-Tension Relation – The Experiment (preload) total passive (afterload = ∞) Tension(%age max contraction) active

26. Normal operating range 1 0.5 0 0 1 2 3 4 sarcomere length (mm) Tension as a Function of Sarcomere Length Stress is used to compare tension (force) generated by different sized muscles stress = force/cross-sectional area of muscle; units kg/cm2) In skeletal muscle, maximal active stress is developed at normal resting length ~ 2 mm At longer lengths, stress declines - At shorter lengths stress also declines - Cardiac muscle normally operates at lengths below optimal length - active stress (tension)

29. High Vmax(fast, white)

30. rapid cross bridge cycling

31. rapid rate of shortening (fast fiber)

32. Low Vmax (slow, red)

33. slow cross bridge cycling

34. slow rate of shortening (slow fiber)

35. Most muscles contain both types of fiber but proportions differ

36. All fibers in a particular motor unit will be of the same type i.e., fast or slow.Figure 6-12; Guyton & Hall

37. Types of Skeletal Muscle - resistance to fatigue - Slow (red muscle) fast and slow fibers show different resistance to fatigue slow fibers oxidative small diameter high myoglobin content high capillary density many mitochondria low glycolytic enzyme content fast fibers glycolytic large diameter low myoglobin content low capillary density few mitochondria high glycolytic enzyme content force (% initial) Fast (white muscle) 5 0 60 time (min)

39. There is little evidence that training alters these proportions in humans.Marathon 18% 82% Runners Swimmers 26 74 Average 55 45 man Weight 55 45 Lifters Sprinters 64 37 Jumpers 63 37

41. Predominantly fast twitch (upper)

42. Stains light: few mitochondria

43. Few, small capillaries

44. Large fibers

45. Electrical stimulation (10 Hz) via motor nerve (60 days)

46. Stimulating fast muscle at the pace of a slow muscle converts fast twitch fibers to predominantly slow twitch fibers (lower)

47. Stains dark: more mitochondria

48. Many, large capillaries

49. Larger fibersright AT

51. as few as 10 fibers/unit

52. precise control

53. rapid reacting

54. Large motor units(eg, quadriceps muscles)

55. as many as 1000 fibers/unit

56. coarse control

57. slower reacting

59. Muscle Contraction - force summation Force summation:increase in contraction intensity as a result of the additive effect of individual twitch contractions (1) Multiple fiber summation:results from an increase in the number of motor units contracting simultaneously (fiber recruitment) Figure 6-13; Guyton & Hall (2) Frequency summation:results from an increase in the frequency of contraction of a single motor unit

60. Myoplasmic [Ca2+] Fused tetanus Force AP Time (1 second) Frequency Summation of Twitches and Tetanus Myoplasmic Ca2+ falls (initiating relaxation) before development of maximal contractile force If the muscle is stimulated before complete relaxation has occurred the new twitch will sum with the previous one etc. If action potential frequency is sufficiently high, the individual contractions are not resolved and a ‘fused tetanus’ contraction is recorded.

62. Caused by near maximal force development (eg. weight lifting)

63. Increase in actin and myosin

64. Myofibrils split

65. Hyperplasia (rare)

66. Formation of new muscle fibers

67. Can be caused by endurance training

68. Hypertrophy and hyperplasia

69. Increased force generation

70. No change in shortening capacity or velocity of contraction

71. Lengthening (normal)

72. Occurs with normal growth

73. No change in force development

74. Increased shortening capacity

76. Denervation/neuropathy

77. Tenotomy

78. Sedentary life style

79. Plaster cast

80. Space flight (zero gravity)

81. Muscle performance

82. Degeneration of contractile proteins

83. Decreased max force of contraction

84. Decreased velocity of contraction

85. Atrophy with fiber loss

86. Disuse for 1-2 years

87. Very difficult to replace lost fibersweeks months/ years