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Skeletal Muscle

Skeletal Muscle

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Skeletal Muscle

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  1. Chapter 8 Skeletal Muscle Structure and Function

  2. Skeletal Muscle • Human body contains over 400 skeletal muscles • 40-50% of total body weight • Functions of skeletal muscle • Force production for locomotion and breathing • Force production for postural support • Heat production during cold stress

  3. Connective Tissue Covering of Muscle

  4. Structure of Skeletal Muscle:Microstructure • Sarcolemma • Muscle cell membrane • Myofibrils • Threadlike strands within muscle fibers • Actin (thin filament) • Troponin • Tropomyosin • Myosin (thick filament) • Myosin head • Crossbridge location

  5. Microstructure of Skeletal Muscle

  6. Structure of Skeletal Muscle:The Sarcomere • Functional unit • Z-line – “anchor” points for actin filaments • Sarcomere is Z-line to Z-line • Within the sarcoplasm • Transverse tubules • Communication into the cell • Sarcoplasmic reticulum • Storage sites for calcium • Terminal cisternae • Storage sites for calcium

  7. Within the Sarcoplasm

  8. The Neuromuscular Junction • Site where motor neuron meets the muscle fiber • Separated by gap called the neuromuscular cleft • Motor end plate • Pocket formed around motor neuron by sarcolemma • Acetylcholine is released from the motor neuron • Causes an end-plate potential • Depolarization of muscle fiber

  9. Illustration of the Neuromuscular Junction

  10. Muscular Contraction • The sliding filament model • Muscle shortening is due to movement of the actin filament over the myosin filament • Reduces the distance between Z-lines

  11. The Sliding Filament Model of Muscle Contraction

  12. Cross-Bridge Formation in Muscle Contraction

  13. Energy for Muscle Contraction • ATP is required for muscle contraction • Myosin ATPase breaks down ATP as fiber contracts

  14. Excitation-Contraction Coupling • Depolarization of motor end plate (excitation) is coupled to muscular contraction • Nerve impulse travels along sarcolemma and down T-tubules to cause a release of Ca++ from SR • Ca++ binds to troponin and causes position change in tropomyosin, exposing active sites on actin • Permits strong binding state between actin and myosin and contraction occurs • ATP is hydrolyzed and energy goes to myosin head which releases from actin

  15. Steps Leading to Muscular Contraction

  16. CD ROM

  17. Properties of Muscle Fibers • Biochemical properties • Oxidative capacity • Type of ATPase on myosin head • Contractile properties • Maximal force production • Speed of contraction / speed of stimulation • Muscle fiber efficiency

  18. Slow fibers Type I fibers Slow-twitch fibers Slow-oxidative fibers Fastest fibers Type IIb(x) fibers Fast-twitch fibers Fast-glycolytic fibers Individual Fiber Types Fast fibers • Type IIa fibers • Intermediate fibers • Fast-oxidative glycolytic fibers

  19. Muscle Fiber Types (x)

  20. Comparison of Maximal Shortening Velocities Between Fiber Types (x)

  21. Histochemical Staining of Fiber Type Type IIb (x) Type IIa Type I

  22. Fiber Types and Performance • Successful Power athletes • Sprinters, jumpers, strikers • Possess high percentage of fast fibers • Successful Endurance athletes • Distance runners, cyclists, swimmers • Have high percentage of slow fibers • Others • Skill sport athletes and probably non-athletes • Have about 50% slow and 50% fast fibers

  23. How Is Fiber Type Distributed In a Population? 50% Each 100% ST 100% FT

  24. How do I get more FT muscle fibers? • Grow more? • Change my ST to FT? • Pick the right parents?

  25. Alteration of Fiber Type by Training • Endurance and resistance training • Cannot make FT into ST or ST into FT • Can make a shift in IIa…. • toward more oxidative properties • toward more glycolytic properties

  26. So how do I gain strength?

  27. Protein Arrangement and Strength • Increase sarcomeres (myofibrils) in parallel • more cross bridges to pull • requires more room in diameter • requires more fluid • hypertrophy

  28. Protein Arrangement and Strength • Increase in sarcomeres in series? • Only with growth in height / limb length • Sarcomeres would crowd each other • Would result in decreased force production

  29. Protein Arrangement and Strength • Hyperplasia? • more cells • stem cells – for injury repair

  30. Protein Arrangement and Strength • Fiber arrangement • Fusiform • penniform

  31. Protein Arrangement and Strength • More sarcomeres in parallel, more tension produced • More cross bridges generating tension

  32. Age-Related Changes in Skeletal Muscle • Aging is associated with a loss of muscle mass (sarcopoenia) • independent of training status • associated with less testosterone • Regular exercise / training can improve strength and endurance • Cannot completely eliminate the age-related loss in muscle mass

  33. Types of Muscle Contraction • Isometric • Muscle exerts force without changing length • Pulling against immovable object • Stabilization • Postural muscles

  34. Types of Muscle Contraction • Isotonic (dynamic) • Concentric • Muscle shortens during force production • Eccentric • Muscle produces force but length increases

  35. Types of Muscle Contraction • Isokinetic (dynamic) • Constant speed of contraction • Machine assisted

  36. Isotonic and Isometric Contractions

  37. Force and Speed of Contraction Regulation in Muscle • Types and number of motor units utilized • More motor units = greater force • “Recruitment” • More FT motor units = even greater force • “Contribution”

  38. Histochemical Staining of Fiber Type Type IIb Type IIa Type I

  39. Force Regulation in Muscle • Initial muscle length • “Ideal” length for force generation • Maximum vs. minimum cross bridging

  40. Length-Tension Relationship in Skeletal Muscle

  41. Force Regulation in Muscle • Nature of the motor units’ neural stimulation • Frequency of stimulation • Simple twitch, summation, and tetanus

  42. Illustration of a Simple Twitch

  43. Simple Twitch, Summation, and Tetanus

  44. Relationship Between Stimulus Frequency and Force Generation

  45. Determinants of Contraction Velocity- Single Fiber • Nerve conduction velocity • Myofibrillar ATPase activity

  46. Determinants of Contraction Velocity- Whole Muscle • Same factors as for single fiber • Nerve conduction velocity • ATPase activity • Number of FT fibers recruited

  47. Force-Velocity Relationship • At any absolute force the speed of movement is greater in muscle with higher percent of fast-twitch fibers • The fastest fibers can contribute while the slow cannot • The maximum force is greatest at the lowest velocity of shortening • True for both slow and fast-twitch fibers • All can contribute at slow velocities

  48. Force-Velocity Relationship Force (Newtons) FT ST Velocity (deg./sec)

  49. Force-Power Relationship • At any given velocity of movement the power generated is greater in a muscle with a higher percent of fast-twitch fibers • The peak power increases with velocity up to movement speed of 200-300 degrees•second-1 • Force decreases with increasing movement speed beyond this velocity

  50. Force-Power Relationship