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Muscle

Muscle. Skeletal muscle Unit Cell Structure Architecture Series/parallel Force/velocity Stimulation Summation/tetanus/rate-coding Muscle mechanics Force-length relation Force velocity relation Pre-stretch. Skeletal Muscle. Striated and voluntary Cardiac muscle is striated

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Muscle

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  1. Muscle • Skeletal muscle • Unit Cell Structure • Architecture • Series/parallel • Force/velocity • Stimulation • Summation/tetanus/rate-coding • Muscle mechanics • Force-length relation • Force velocity relation • Pre-stretch

  2. Skeletal Muscle • Striated and voluntary • Cardiac muscle is striated • Smooth muscle is unstriated and involuntary • Attaches to skeleton via tendons • Most abundant tissue in the body • 45-75% of body weight

  3. Structure of a muscle cell A. Fascicles • fiber bundles B. Fibers • muscle cell • bundles of myofibrils C. Myofibrils D. Sarcomeres (series) E. Actin & Myosin Filaments

  4. Fascicles • A muscle is composed of multiple fascicles in parallel • A sheath of connective tissue surrounds the muscle (epimysium) • Each fascicle is surrounded by connective tissue (perimysium) • Fascicles composed of bundles of muscle fibers

  5. Muscle Fiber • Long, cylindrical, multinucleated cells • Between fibers are blood vessels • Surrounded by endomysium • Composed of myofibrils

  6. Myofibrils • Literally (muscle thread) • Contractile element of muscle • Made up of filaments • Aligned in parallel • filaments make striations • Banding pattern • One repeating unit is called a sarcomere • string of sarcomeres in series

  7. Sarcomeres • Functional unit of muscle contraction • Literally ‘muscle segment’ • Number of sarcomeres in a fiber is very important to muscle function • When each sarcomere shortens the same amount, the fiber with more sarcomeres will shorten more. • Made up of myofilaments • Thick and thin filaments

  8. Myofilaments • Myosin(thick) • In central region • Dark bands • Globular heads • Arranged in both directions • Actin(thin)

  9. Banding Pattern

  10. Based on myofilaments: • Z-Disc • I-Band • A-Band • H-zone • M-line

  11. Sarcomere: <--I-Band---> <--------------------A-Band---------------> <--I-Band---> <-H-Zone-> Z-Disc M-line

  12. Muscle contraction • Sliding filament theory • AF Huxley and HE Huxley • Light and Electron microscopy • Both published results same time in Nature • Does not explain lengthening contractions

  13. Sliding Filament Theory • The exertion of force by muscle is accompanied by the sliding of thick and thin filaments past one another • Commonly explained by cross-bridges

  14. cross-bridge theory: • muscle force is proportional to the number of cross bridges attached

  15. Sliding filament theory • A band stay the same • I band shorten

  16. A single functional unit in a muscle contraction is a • fascicle • fiber • myofibril • sarcomere

  17. According to sliding filament theory, during a contraction the distance between the M and Z lines • increases • decreases • stays the same • need more information

  18. Muscle • Skeletal muscle • Unit Cell Structure • Architecture • Series/parallel • Force/velocity • Stimulation • Summation/tetanus/rate-coding • Muscle mechanics • Force-length relation • Force velocity relation • Pre-stretch

  19. Muscle architecture • Organization of muscle fibers • Muscle also organized at macro level • Architecture is the arrangement of muscle fibers relative to the axis of force generation • Muscle fibers have fairly consistent diameters among muscle of different size, but arrangement can be very different • So cannot tell force capacity of a muscle from a biopsy • Need number of fibers and how arranged

  20. 3 types of arrangements • Longitudinal (parallel) • Fibers run parallel to force generating axis • Pennate • Fibers at a single angle • shallow • Multipennate • several angles

  21. What are advantages/disadvantages of • longitudinal arrangement? • pennate arrangement?

  22. Muscle architecture • Determines • Max muscle force • Fibers in parallel • Pennation angle • Max muscle shortening velocity • no of sarcomeres in series

  23. Hill Muscle Model CE: Contractile Element (active force generation) SE: Series Elastic Element represents elasticity in: cross-bridges and myofilaments tendon and aponeuroses PE: Parallel Elastic Element connective tissue surrounding muscle fibers

  24. Can use Hill muscle model to illustrate effects of muscle length and width on muscle’s • maximum force • maximum shortening velocity

  25. f, Dl f, Dl f, Dl Series f, Dl f, Dl f, Dl f, Dl Parallel

  26. F=? DL=? f, Dl f, Dl f, Dl Series • F = f ; DL = Dl • F = 3f ; DL = 3Dl • F = 3f ; DL = Dl • F = f ; DL = 3Dl • don’t understand f, Dl

  27. f, DL DL=nDl f, Dl f, Dl f, Dl Series f, Dl f, Dl F,Dl F=nf • F = f ; DL = Dl • F = 3f ; DL = 3Dl • F = 3f ; DL = Dl • F = f ; DL = 3Dl • don’t understand f, Dl f, Dl Parallel

  28. Pennation Angle

  29. Pennation Angle • Pennation angle is a space saving strategy • Allows you to pack more fibers into a smaller space • Doesn’t hurt b/c cos0=1, cos 30=0.87 (13% force loss)

  30. Muscle architecture • Determines • Max muscle force • Fibers in parallel • Pennation angle • Max muscle shortening velocity • no of sarcomeres in series

  31. Physiological Cross-Sectional Area • PCSA ~ max muscle force • M=muscle mass (g) • r=muscle density (g/cm3) = 1.056 g/cm3 • l=fiber length (cm) • V= Muscle volume = M/r

  32. How do we measure PCSA?

  33. More on PCSA • Not proportional to muscle mass • Not proportional to anatomical cross-sectional area

  34. Muscle architecture • Determines • Max muscle force (~PCSA) • Fibers in parallel • Pennation angle • Max muscle shortening velocity • no of sarcomeres in series

  35. Muscle fiber length • Assumed that fiber length ~fiber velocity • Fiber length ~ no. of sarcomeres in series

  36. Muscle architecture • Determines • Max muscle force (~PCSA) • Fibers in parallel • Pennation angle • Max muscle shortening velocity (~Fiber length) • no of sarcomeres in series

  37. What are advantages/disadvantages of • longitudinal arrangement? • pennate arrangement?

  38. Significance of Architecture • Clever design • Same functional component can yield so many different motors • Muscles designed for a purpose • Perhaps this simplifies the control

  39. Problem Imagine you have 10 sarcomeres; each generates a maximum of 1 unit of force, and shortens with a maximum velocity of 1 unit/s. Diagram an arrangement of sarcomeres that will create a muscle fiber with the following force and velocity characteristics. Use I to represent individual sarcomeres, and draw ellipses around sarcomeres to specify fibers. i) Fmax= 5 units; Vmax= 2 units/s ii) Fmax= 2 units; Vmax=5 units/s iii) Fmax=5cos10o units; Vmax=2cos10o units/s

  40. Net muscle force Vector math can illustrate the effect of coactivating different parts of the pectoralis major muscle. Suppose clavicular component exerted a force of 224N at 0.55 rad above horizontal, and the sternal portions has a magnitude of 251N at 0.35 rad below horizontal. What is the resultant force? • F = 472 N, angle = 64.5 deg • F = 472 N, angle = 25.4 deg • F = 428 N, angle = 4.17 deg • F = 428 N, angle = 85.82 • I don’t understand Enoka Fig 1.6

  41. Enoka Fig 1.6

  42. Muscle • Skeletal muscle • Unit Cell Structure • Architecture • Series/parallel • Force/velocity • Stimulation • Summation/tetanus/rate-coding • Muscle mechanics • Force-length relation • Force velocity relation • Pre-stretch

  43. Temporal Summation • Excitation fast (~1-2ms) • Contraction/relaxation slow (100ms) • Muscle twitch lags because slack in the elastic components must be taken up. • Contraction time: • Relaxation time: • Summation • If second impulse comes along before the first one has relaxed, they sum • Get more force with multiple impulses then alone • Tetanic Summation • maximum tension is sustained because rapidity of stimulation outstrips the contraction-relaxation time of the muscle

  44. Fused Tetanus Unfused Tetanus Twitch Single Low frequency High frequency Neural Stimulation Force Stimulation (Action potentials) Time

  45. If the contraction-relaxation time for a muscle twitch is 100 ms, at what stimulation frequency will we begin to see summation? NB: 1 Hz corresponds to 1 stimulus/second • 100 Hz and greater • 5 Hz and greater • 10 Hz and greater • I don’t understand

  46. Max Force Max Shortening Velocity No. of sarcomeres in series Muscle fiber length • PCSA • No. sarcomeres in parallel • Pennation angle • Stimulation

  47. Muscle • Skeletal muscle • Unit Cell Structure • Architecture • Series/parallel • Force/velocity • Stimulation • Summation/tetanus/rate-coding • Muscle mechanics • Force-length relation • Force velocity relation • Pre-stretch • WorkLoops

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