SKELETAL MUSCLES & PHYSICAL PERFORMANCE

1. SKELETAL MUSCLES & PHYSICAL PERFORMANCE

2. 3. Skeletal muscle fibers are comprised of two primary constituents: • Sarcoplasm (muscle cytoplasm) • Myofibrils (sarcomeres in series– contractile elements) (continued) Musculoskeletal/Neuromuscular System 1. Myology  scientific study of muscles & their parts • Structural Unit  muscle fiber or muscle cell • Functional Unit  motor unit 2. Gross muscles may be comprised of only a few muscle fibers or as many as 1 million.

3. MOTOR UNIT

4. Musculoskeletal/Neuromuscular System • Sarcoplasm ~ 25% protein in a non-contractile configuration & ~ 75% fluid • Myofibrils various protein filaments configured to enable the contractile process • Myosin & actin – main filaments involved in contractions • Troponin & tropomyosin – loosely bound to actin when the myofibril is relaxed

5. (continued) Musculoskeletal/Neuromuscular System 4. Muscle fibers are multinucleated 5. Muscle fibers have a cell wall called the sarcolemma that has 3 major substructures: • Transverse Tubules • Terminal Cisternae • Sarcoplasmic Reticulum

6. (continued) Musculoskeletal/Neuromuscular System • Transverse Tubules opening or invagination of the sarcolemma that provides for neural input • Run roughly perpendicular to sarcomere & are located @ junction of A & I bands • Terminal Cisternae Storage vesicles for calcium • Located parallel & contiguous to transverse tubules • Sarcoplasmic Reticulum  conduit for calcium • Oriented roughly parallel to sarcomere

7. Within the Sarcoplasm (c) 2004 The McGraw-Hill Companies, Inc. All rights reserved.

9. ARRANGEMENT OF FILAMENTS in SINGLE MUSCLE FIBER

10. (continued) Musculoskeletal/Neuromuscular System • Connective tissue is found w/in each muscle fiber & @ multiple levels outside of it: • Z line or Z disc (connects sarcomeres) • Endomysium (surrounds muscle fiber) • Perimysium (surrounds fasciculus) • Epimysium (surrounds whole muscle) • Tendon (coalescing of epimysium that connects muscle to bone)

11. Connective Tissue Covering Skeletal Muscle (c) 2004 The McGraw-Hill Companies, Inc. All rights reserved.

12. (continued) Musculoskeletal/Neuromuscular System • Tissues affecting elasticity of musculo-tendinous unit: • Collagen • Elastin • Titin • Myomesin

13. Musculoskeletal/Neuromuscular System • Collagen (fibrous insoluble protein found in the connective tissue within muscle, skin, bone, ligaments & cartilage that stiffens tissue– at least 20 isoforms exist) • Elastin (an albuminoid protein that is the principal constituent of yellow elastic tissue found in connective tissue) • Titin (Long intramuscular protein that fixes the large myosin filaments in place – surrounds myosin filament and runs in series with it to the Z-line – primary source of passive tension in skeletal muscle) • Myomesin (One of the small proteins that fixes the large myosin filaments in place – located at H-zone and runs perpendicular to the myosin filament)

14. ARRANGEMENT OF FILAMENTS IN A SARCOMERE

15. Muscle Contraction & Relaxation (Sliding Filament Theory) • Relaxed muscle • Few or no nerve impulses reach muscle • Calcium ions bound to sarcoplasmic reticulum @ terminal cisternae • Tropomyosin-troponin complex blocks attachment sites for myosin on actin • ATP bound to myosin “heads” (myosin-ATP)

16. ACTIN FILAMENT

17. Summary of Muscle Contraction & Relaxation • Muscle Contraction • Nerve impulse arrives at Granules of Kuhn (synaptic vesicles) causing release of acetylcholine @ neuromuscular junction (myoneural junction or motor end plate) • Acetylcholine crosses gap & depolarization spreads across sarcolemma & down transverse tubules • Depolarization of transverse tubules triggers release of calcium from terminal cisternae. • Calcium spreads to myofibrils, especially at A bands

18. EVENTS LEADING TO MUSCLE ACTION

19. Summary of Muscle Contraction & Relaxation • Muscle Contraction • Calcium is bound by troponin. Troponin changes shape & pulls tropomyosin away from attachment sites on actin. • Myosin-ATP attaches to actin at unblocked sites & at various angles to form actomyosin-ATP (cross bridge) • Actomyosin-ATP is hydrolyzed via ATPase to actomyosin plus ADP plus P plus energy resulting in myosin heads pulling actin filaments toward center of sarcomere.

20. Summary of Muscle Contraction & Relaxation • Continuation of Muscle Contraction • New ATP molecules bind to myosin heads causing release of myosin heads from attachments to actin • Concurrently, other myosin heads have been bound to other attachment sites to maintain contraction • If other nearby attachment sites on actin are unblocked, the previously-released myosin heads may be reattached • Cross bridge cycling continues as long as nerve impulses continue and ATP is available

21. Summary of Muscle Contraction & Relaxation • Return to Relaxed Muscle State • Nerve impulse stops and acetylcholinesterase is released into neuromuscular junction causing acetylcholine to decompose • This eliminates depolarization stimulus across sarcolemma & down transverse tubules & stops release of calcium from terminal cisternae • In absence of nerve stimulus, terminal cisternae withdraw calcium via active transport from troponin & stores it for future contractile activity (recycling) • In absence of calcium, troponin changes shape & allows tropomyosin to block binding sites for myosin

22. Key Points Myofibrils w Myofibrils are the contractile elements of skeletal muscle, with several hundred to several thousand composing a single muscle fiber. w Myofibrils are made up of sarcomeres. w A sarcomere is composed of filaments of two proteins, myosin and actin, which are responsible for muscle contraction. w Myosin is a thick filament with a globular head at one end. w An actin filament—composed of actin, tropomyosin, & troponin—is attached to a Z disk when the myofibril is in a relaxed state.

23. 5. The Ca2+ binds to troponin on the actin filament, and the troponin pulls tropomyosin off the active sites, allowing myosin heads to attach to the actin filament. (continued) Excitation/Contraction Coupling 1. A motor neuron, with signals from the brain or spinal cord, releases the neurotransmitter acetylcholine (Ach) at the neuromuscular junction. 2. ACh crosses the junction and binds to receptors on the sarcolemma. 3. This initiates an action potential if there is sufficient ACh. 4. The action potential (MAP) travels along the sarcolemma and through the T tubules to the SR releasing Ca2+.

24. Excitation/Contraction Coupling 6. Once a strong binding state is established with actin, the myosin head tilts, pulling the actin filament (power stroke). 7. The myosin head binds to ATP, and ATPase found on the head splits ATP into ADP and Pi, releasing energy. 8. Muscle action ends when calcium is actively pumped out of the sarcoplasm back into the sarcoplasmic reticulum for storage.

25. Sliding Filament Theory w When myosin cross-bridges are activated, they bind strongly with actin, resulting in a change in the cross-bridge. w The change in the cross-bridge causes the myosin head to tilt toward the arm of the cross-bridge and drag the actin and myosin filaments in opposite directions. w The tilt of the myosin head is known as a power stroke. w The pulling of the actin filament past the myosin results in muscle shortening and generation of muscle force.

26. CONTRACTING MUSCLE FIBER

27. w Ca2+ ions bind with troponin, which lifts the tropomyosin molecules off the active sites on the actin filament. These open sites allow the myosin heads to bind to them. (continued) Key Points Muscle Fiber Action w Muscle action is initiated by a nerve impulse. w The nerve releases ACh, which allows sodium to enter and depolarize the cell. If the cell is sufficiently depolarized, an action potential occurs which releases stored Ca2+ ions.

28. Key Points Muscle Fiber Action w Once myosin binds with actin, the myosin head tilts and pulls the actin filament so they slide across each other. w Muscle action ends when calcium is pumped out of the sarcoplasm to the sarcoplasmic reticulum for storage. w Energy for muscle action is provided when the myosin head binds to ATP. ATPase on the myosin head splits the ATP into a usable energy source.

29. Muscle Biopsy w Hollow needle is inserted into muscle to take a sample. w Sample is mounted, frozen, thinly sliced, and examined under a microscope. w Allows study of muscle fibers and the effects of acute exercise and exercise training on fiber composition.

30. SLOW- AND FAST-TWITCH FIBERS

31. Skeletal Muscle Fiber Types w Different classification schemes exist for muscle fibers. • Functional Characteristics: • Slow-twitch oxidative • Fast-twitch oxidative-glycolytic • Fast-twitch glycolytic • Fast-twitch unclassified (primarily in fetuses) • Others exist on a continuum sharing characteristics wMyofibrillar adenosine triphospatase (mATPase) • Type I • Type IIa • Type IIb (IIx) • Type II ab • Others exist on a continuum sharing characteristics

32. Slow-Twitch Oxidative (ST) Muscle Fibers w High aerobic (oxidative) capacity and fatigue resistance w Low anaerobic (glycolytic) capacity and motor unit strength w Slow contractile speed (110 ms to reach peak tension) and myosin ATPase w10–180 fibers per motor neuron w Low sarcoplasmic reticulum development

33. Skeletal Muscle Fiber Types wSlow-twitch oxidative • Small diameter and cross-sectional area • Low threshold for activation (easily recruited) • Non-extensive sarcoplasmic reticulum • Relatively long sarcomeres & fewer myosin heads • Relatively low force-generating capacity • Relatively large & numerous mitochondria & associated enzymes • Great capillary density • Substantial intramuscular fat storage • Great endurance capacity

34. Skeletal Muscle Fiber Types wSlow-twitch oxidative • Low glycogen & PCr storage • Glycolytic enzyme presence favors pyruvic acid production • Low level of muscle-specific lactate dehydrogenase (LDHms) • High level of heart-specific lactate dehydrogenase (LDHhs) • Contain relatively high level of collagen and less elastin so fibers tend to be stiffer or less elastic

35. Fast-Twitch Glycolytic (FTb) Muscle Fibers w Low aerobic (oxidative) capacity and fatigue resistance w High anaerobic (glycolytic) capacity and motor unit strength w Fast contractile speed (50 ms to reach peak tension) and myosin ATPase w300–800 fibers per motor neuron w High sarcoplasmic reticulum development

36. Skeletal Muscle Fiber Types wFast-twitch glycolytic • Large diameter and cross-sectional area • High threshold for activation (more difficult to recruit) • Extensive sarcoplasmic reticulum • Relatively short sarcomeres & many myosin heads • Relatively high force-generating capacity • Relatively small & few mitochondria & associated enzymes • Low capillary density • Proportionally little intramuscular fat storage • Low endurance capacity

37. Skeletal Muscle Fiber Types wFast-twitch glycolytic • High glycogen & PCr storage • Glycolytic enzyme presence favors lactic acid production • High level of muscle-specific lactate dehydrogenase (LDHms) • Low level of heart-specific lactate dehydrogenase (LDHhs) • Contain relatively low level of collagen and more elastin so fibers tend to be less stiff or more elastic

38. Fast-Twitch (FTa) Oxidative-glycolytic Muscle Fibers w Moderate aerobic (oxidative) capacity and fatigue resistance w High anaerobic (glycolytic) capacity and motor unit strength w Fast contractile speed (50 ms to reach peak tension) and myosin ATPase w300–800 fibers per motor neuron w High sarcoplasmic reticulum development

39. Skeletal Muscle Fiber Types wFast-twitch oxidative-glycolytic • Large diameter and cross-sectional area • High threshold for activation (more difficult to recruit) • Extensive sarcoplasmic reticulum • Relatively short sarcomeres & many myosin heads • Relatively high force-generating capacity • Moderate size & amount of mitochondria & associated enzymes • Moderate capillary density • Proportionally moderate intramuscular fat storage • Moderate endurance capacity

40. Skeletal Muscle Fiber Types wFast-twitch oxidative-glycolytic • High glycogen & PCr storage • Glycolytic enzyme presence favors lactic acid production • High level of muscle-specific lactate dehydrogenase (LDHms) • Low level of heart-specific lactate dehydrogenase (LDHhs) • Contain relatively low level of collagen and more elastin so fibers tend to be less stiff or more elastic

41. Skeletal Muscle Fiber Types wMotor Units • Homogeneous for fiber type wWhole muscles are variously heterogeneous wHomogeneous whole muscles are likely to be postural muscles, and if so, likely to be STO

43. The difference in force development between FT and ST motor units is due to the number of muscle fibers per motor unit and the larger diameter of the FT fibers. Fiber composition varies w/in different areas of the same muscle, between different muscles w/in the same person, & among identical muscles in different people.

44. What Determines Fiber Type? w Genetics determine which type of motor neurons innervate our individual muscle fibers. w Muscle fibers become specialized according to the type of neuron that stimulates them. w Endurance training, strength training, and muscular inactivity may result in small changes (less than 10%) in the percentage of FT and ST fibers. w Endurance training has been shown to reduce the percentage of FTb fibers, while increasing the fraction of FTa fibers. • Aging may result in changes in the percentage of FT and ST fibers.

45. w FT fibers have a more highly developed sarcoplasmic reticulum enhancing calcium delivery. (continued) Key Points Slow- and Fast-Twitch Muscle Fibers w Skeletal muscles contain both ST and FT fibers. w ATPase in FT fibers acts faster providing energy for muscle action more quickly than ATPase in ST fibers.

46. Key Points Slow- and Fast-Twitch Muscle Fibers w Motor units comprised of FT fibers are larger (e.g., more fibers per motor neuron) than those comprised of ST fibers; thus, FT motor units can recruit more fibers. w ST fibers have high aerobic endurance and are suited to low-intensity endurance activities. w FT fibers are better for anaerobic or explosive activities.

47. All-Or-None-Response w For a motor unit to be recruited into activity the efferent nerve impulse must meet or exceed the threshold. w When this occurs, all muscle fibers in the motor unit act maximally. w If the threshold is not met no fibers in that unit act. w More force is produced by activating more motor units.

48. Orderly Recruitment of Muscle Fibers • Principle of orderly recruitment states that motor units are activated in a fixed order, based on their ranking in the muscle. • Size principle states that the order of recruitment is directly related to their motor neuron size. • Slow-twitch fibers, which have smaller motor neurons, are recruited before fast-twitch fibers.

49. Size Principle of Motor Unit Recruitment • Henneman et al, 1965 • Low threshold, small, low force-producing motor units recruited 1st in along a continuum • Higher threshold, larger, higher force-producing motor units recruited progressively later • For low-intensity or low force-output training activities, selective hypertrophy may occur • For slow-moving activities requiring great force output, it appears MOST motor units are recruited • For high force-output, ballistic activities, the FT motor units may be selectively recruited (cat’s paw in water) with STO motor units inhibited • Potential implications for humans

50. RAMPLIKE RECRUITMENT OF FIBERS