Chapter 12

1. Chapter 12 Muscle Mechanisms of Contraction and Neural Control 12-1

2. Voluntary, Striated Skeletal Muscle

3. Skeletal Muscle Structure • Fibrous connective tissue forms sheaths (epimysium) that extend around and into skeletal muscle • Inside muscle this connective tissue divides it into sections termed fascicles • Connective tissue around fascicles is called perimysium 12-5

4. Skeletal Muscle Structure • Muscle fibers are muscle cells • Each ensheathed by thin connective tissue layer called endomysium • Plasma cell membrane is called sarcolemma 12-6

5. Structure of Skeletal Muscle Gross Anatomy

6. Structure of Muscle Fiber • Each muscle fiber is packed with myofibrils 12-16

7. Structure of a Muscle Fiber Sarcoplasmic reticulum surrounds each myofibril. Transverse tubules extend from sarcolemma down into muscle fiber. Terminal cisternae run along-side and parallel to transverse tubules. 12-30

8. Ca++ Control of Contraction Most Ca++ stored in sarcoplasmic reticulum is actually in the terminal cisternae 12-30

9. Structure of Muscle Fiber Myofibrils are composed of myofilaments Myofilaments are thick and thinand give rise to bands which show as striations 12-16

10. Sarcomeres • Myofilaments are arranged in sarcomeres • Sarcomeres are contractile units of skeletal muscle consisting of the components between 2 Z discs • Myofibrils are a single file line of repeating sarcomeres 12-19

11. Thick and Thin Myofilaments -Thin myofilaments composed of the proteins actin, troponin, and tropomyosin -Thick myofilaments composed of the protein myosin

12. Sarcomeres • A band is dark area, contains thick myofilaments • Light area at center of A band is H zone • = area where there are no thin myofilaments • I band is light area, only contains thin myofilaments • At center of I band is Z line/discwhere thin myofilaments attach 12-17

13. 3-D Structure of Sarcomeres 12-18

14. The Motor Unit • Each motor neuron branches to innervate a variable # of muscle fibers • A motor unit includes each motor neuron and all the fibers it innervates 12-12

15. The Motor Unit • When a motor neuron is activated, all muscle fibers in its motor unit contract together • Number of muscle fibers in motor unit varies • Innervation ratio is # muscle fibers per motor neuron • Varies from 1:100 to 1:2000 muscle fibers

16. The Motor Unit • Fine (precise) control occurs when motor units are small (i.e. 1 motor neuron per a few fibers-like the eye muscles) • Larger motor units require stronger stimuli (more APs) and produce more force

17. Motor Unit continued • Recruitment: • Brain estimates number of motor units required to generate force needed, and stimulates them to contract simultaneously • It keeps recruiting more motor units until desired movement is accomplished in smooth fashion • More and larger motor units are progressively activated to produce greater strength 12-14

18. The Neuromuscular Junction =Junction between axon terminal and muscle fiber

19. Neuromuscular Junction (NMJ) continued • Place on sarcolemma where NMJ occurs is the motor end plate • Neurotransmitter is stored in synaptic vesicles of synaptic end bulb 12-10

20. Neuromuscular Junction (NMJ) continued • Gap between synaptic end bulb and motor end plate is synaptic cleft • Motor end plate contains receptors for acetylcholine that are chemically regulated channels for Na+ 12-10

21. Sliding Filament Mechanism of Skeletal Muscle Contraction • Muscle contracts because myofibrils get shorter • Occurs because thin myofilaments slide along and between thick myofilaments toward center of sarcomere • Shortens distance between adjacent Z discs thus sarcomeres get shorter 12-21

22. Cross Bridges • Are formed by heads of myosin molecules that extend toward and interact with actin • Sliding of myofilaments is produced by actions of cross bridges • Each myosin head contains an ATP-binding site which functions as an ATPase 12-24

23. Cross Bridges continued • Myosin can’t bind to actin unless it is “loaded” by ATP • After binding, myosin undergoes conformational change (power stroke/bends) which exerts force on actin • After power stroke myosin detaches 12-25

24. 12-26

25. Regulation of Contraction • Control of cross bridge attachment to actin is via troponin-tropomyosin system • The filament tropomyosin lies in groove between double row of G-actins of thin myofilament • Troponin complex is attached to tropomyosin at intervals 12-27

26. Regulation of Contraction continued • In relaxed muscle, tropomyosin blocks binding sites on actin so crossbridges can’t connect • This occurs when sarcoplasmic Ca++ levels are low • Binding, thus contraction, can only occur when binding sites are exposed 12-28

27. Ca++ Control of Contraction • When cytoplasmic Ca++ levels rise, Ca++ binds to troponin causing conformational change which moves tropomyosin and exposes binding sites • Allows crossbridges to connect and contraction to occur • Crossbridge cycles stop when Ca++ levels decrease 12-29

28. Excitation-Contraction Coupling • Skeletal muscle sarcolemma is excitable • Conducts APs just like axons • Release of ACh at NMJ causes large depolarizing motor end-plate potentials and APs by opening sodium channels • APs spread over sarcolemma and down into muscle via T tubules 12-31

29. Excitation-Contraction Coupling continued • T tubules are extensions of sarcolemma • APs in T tubules cause release of Ca++ from terminal cisternae 12-32

30. Excitation-Contraction Coupling continued 12-33

31. Muscle Relaxation • When APs cease because thought to move stops, muscle relaxes • Because Ca++ channels close and Ca++ is pumped back into SR • Cramps due to lack of sufficient ATP for relaxation (ie: release of cross bridges from troponin) 12-34

32. Summary of Skeletal Muscle Contraction 1. Action potential (AP) travels down axon to neuromuscular junction. 2. AP causes calcium channels to open at synaptic end bulb which, in turn, causes synaptic vesicles to undergo exocytosis thus releasing their neurotransmitter (Ach). 3. ACh diffuses across synaptic cleft and binds to membrane receptors on motor end (muscle cell). 4. Binding of ACh causes sodium channels to open and thus a depolarization of motor end plate. 5. Acetylcholinesterase breaks down the ACh. 6. At threshold an action potential ensues and spreads from motor end plate outward along sarcolemma.

33. Summary of Skeletal Muscle Contraction 7. Action potential travels down into muscle cell via transverse tubules. 8. AP causes calcium channels to open on terminal cisternae and calcium diffuses out throughout sarcoplasm. 9. Calcium binds to troponin causing the troponin/tropomyosin complex to move such that binding sites on actin for cross bridges are exposed. 10. Cross bridges connect to actin. 11. ATP is then released and the crossbridge bends pulling the thin myofilaments inward toward the center of sarcomere. 12. Z lines are pulled toward one another and the sarcomere shortens. 13. Another ATP binds to the crossbridge; crossbridge releases from actin; ATP is then split and the energy causes crossbridge to straighten again (energized state). 14. Entire process is repeated so long as there are stimulatory action potentials.

34. Twitch, Summation, and Tetanus • A single rapid contraction and relaxation of a muscle fiber is a twitch • If 2nd stimulus occurs before muscle relaxes from 1st, the 2nd twitch will be greater (summation) • Contractions of varying strength (graded contractions) are obtained by stimulation of varying numbers of fibers 12-36

35. Muscle Fatigue • Is exercise-induced reduction in ability of muscle to generate force: • Neural fatigue: due to accumulation of extracellular K+ • From K+ efflux during action potentials (repolarization) • Muscular fatigue: • occurs as slow-twitch fibers deplete glycogen stores • and as fast twitch fibers are increasingly recruited, converting glucose to lactic acid (during anaerobic respiration) which interferes with Ca2+ transport (pH change) out of sarcoplasmic reticulum 12-57