1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 12 Muscle Mechanisms of Contraction and Neural Control 12-1

2. Skeletal Muscle Structure 12-3

3. Neuromuscular Junction 12-8

4. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Neuromuscular Junction (NMJ)  Includes the single synaptic ending of the motor neuron innervating each muscle fiber and underlying specializations of sarcolemma 12-9

5. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Place on sarcolemma where NMJ occurs is the motor end plate Neuromuscular Junction (NMJ) continued 12-10

6. Motor Unit 12-11

7. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Each motor neuron branches to innervate a variable # of muscle fibers  A motor unit includes each motor neuron and all fibers it innervates Motor Unit 12-12

8. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  When a motor neuron is activated, all muscle fibers in its motor unit contract  Number of muscle fibers in motor unit varies according to degree of fine control capability of the muscle  Innervation ratio is # motor neurons : : muscle fibers  Vary from 1:100 to 1:2000  Fine control occurs when motor units are small, i.e. 1 motor neuron innervates small # of fibers Motor Unit continued 12-13

9. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Since individual motor units fire "all-or-none," how do skeletal muscles perform smooth movements?  Recruitment is used:  Brain estimates number of motor units required and stimulates them to contract  It keeps recruiting more units until desired movement is accomplished in smooth fashion  More and larger motor units are activated to produce greater strength Motor Unit continued 12-14

10. Structure of Muscle Fiber 12-15

11. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Structure of Muscle Fiber  Each fiber is packed with myofibrils  Myofibrils are 1  in diameter and extend length of fiber  Packed with myofilaments  Myofilaments are composed of thick and thin filaments that give rise to bands which underlie striations 12-16

12. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  A band is dark, contains thick filaments (mostly myosin)  Light area at center of A band is H band  = area where actin and myosin don’t overlap  I band is light, contains thin filaments (mostly actin)  At center of I band is Z line/disc where actins attach Structure of Myofibril 12-17

13. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Are contractile units of skeletal muscle consisting of components between 2 Z discs  M lines are structural proteins that anchor myosin during contraction  Titin is elastic protein attaching myosin to Z disc that contributes to elastic recoil of muscle Sarcomeres 12-19

14. How Fiber Contracts 12-20

15. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sliding Filament Theory of Contraction  Muscle contracts because myofibrils get shorter  Occurs because thin filaments slide over and between thick filaments towards center  Shortening distance from Z disc to Z disc 12-21

16. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Sliding Filament Theory of Contraction continued  During contraction:  A bands (containing actin) move closer together, do not shorten  I bands shorten because they define distance between A bands of successive sarcomeres  H bands (containing myosin) shorten 12-22

17. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-23

18. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cross Bridges  Are formed by heads of myosin molecules that extend toward and interact with actin  Sliding of filaments is produced by actions of cross bridges  Each myosin head contains an ATP-binding site which functions as an ATPase 12-24

19. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cross Bridges continued  Myosin can’t bind to actin unless it is “cocked” by ATP  After binding, myosin undergoes conformational change (power stroke) which exerts force on actin  After power stroke myosin detaches 12-25

20. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-26

21. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Excitation-Contraction Coupling  Skeletal muscle sarcolemma is excitable  Conducts APs just like axons  Release of ACh at NMJ causes large depolarizing end-plate potentials and APs in muscle  APs race over sarcolemma and down into muscle via T tubules 12-31

22. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Excitation-Contraction Coupling continued  T tubules are extensions of sarcolemma  Ca ++ channels in SR are mechanically linked to channels in T tubules  APs in T tubules cause release of Ca ++ from cisternae via V-gated and Ca ++ release channels  Called electromechanical release  channels are 10X larger than V-gated channels 12-32

23. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Excitation-Contraction Coupling continued 12-33

24. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Muscle Relaxation  Ca ++ from SR diffuses to troponin to initiate crossbridge cycling and contraction  When APs cease, muscle relaxes  Because Ca ++ channels close and Ca ++ is pumped back into SR 12-34

25. Characteristics of Contractions 12-35

26. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Twitch, Summation, and Tetanus  A single rapid contraction and relaxation of muscle fibers 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

27. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Twitch, Summation, and Tetanus continued  If muscle is stimulated by an increasing frequency of electrical shocks, its tension will increase to a maximum (incomplete tetanus)  If frequency is so fast that no relaxation occurs, a smooth sustained contraction results called complete tetanus or tetany 12-37

28. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Twitch, Summation, and Tetanus continued  If muscle is repeatedly stimulated with maximum voltage to produce individual twitches, successive twitches get larger  This is Treppe or staircase effect  Caused by accumulation of intracellular Ca ++ 12-38

29. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Velocity of Contraction  For muscle to shorten it must generate force greater than the load  The lighter the load the faster the contraction and vice versa 12-39

30. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Isotonic, Isometric, and Eccentric Contractions  During isotonic contraction, force remains constant throughout shortening process  During isometric contraction, exerted force does not cause load to move and length of fibers remains constant  During eccentric contraction, load is greater than exerted force and fibers lengthen 12-40

31. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Series-Elastic Component  Tendons and connective tissue are elastic and absorb tension as muscle contracts  They recoil as muscle relaxes and spring back to resting length 12-41

32. Metabolism of Skeletal Muscle 12-44

33. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Metabolism of Skeletal Muscles  Skeletal muscles respire anaerobically 1st 45-90 sec of moderate-to-heavy exercise  Cardiopulmonary system requires this time to increase O 2 supply to exercising muscles  If exercise is moderate, aerobic respiration contributes majority of muscle requirements after 1st 2 min 12-45

34. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Maximum Oxygen Uptake  Maximum oxygen uptake (aerobic capacity) is maximum rate of oxygen consumption (V 02 max)  Determined by age, gender, and size  Lactate (anaerobic) threshold is % of max O 2 uptake at which there is significant rise in blood lactate levels  In healthy individuals this is at 50–70% V 02 max 12-46

35. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  During light exercise, most energy is derived from aerobic respiration of fatty acids  During moderate exercise, energy derived equally from fatty acids and glucose  During heavy exercise, glucose supplies 2/3 of energy  Liver increases glycogenolysis  GLUT-4 carrier is moved to muscle cell’s plasma membrane Metabolism of Skeletal Muscles 12-47

36. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-48

37. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Oxygen Debt  When exercise stops, rate of oxygen uptake does not immediately return to pre-exercise levels  Because of oxygen debt accumulated during exercise  When oxygen is withdrawn from hemoglobin and myoglobin  And because of O 2 needed for metabolism of lactic acid produced by anaerobic respiration 12-49

38. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Phosphocreatinine  During exercise ATP can be used faster than can be generated by respiration  Phosphocreatine (creatine phosphate) is source of high energy Ps to regenerate ATP from ADP  Phosphocreatine levels are 3X ATP 12-50

39. Types of Skeletal Muscle 12-51

40. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Slow- and Fast-Twitch Fibers  Skeletal muscle fibers can be divided on basis of contraction speed and resistance to fatigue:  Slow-twitch, slow fatigue (Type I fibers)  Fast-twitch, fast fatigue (Type IIA and IIX fibers)  Twitch speed due to different myosin ATPases that are slow or fast 12-52

41. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Also called red slow oxidative  Adapted to contract slowly without fatiguing  Uses mostly aerobic respiration  Has rich capillary supply, many mitochondria, and aerobic enzymes  Has lots of myoglobin (O 2 storage molecule)  Gives fibers red color  Have small motor neurons with small motor units Type I Fibers 12-53

42. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Type IIX fibers also called white fast glycolytic  Adapted to contract fast using anaerobic metabolism  Has large stores of glycogen, few capillaries and mitochondria, little myoglobin  Type II A fibers also called white fast oxidative  Adapted to contract fast using aerobic metabolism  Intermediate to Type I and Type IIX  Have large motor neurons with large motor units Type II Fibers 12-54

43. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-55

44. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  People vary genetically in proportion of fast- and slow- twitch fibers 12-56

45. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Muscle Fatigue  Is exercise-induced reduction in ability of muscle to generate force  Sustained muscle contraction fatigue is due to accumulation of extracellular K +  From K + efflux during AP  Occurs in moderate exercise as slow-twitch fibers deplete glycogen stores  Fast twitch fibers are then recruited, converting glucose to lactic acid which interferes with Ca 2+ transport  Central fatigue occurs as brain is less able to activate muscles even when muscle is not fatigued 12-57

46. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Adaptations of Muscles to Exercise Training  Aerobic training improves aerobic capacity (by 20%) and lactate threshold (by 30%)  Weight training increases muscle size by increasing # of myofibrils/fiber  And in some cases fibers can split 12-58

47. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12-59

48. Neural Control 12-60

49. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Upper Motor Neuron Control of Skeletal Muscles  Influence lower motor neurons  Axons of neurons in precentral gyrus form pyramidal tracts  Extrapyramidal tracts arise from neurons in other areas of brain 12-74

50. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.  Cerebellum receives sensory input from spindles, Golgi tendon organs, and areas of cortex devoted to vision, hearing, and equilibrium  No descending tracts arise from cerebellum  Influences motor activity indirectly  All output from cerebellum is inhibitory  Aids motor coordination  Basal ganglia exert inhibitory effects on activity of lower motor neurons Upper Motor Neuron Control of Skeletal Muscles continued 12-75