1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 12 Lecture Outline

2. Chapter 12 Outline  Skeletal Muscles  Mechanisms of Contraction  Contractions of Skeletal Muscle  Energy Requirements of Skeletal Muscle  Neural Control of Skeletal Muscles  Cardiac and Smooth Muscle 12-2

3. Skeletal Muscles 12-3

4. Skeletal Muscles  Are attached to bone on each end by tendons  Insertion is the more movable attachment; is pulled toward origin-the less moveable attachment  Contracting muscles cause tension on tendons which move bones at a joint  Flexors decrease angle of joint  Extensors increase angle of joint  Prime mover of any skeletal movement is agonist muscle  Antagonistic muscles are muscles (flexors and extensors) that act on the same joint to produce opposite actions 12-4

5. Skeletal Muscle Structure  Fibrous connective tissue from tendons forms sheaths (epimysium) that extend around and into skeletal muscle  Inside the muscle this connective tissue divides muscle into columns called fascicles  Connective tissue around fascicles is called perimysium 12-5

6.  Muscle fibers are muscle cells  Ensheathed by thin connective tissue layer called endomysium  Plasma membrane is called sarcolemma  Muscle fibers are similar to other cells except are multinucleate and striated Skeletal Muscle Structure continued 12-6

7.  Most distinctive feature of skeletal muscle is its striations Skeletal Muscle Structure continued 12-7

8. Neuromuscular Junction 12-8

9. Neuromuscular Junction (NMJ)  Includes the single synaptic ending of the motor neuron innervating each muscle fiber and underlying specializations of sarcolemma 12-9

10.  Place on sarcolemma where NMJ occurs is the motor end plate Neuromuscular Junction (NMJ) continued 12-10

11. Motor Unit 12-11

12.  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

13.  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

14.  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

15. Mechanisms of Contraction 12-15

16. 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

17.  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

18. 12-18

19.  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

20. How Fiber Contracts 12-20

21. 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

22. 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

23. 12-23

24. 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

25. 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

26. 12-26

27. Control of Contraction  Control of cross bridge attachment to actin is via troponin- tropomyosin system  Serves as a switch for muscle contraction and relaxation  The filament tropomyosin lies in grove between double row of G-actins (that make up actin thin filament)  Troponin complex is attached to tropomyosin at intervals of every 7 actins 12-27

28. Control of Contraction continued  In relaxed muscle, tropomyosin blocks binding sites on actin so crossbridges can’t occur  This occurs when Ca ++ levels are low  Contraction can occur only when binding sites are exposed 12-28

29. Role of Ca ++ in Muscle Contraction  When Ca ++ levels rise, Ca ++ binds to troponin causing conformational change which moves tropomyosin and exposes binding sites  Allowing crossbridges and contraction to occur  Crossbridge cycles stop when Ca ++ levels decrease 12-29

30. Role of Ca ++ in Muscle Contraction  Ca ++ levels decrease because it is continually pumped back into the sarcoplasmic reticulum (SR - a calcium reservoir in muscle)  Most Ca ++ in SR is in terminal cisternae  Running along terminal cisternae are T tubules 12-30

31. Excitation-Contraction Coupling  Skeletal muscle sarcolemma is excitable  Conducts Action Potentials  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

32. 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

33. Excitation-Contraction Coupling continued 12-33

34. 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 Sarcolplamic Reticulum by Ca++-ATPase pumps.  Therefore, ATP is needed for relaxation as well as contraction. 12-34

35. Contractions of Skeletal Muscles 12-35

36. 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

37. 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

38. 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

39. 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

40. Isotonic, Isometric, Eccentric, and Concentric Contractions  During isotonic contraction, force remains constant throughout shortening process, length changes  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 despite its contraction  During concentric contraction, muscle tension is greater than the load and muscle shortens 12-40

41. 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

42. Length-Tension Relationship  Strength of muscle contraction influenced by:  Frequency of stimulation  Thickness of each muscle fiber  Initial length of muscle fiber  Ideal resting length is that which can generate maximum force 12-42

43. Length-Tension Relationship  Too little overlap yields less tension because fewer cross bridges can form  With no overlap force cannot be generated because cross bridges cannot form 12-43

44. Energy Requirements of Skeletal Muscles 12-44

45. 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

46. 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

47.  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

48. 12-48

49. 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

50. Phosphocreatinine  During exercise ATP can be used faster than can be generated by respiration  Phosphocreatine (creatine phosphate) is source of high energy phosphate to regenerate ATP from ADP  So efficient that muscle ATP concent. dec. only slightly from rest to heavy exercise 12-50

51. Types of Skeletal Muscle 12-51

52. 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)  a=fast twitch extraocular, b=gastrocnemius muscle, and c=slow-twitch soleus 12-52

53.  Also called red slow oxidative  Adapted to contract slowly without fatigue  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

54.  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

55. 12-55

56. 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 caused by changes in CNS rather than by fatigue in muscles themselves 12-57

57. Adaptations of Muscles to Exercise Training  Endurance training improves aerobic capacity (by 20%) and lactate threshold (by 30%)  Resistance training increases muscle size by increasing # of myofibrils/fiber (hypertrophy)  Once a myofibril has attained a certain thickness, it may split into two myofibrils. 12-58

58. 12-59

59. Neural Control of Skeletal Muscles 12-60

60. Neural Control of Skeletal Muscles  Motor neuron cell bodies are in ventral horn of spinal cord; axons leave in ventral root  Called lower motor neurons and final common pathway  Activity influenced by sensory feedback from muscles and tendons  And facilitory and inhibitory activity from upper motor neurons 12-61

61. Sensory Feedback  To control skeletal muscle movements, NS must receive continuous sensory feedback  Including information on tension from Golgi tendon organs  And on length of muscle from muscle spindle apparatus 12-62

62. Muscle Spindle Apparatus  Consists of modified thin muscle cells called intrafusal fibers  Regular muscle fibers are extrafusal fibers  Spindles are arranged in parallel with extrafusal fibers  Insert into tendons at each end of muscle 12-63

63. Muscle Spindle Apparatus  Intrafusal fibers have nuclei in central region instead of contractile filaments  Nuclear bag fibers have nuclei arranged in loose aggregate  Nuclear chain fibers have nuclei arranged in rows 12-64

64.  Nuclear bag and chain fibers are innervated by primary, annulospiral sensory endings  Which wrap around central regions  Respond most at onset of stretch Muscle Spindle Apparatus continued 12-65

65.  Nuclear chain fibers additionally have secondary, flower-spray endings located at ends  Respond to sustained stretch Muscle Spindle Apparatus continued 12-66

66.  Both nuclear bag and chain fibers respond strongly to sudden, rapid stretching  Their activation causes a reflex contraction of muscle Muscle Spindle Apparatus continued 12-67

67. Alpha and Gamma Motor Neurons  Fast conducting alpha motor neurons innervate extrafusal fibers and cause muscle contraction  Slower conducting gamma motor neurons innervate and induce tension in intrafusal fibers (=active stretch)  Increases sensitivity of muscle to passive stretch 12-68

68. Coactivation of Alpha and Gamma Motor Neurons  Upper motor neurons usually stimulate alpha and gamma motor neurons simultaneously (coactivation)  Stimulation of alpha motor neurons results in muscle contraction and shortening  Stimulation of gamma motor neurons causes intrafusal fibers to take up slack  Activity of gamma motor neurons maintains normal muscle tone 12-69

69. Monosynaptic-Stretch Reflex  Consists of only 1 synapse within CNS  Striking patellar ligament passively stretches spindles activating annulospiral sensory neurons  Which synapse on alphas causing them to stimulate extrafusals  Produces knee-jerk reflex 12-70

70. Golgi Tendon Organ Reflex  Involves 2 synapses in the CNS (=disynaptic reflex)  Sensory axons from Golgi tendon organ synapse on interneurons  Which make inhibitory synapses on motor neurons  Prevents excessive muscle contraction or passive muscle stretching 12-71

71. Reciprocal Innervation  Occurs in stretch reflexes because sensory neurons stimulate motor neuron and interneuron  Interneuron inhibits motor neurons of antagonistic muscles  When limb is flexed, antagonistic extensors are inhibited from doing stretch reflex 12-72

72. Crossed-Extensor Reflex  Involves double reciprocal innervation  Affecting muscles on contralateral side of cord  e.g. if step on tack, foot is withdrawn by contraction of flexors and relaxation of extensors  And contralateral leg extends to support body (crossed extensor reflex) 12-73

73. 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

74.  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  Cerebral ganglia exert inhibitory effects on activity of lower motor neurons Upper Motor Neuron Control of Skeletal Muscles continued 12-75

75. Cardiac and Smooth Muscles 12-76

76. Cardiac Muscle (Myocardium)  Contractile apparatus similar to skeletal  Striated like skeletal but involuntary like smooth  Branched; adjacent myocardial cells joined by intercalated disks (gap junctions)  Allow Action Potentials to spread throughout cardiac muscle 12-77

77. Smooth Muscle  Has no sarcomeres  Has gap junctions  Contains 16X more actin than myosin  Allows greater stretching and contracting  Actin filaments are anchored to dense bodies 12-78

78. Smooth Muscle Contraction  Controlled by Ca ++ but different from striated  Has little SR and no troponin/tropomyosin  Ca ++ enters thru voltage gated channels in plasma membrane  Binds with calmodulin  Ca ++ -calmodulin complex activates myosin light chain kinase (MLCK)  Which phosphorylates and activates myosin  Myosin forms crossbridges with actin 12-79

79. Smooth Muscle Contraction  Relaxation occurs when Ca ++ concentration decreases  Myosin is dephosphorylated by myosin phosphatase  Myosin can no longer form crossbridges  Smooth muscle has slower contractions than striated  Can form a state of prolonged binding of myosin to actin (latch state)  Maintains force using little energy 12-80

80. 12-81

81. Single and Multiunit Smooth Muscle  Single unit is spontaneously active (myogenic)  Some cells are pacemakers  Has gap junctions to spread electrical activity  Multiunit requires nerve stimulation by ANS  NT released along a series of synapses called varicosities  Called synapses en passant (in passing) 12-82

82. 12-83