1. Human Anatomy & Physiology Chapter 9 Muscles and Muscle Tissue

2. Muscle Overview The three types of muscle tissue are skeletal, cardiac, and smooth These types differ in structure, location, function, and means of activation 2

3. Muscle Function Movement – All body locomotion and manipulation Posture Maintenance – Function Constantly Heat Generation – A by-product of muscle metabolism and contraction 3

4. Functional Characteristics of Muscles Excitability, or Irritability – the ability to receive and respond to stimuli Contractility – the ability to shorten forcibly Extensibility – the ability to be stretched or extended Elasticity – the ability to recoil and resume the original resting length 4

5. Muscle Similarities Skeletal and smooth muscle cells are elongated and are called muscle fibers Muscle contraction depends on two kinds of myofilaments – actin and myosin Muscle terminology is similar Sarcolemma – muscle plasma membrane Sarcoplasm – cytoplasm of a muscle cell Prefixes – myo, mys, and sarco all refer to muscle 5

6. Skeletal Muscle Tissues Packaged in skeletal muscles that attach to and cover the skeleton Has obvious stripes called striations Is controlled voluntarily (i.e., by conscious control) Contracts rapidly but tires easily 6

8. Skeletal Muscle Tissues (Cont) Is responsible for overall body motility Is extremely adaptable and can exert forces over a range from a fraction of an ounce to over 70 pounds 8

9. Cardiac Muscle Tissue Occurs only in the heart Is striated like skeletal muscle but is not voluntary Contracts at a fairly steady rate set by the heart’s pacemaker Neural controls allow the heart to respond to changes in bodily needs 9

10. Smooth Muscle Tissue Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages Forces food and other substances through internal body channels It is not striated and is involuntary 10

12. Skeletal Muscle Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue The three connective tissue wrappings are: Epimysium – a covering of dense regular CT that surrounds the entire muscle Perimysium – fibrous CT that surrounds groups of muscle fibers called fascicles Endomysium – fine sheath of CT composed of reticular fibers surrounding each muscle fiber 12

13. Skeletal Muscle Figure 9.1

14. Skeletal Muscle: Nerve and Blood Supply Each muscle is served by one nerve, an artery, and one or more veins Each skeletal muscle fiber is supplied with a nerve ending that controls contraction Contracting fibers require continuous delivery of oxygen and nutrients via arteries Wastes must be removed via veins 14

15. Skeletal Muscle: Attachments Muscles span joints and are attached to bone in at least two places When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone – the muscle’s origin Muscles attach: Directly – The muscle is fused to the periosteum of a bone Indirectly (Aponeurosis) – CT wrappings extend beyond the muscle as a tendon 15

16. Microscopic Anatomy of a Skeletal Muscle Fiber Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma Fibers are 10 to 100  m in diameter, and up to hundreds of centimeters long Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules 16

17. Myofibrils Myofibrils are densely packed, rodlike contractile elements They make up most of the muscle volume The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident 17 Figure 9.2b

18. Sarcomeres-basic unit of muscle The smallest contractile unit of a muscle The region of a myofibril between two successive Z discs Composed of myofilaments made up of contractile proteins Myofilaments are in two types – thick & thin 18 Figure 9.2c

19. Myofilaments: Banding Pattern Thick filaments – extend the entire length of an A band Thin filaments – extend across the I band and partway into the A band 19

20. Ultrastructure of Myofilaments: Thick Filaments Thick filaments are composed of the protein myosin 20 Figure 9.3a, b

21. Ultrastructure of Myofilaments: Thin Filaments Thin filaments are chiefly composed of the protein actin 21 Figure 9.3c

22. Sarcoplasmic Reticulum (SR) SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril Functions in the regulation of intracellular calcium levels 22

23. Sarcoplasmic Reticulum (SR) Figure 9.4

24. Contraction of Skeletal Muscle Fibers Contraction – refers to the activation of myosin’s cross bridges (force generating sites) Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced 24

25. Sliding Filament Mechanism of Contraction Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree In the relaxed state, thin and thick filaments overlap only slightly Upon stimulation, myosin heads bind to actin and sliding begins 25

26. Sliding Filament Mechanism of Contraction Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere As this event occurs throughout the sarcomeres, the muscle shortens 26

27. Changing Ca 2+ level concentration increase or decrease the ability of a muscle to enter contraction 27 Figure 9.6b Role of Ionic Calcium (Ca 2+ ) in the Contraction Mechanism

28. Sequential Events of Contraction Cross bridge attachment – myosin cross bridge attaches to actin filament Power stroke – myosin head pivots and pulls actin filament toward M line Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches “Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high energy state 28

29. Regulation of Contraction In order to contract, a skeletal muscle must: Be stimulated by a nerve ending Propagate an electrical current, or action potential, along its sarcolemma Have a rise in intracellular Ca 2+ levels, the final trigger for contraction 29

30. Nerve Stimulus of Skeletal Muscle Skeletal muscles are stimulated by motor neurons of the nervous system Axons of these neurons travel in nerves to muscle cells Axons of motor neurons branch profusely as they enter muscles Each axonal branch forms a neuromuscular junction with a single muscle fiber 30

31. Action Potential A transient depolarization event that includes polarity reversal of a sarcolemma (or nerve cell membrane) and the propagation of an action potential along the membrane 31

32. The outside (extracellular) face is positive, while the inside face is negative This difference in charge is the resting membrane potential 32 Action Potential: Electrical Conditions of a Polarized Sarcolemma Figure 9.9a

33. Action Potential: Repolarization Repolarization occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period) The ionic concentration of the resting state is restored by the Na + -K + pump 33 Figure 9.9d

34. Contraction of Skeletal Muscle (Organ Level) Contraction of muscle fibers (cells) and muscles (organs) is similar The two types of muscle contractions are: Isometric contraction – increasing muscle tension (muscle does not shorten) Isotonic contraction – decreasing muscle length (muscle shortens during contraction) 34

35. Motor Unit: The Nerve-Muscle Functional Unit A motor unit is a motor neuron & all the muscle fibers it supplies The number of muscle fibers per motor unit can vary from four to several hundred Muscles that control fine movements (fingers, eyes) have small motor units 35 Figure 9.11a

36. Muscle Twitch A muscle twitch is the response of a muscle to a single brief threshold stimulus Muscle twitch has three phases: Latent period – first few milliseconds after stimulation when excitation-contraction coupling is taking place 36 Figure 9.12a

37. Muscle Twitch Period of contraction – cross bridges actively form and the muscle shortens Period of relaxation – Ca 2+ is reabsorbed into the SR, and muscle tension goes to zero 37 Figure 9.12a

38. Graded Muscle Response Graded muscle responses are: Variations in the degree of muscle contraction Required for proper control of skeletal movement Responses are graded by: Changing the frequency of stimulation Changing the strength of the stimulus 38

39. Muscle Response to Varying Stimuli Muscle Twitch: A single stimulus results in a single contractile response Wave Summation: Frequently delivered stimuli (muscle does not have time to completely relax) increases contractile force 39 Figure 9.13

40. Muscle Response to Varying Stimuli More rapidly delivered stimuli result in incomplete tetanus Tetanus: Sustained muscle contraction If stimuli are given quickly enough, complete tetanus results 40 Figure 9.13

41. Muscle Response: Stimulation Strength Threshold stimulus – the stimulus strength at which the first observable muscle contraction occurs Beyond threshold, muscle contracts more vigorously as stimulus strength is increased 41

42. Treppe: The Staircase Effect Staircase – increased contraction in response to same strength multiple stimuli Contractions increase because: There is increasing availability of Ca 2+ in the sarcoplasm Muscle enzyme systems become more efficient because heat is increased as muscle contracts 42 Figure 9.14

43. Muscle Metabolism: Energy for Contraction ATP is the only source used directly for contractile activity (Muscle cells contain many mitochondria) ATP production is dependent upon the type of respiration Aerobic: produces the most ATP’s Anaerobic: Less ATP’s, Less efficent and produces lactic acid as a by- product of glycolysis 43

44. Muscle Fatigue Muscle fatigue – the muscle is in a state of physiological inability to contract Muscle fatigue occurs when: ATP production is lower than use, lactic acid builds up and ionic imbalances are present. 44

45. Oxygen Debt For a muscle to return to a resting state: Oxygen reserves must be replenished Lactic acid must be converted to pyruvic acid Glycogen stores must be replaced ATP reserves must be resynthesized Oxygen debt – the extra amount of O 2 needed to restore the above processes 45

46. Heat Production During Muscle Activity Only 40% of the energy released in muscle activity is useful as work The remaining 60% is given off as heat Dangerous heat levels are prevented by radiation of heat from the skin and sweating 46

47. Force of Contraction The force of contraction is affected by: The number of muscle fibers contracting The relative size of the muscle Degree of muscle stretch 47 Figure 9.19a

48. Muscle Fiber Type: Functional Characteristics Speed of contraction – determined by speed in which ATP is used. The two types of fibers: Slow twitch (react slowly to stimulus) Fast twitch (react quickly to stimulus) 48

49. Smooth Muscle Composed of spindle- shaped fibers, shorter in length and with a single centrally located nuclei 49 Figure 9.23

50. Smooth Muscle Found in walls of hollow organs (except the heart) Have essentially the same contractile mechanisms as skeletal muscle 50 Figure 9.23

51. Peristalsis Peristalsis – alternating contractions and relaxations of smooth muscles that mix and squeeze substances through the lumen of hollow organs 51

52. Innervation of Smooth Muscle Smooth muscle lacks neuromuscular junctions Innervating nerves have bulbous swellings called varicosities Varicosities release neurotransmitters into wide synaptic clefts called diffuse junctions 52

53. Microscopic Anatomy of Smooth Muscle SR is less developed than in skeletal muscle and lacks a specific pattern T tubules are absent There are no visible striations and no sarcomeres Thick and thin filaments are present 53

54. Thick and thin filaments are arranged diagonally, causing smooth muscle to contract in a corkscrew manner 54 Figure 9.25 Proportion and Organization of Myofilaments in Smooth Muscle

55. Contraction of Smooth Muscle Whole sheets of smooth muscle exhibit slow, synchronized contraction They contract in unison, reflecting their electrical coupling with gap junctions Action potentials transmit from cell to cell Some smooth muscle cells: Act as pacemakers and set the contractile pace for whole sheets of muscle or they are self- excitatory and depolarize without external stimuli 55

56. Special Features of Smooth Muscle Contraction Unique characteristics of smooth muscle include: Smooth muscle tone Slow, prolonged contractile activity Low energy requirements Response to stretch 56

57. Response to Stretch Smooth muscles exhibit a phenomenon called stress-relaxation response : Smooth muscle responds to stretch only briefly, then adapt to its new length The new length, however, retains its ability to contract This enables organs such as the stomach and bladder to temporarily store contents 57

58. Types of Smooth Muscle: Single Unit The cells of single unit smooth muscle, commonly called visceral muscle: Contract rhythmically as a unit Are arranged in opposing sheets and exhibit stress-relaxation response 58

59. Types of Smooth Muscle: Multiunit Their characteristics include: Structurally independent muscle fibers A rich nerve supply; in which a number of muscle fibers, forms motor units Graded contractions in response to neural stimuli 59

60. Types of Smooth Muscle: Multiunit Multiunit smooth muscles are found in: Large airways to the lungs Large arteries Arrector pili muscles Attached to hair follicles Internal eye muscles 60

61. Muscular Dystrophy Muscular dystrophy – group of inherited muscle-destroying diseases where muscles enlarge due to fat and connective tissue deposits, but muscle fibers atrophy 61

62. Muscular Dystrophy Duchenne muscular dystrophy (DMD) Inherited, sex-linked disease carried by females and expressed in males (1/3500) Victims become clumsy and fall frequently as their muscles fail Diagnosed between ages 2-10, it then progresses from the extremities upward, and victims die of respiratory failure in their 20s There is no cure, but myoblast transfer therapy shows promise 62

63. Homeostatic Imbalance: Age Related With age, connective tissue increases and muscle fibers decrease Muscles become stringier and more sinewy By age 80, 50% of muscle mass is lost (This is called Sarcopenia) Regular exercise reverses sarcopenia Aging of the cardiovascular system affects every organ in the body 63

64. Developmental Aspects Muscle tissue develops from embryonic mesoderm called myoblasts Multinucleated skeletal muscles form by fusion of myoblasts As muscles are better controlled by the somatic nervous system, the numbers of fast and slow fibers are also determined Smooth muscle has good regenerative ability 64

65. Developmental Aspects: Atrophy Atrophy: Muscules loss in size & strength due to lack of use (usually due to injury) 65

66. Developmental Aspects: After Birth Muscular development reflects neuromuscular coordination Development occurs head-to-toe, and proximal-to-distal Peak natural neural control of muscles is achieved by mid-adolescence Athletics and training can improve neuromuscular control 66

67. Developmental Aspects: Male and Female There is a biological basis for greater strength in men than in women Women’s skeletal muscle makes up 36% of their body mass, 42% in men These differences are due primarily to the male sex hormone testosterone With more muscle mass, men are generally stronger than women Body strength per unit muscle mass, however, is the same 67