1. Structure and Function
Chapter 8
Skeletal Muscle
Structure and Function

2. Skeletal Muscle Human body contains over 400 skeletal muscles
40-50% of total body weight
Functions of skeletal muscle
Force production for locomotion and breathing
Force production for postural support
Heat production during cold stress

3. Connective Tissue Covering of Muscle

4. Structure of Skeletal Muscle: Microstructure
Sarcolemma
Muscle cell membrane
Myofibrils
Threadlike strands within muscle fibers
Actin (thin filament)
Troponin
Tropomyosin
Myosin (thick filament)
Myosin head
Crossbridge location

5. Microstructure of Skeletal Muscle

6. Structure of Skeletal Muscle: The Sarcomere
Functional unit
Z-line – “anchor” points for actin filaments
Sarcomere is Z-line to Z-line
Within the sarcoplasm
Transverse tubules
Communication into the cell
Sarcoplasmic reticulum
Storage sites for calcium
Terminal cisternae

7. Within the Sarcoplasm

8. The Neuromuscular Junction
Site where motor neuron meets the muscle fiber
Separated by gap called the neuromuscular cleft
Motor end plate
Pocket formed around motor neuron by sarcolemma
Acetylcholine is released from the motor neuron
Causes an end-plate potential
Depolarization of muscle fiber

9. Illustration of the Neuromuscular Junction

10. Muscular Contraction The sliding filament model
Muscle shortening is due to movement of the actin filament over the myosin filament
Reduces the distance between Z-lines

11. The Sliding Filament Model of Muscle Contraction

12. Cross-Bridge Formation in Muscle Contraction

13. Energy for Muscle Contraction
ATP is required for muscle contraction
Myosin ATPase breaks down ATP as fiber contracts

14. Excitation-Contraction Coupling
Depolarization of motor end plate (excitation) is coupled to muscular contraction
Nerve impulse travels along sarcolemma and down T-tubules to cause a release of Ca++ from SR
Ca++ binds to troponin and causes position change in tropomyosin, exposing active sites on actin
Permits strong binding state between actin and myosin and contraction occurs
ATP is hydrolyzed and energy goes to myosin head which releases from actin

15. Steps Leading to Muscular Contraction

16. CD ROM

17. Properties of Muscle Fibers
Biochemical properties
Oxidative capacity
Type of ATPase on myosin head
Contractile properties
Maximal force production
Speed of contraction / speed of stimulation
Muscle fiber efficiency

18. Individual Fiber Types
Slow fibers
Type I fibers
Slow-twitch fibers
Slow-oxidative fibers
Fastest fibers
Type IIb(x) fibers
Fast-twitch fibers
Fast-glycolytic fibers
Fast fibers
Type IIa fibers
Intermediate fibers
Fast-oxidative glycolytic fibers

19. Muscle Fiber Types
(x)

20. Comparison of Maximal Shortening Velocities Between Fiber Types

21. Histochemical Staining of Fiber Type
Type IIb (x)
Type IIa
Type I

22. Fiber Types and Performance
Successful Power athletes
Sprinters, jumpers, strikers
Possess high percentage of fast fibers
Successful Endurance athletes
Distance runners, cyclists, swimmers
Have high percentage of slow fibers
Others
Skill sport athletes and probably non-athletes
Have about 50% slow and 50% fast fibers

23. How Is Fiber Type Distributed In a Population?
50% Each
100% ST
100% FT

24. How do I get more FT muscle fibers?
Grow more?
Change my ST to FT?
Pick the right parents?

25. Alteration of Fiber Type by Training
Endurance and resistance training
Cannot make FT into ST or ST into FT
Can make a shift in IIa….
toward more oxidative properties
toward more glycolytic properties

26. So how do I gain strength?

27. Protein Arrangement and Strength
Increase sarcomeres (myofibrils) in parallel
more cross bridges to pull
requires more room in diameter
requires more fluid
hypertrophy

28. Protein Arrangement and Strength
Increase in sarcomeres in series?
Only with growth in height / limb length
Sarcomeres would crowd each other
Would result in decreased force production

29. Protein Arrangement and Strength
Hyperplasia?
more cells
stem cells – for injury repair

30. Protein Arrangement and Strength
Fiber arrangement
Fusiform
penniform

31. Protein Arrangement and Strength
More sarcomeres in parallel, more tension produced
More cross bridges generating tension

32. Age-Related Changes in Skeletal Muscle
Aging is associated with a loss of muscle mass (sarcopoenia)
independent of training status
associated with less testosterone
Regular exercise / training can improve strength and endurance
Cannot completely eliminate the age-related loss in muscle mass

33. Types of Muscle Contraction
Isometric
Muscle exerts force without changing length
Pulling against immovable object
Stabilization
Postural muscles

34. Types of Muscle Contraction
Isotonic (dynamic)
Concentric
Muscle shortens during force production
Eccentric
Muscle produces force but length increases

35. Types of Muscle Contraction
Isokinetic (dynamic)
Constant speed of contraction
Machine assisted

36. Isotonic and Isometric Contractions

37. Force and Speed of Contraction Regulation in Muscle
Types and number of motor units utilized
More motor units = greater force
“Recruitment”
More FT motor units = even greater force
“Contribution”

38. Histochemical Staining of Fiber Type
Type IIb
Type IIa
Type I

39. Force Regulation in Muscle
Initial muscle length
“Ideal” length for force generation
Maximum vs. minimum cross bridging

40. Length-Tension Relationship in Skeletal Muscle

41. Force Regulation in Muscle
Nature of the motor units’ neural stimulation
Frequency of stimulation
Simple twitch, summation, and tetanus

42. Illustration of a Simple Twitch

43. Simple Twitch, Summation, and Tetanus

44. Relationship Between Stimulus Frequency and Force Generation

45. Determinants of Contraction Velocity - Single Fiber
Nerve conduction velocity
Myofibrillar ATPase activity

46. Determinants of Contraction Velocity - Whole Muscle
Same factors as for single fiber
Nerve conduction velocity
ATPase activity
Number of FT fibers recruited

47. Force-Velocity Relationship
At any absolute force the speed of movement is greater in muscle with higher percent of fast-twitch fibers
The fastest fibers can contribute while the slow cannot
The maximum force is greatest at the lowest velocity of shortening
True for both slow and fast-twitch fibers
All can contribute at slow velocities

48. Force-Velocity Relationship
Force (Newtons) FT ST
Velocity (deg./sec)

49. Force-Power Relationship
At any given velocity of movement the power generated is greater in a muscle with a higher percent of fast-twitch fibers
The peak power increases with velocity up to movement speed of degrees•second-1
Force decreases with increasing movement speed beyond this velocity

50. Force-Power Relationship

51. Is there a safety mechanism?

52. Receptors in Muscle Muscle spindle
Detect dynamic and static changes in muscle length
Stretch reflex
Stretch on muscle causes reflex contraction

53. Muscle Spindle

54. Receptors in Muscle Golgi tendon organ (GTO)
Monitor tension developed in muscle
Prevents damage during excessive force generation
Stimulation results in reflex relaxation of muscle

55. Golgi Tendon Organ

56. Questions?

57. End

61. Force-Velocity Relationship

62. Structure of Skeletal Muscle: Connective Tissue Covering
Epimysium
Surrounds entire muscle
Perimysium
Surrounds bundles of muscle fibers
Fascicles
Endomysium
Surrounds individual muscle fibers

63. Speed of Muscle Contraction and Relaxation
Muscle twitch
Contraction as the result of a single stimulus
Latent period
Lasting only ~5 ms
Contraction
Tension is developed
40 ms
Relaxation
50 ms

64. Illustration of a Simple Twitch

65. Illustration of the Steps of Excitation-Contraction Coupling

66. Training-Induced Changes in Muscle Fiber Type