1. Skeletal Muscle: Structure and Function

2. Objectives Draw and label the microstructure of skeletal muscle.
Define satellite cells. How do these cells differ from the nuclei located within skeletal muscle fibers?
List the chain of events that occur during muscular contraction.
Define both dynamic and static exercise. What types of muscle action occur during each form of exercise?

3. Objectives
What three factors determine the amount of force produced during muscular contraction?
List the three human skeletal muscle fiber types. Compare and contrast the major biochemical and mechanical properties of each.
How does skeletal muscle fiber type influence athletic performance?
Graph and describe the relationship between movement velocity and the amount of force exerted during muscular contraction.

4. Outline
Structure of Skeletal Muscle
Neuromuscular Junction
Muscular Contraction
Overview of the Sliding Filament Model
Energy for Contraction
Regulation of Excitation-Contraction Coupling
Fiber Types
Biochemical and Contractile Characteristics of Skeletal Muscle
Characteristics of Individual Fiber Types
Fiber Types and Performance
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
Exercise-Induced Changes in Skeletal Muscles
Muscle Atrophy Due to Inactivity
Age-Related Changes in Skeletal Muscle
Muscle Actions
Speed of Muscle Action and Relaxation
Force Regulation in Muscle
Force-Velocity/Force-Power Relationships

5. Skeletal Muscle Human body contains over 400 skeletal muscles
Structure of Skeletal Muscle
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
Muscle actions
Flexors
Decrease joint angle
Extensors
Increase joint angles

6. Connective Tissue Covering Skeletal Muscle
Structure of Skeletal Muscle
Connective Tissue Covering Skeletal Muscle
Epimysium
Surrounds entire muscle
Perimysium
Surrounds bundles of muscle fibers
Fascicles
Endomysium
Surrounds individual muscle fibers
External lamina
Just below endomysium
Sarcolemma
Muscle cell membrane

7. Connective Tissue Surrounding Skeletal Muscle
Structure of Skeletal Muscle
Connective Tissue Surrounding Skeletal Muscle
Figure 8.1

8. Satellite Cells Play role in muscle growth and repair
Structure of Skeletal Muscle
Satellite Cells
Play role in muscle growth and repair
Increase number of nuclei
Myonuclear domain
Cytoplasm surrounding each nucleus
Each nucleus can support a limited myonuclear domain
More nuclei allow for greater protein synthesis
Important for adaptations to strength training

9. Microstructure of Muscle Fibers
Structure of Skeletal Muscle
Microstructure of Muscle Fibers
Myofibrils
Contain contractile proteins
Actin (thin filament)
Myosin (thick filament)
Sarcomere
Includes Z line, M line, H zone, A band, I band
Sarcoplasmic reticulum
Storage sites for calcium
Terminal cisternae
Transverse tubules
Extend from sarcolemma to sarcoplasmic reticulum

10. Microstructure of Skeletal Muscle
Figure 8.2

11. The Sarcoplasmic Reticulum and Transverse Tubules
Structure of Skeletal Muscle
The Sarcoplasmic Reticulum and Transverse Tubules
Figure 8.3

12. Neuromuscular Junction
Junction between motor neuron and muscle fiber
Motor unit
Motor neuron and all fibers it innervates
Motor end plate
Pocket formed around motor neuron by sarcolemma
Neuromuscular cleft
Short gap between neuron and muscle fiber
Acetylcholine is released from the motor neuron
Causes an end-plate potential (EPP)
Depolarization of muscle fiber

13. The Neuromuscular Junction
Figure 8.4

14. Neuromuscular Junction
In Summary
The human body contains over 400 voluntary skeletal muscles, which constitute 40% to 50% of the total body weight. Skeletal muscle performs three major functions: (1) force production for locomotion and breathing, (2) force production for postural support, and (3) heat production during cold stress.
Individual muscle fibers are composed of hundreds of threadlike protein filaments called myofibrils. Myofibrils contain two major types of contractile protein: (1) actin (part of the thin filaments) and (2) myosin (major component of the thick filaments).

15. Neuromuscular Junction
In Summary
The region of cytoplasm surrounding an individual nucleus is termed the myonuclear domain. The importance of the myonuclear domain is that a single nucleus is responsible for the gene expression for its surrounding cytoplasm.
Motor neurons extend outward from the spinal cord and innervate individual muscle fibers. The site where the motor neuron and muscle cell meet is called the neuromuscular junction. Acetylcholine is the neurotransmitter that stimulates the muscle fiber to depolarize, which is the signal to start the contractile process.

16. The Sliding Filament Model
Muscular Contraction
The Sliding Filament Model
Muscle shortening occurs due to the movement of the actin filament over the myosin filament
Formation of cross-bridges between actin and myosin filaments
Power stroke
Reduction in the distance between Z lines of the sarcomere

17. The Sliding Filament Theory of Contraction
Muscular Contraction
The Sliding Filament Theory of Contraction
Figure 8.5

18. The Relationships Among Troponin, Tropomyosin, Myosin, and Calcium
Muscular Contraction
The Relationships Among Troponin, Tropomyosin, Myosin, and Calcium
Figure 8.6

19. Energy for Muscle Contraction
Muscular Contraction
Energy for Muscle Contraction
ATP is required for muscle contraction
Myosin ATPase breaks down ATP as fiber contracts
ATP  ADP + Pi
Sources of ATP
Phosphocreatine (PC)
Glycolysis
Oxidative phosphorylation

20. Sources of ATP for Muscle Contraction
Muscular Contraction
Sources of ATP for Muscle Contraction
Figure 8.7

21. A Closer Look 8.1 Muscle Fatigue
Muscular Contraction
A Closer Look 8.1 Muscle Fatigue
Decrease in muscle force production
Reduced ability to perform work
Contributing factors:
High-intensity exercise (~60 seconds)
Accumulation of lactate, H+, ADP, Pi, and free radicals
Long-duration exercise (2–4 hours)
Muscle factors
Accumulation of free radicals
Electrolyte imbalance
Glycogen depletion
Central fatigue
Reduced motor drive to muscle from CNS

22. Muscular Contraction
Muscular Fatigue
Figure 8.8

23. Excitation-Contraction Coupling
Muscular Contraction
Excitation-Contraction Coupling
Depolarization of motor end plate (excitation) is coupled to muscular contraction
Action potential travels down transverse tubules and causes release of Ca+2 from SR
Ca+2 binds to troponin and causes position change in tropomyosin
Exposing active sites on actin
Strong binding state formed between actin and myosin
Contraction occurs

24. Step-by-Step Summary of Excitation-Contraction Coupling
Muscular Contraction
Step-by-Step Summary of Excitation-Contraction Coupling
Excitation
Action potential in motor neuron causes release of acetylcholine into synaptic cleft.
Acetylcholine binds to receptors on motor end plate, leads to depolarization that is conducted down transverse tubules, which causes release of Ca+2 from sarcoplasmic reticulum (SR).

25. Step-by-Step Summary of Excitation-Contraction Coupling
Muscular Contraction
Step-by-Step Summary of Excitation-Contraction Coupling
Contraction
At rest, myosin cross-bridges in weak binding state.
Ca+2 binds to troponin, causes shift in tropomyosin to uncover active sites, and cross-bridge forms strong binding state.
Pi released from myosin, cross-bridge movement occurs.
ADP released from myosin.
ATP attaches to myosin, breaking the cross-bridge and forming weak binding state. Then ATP binds to myosin, broken down to ADP+Pi, which energizes myosin. Continues as long as Ca+2 and ATP are present.

26. Muscle Excitation, Contraction, and Relaxation
Muscular Contraction
Muscle Excitation, Contraction, and Relaxation
Figure 8.9

27. Steps Leading to Muscular Contraction
Figure 8.10

28. Muscular Contraction
In Summary
The process of muscular contraction can be best explained by the sliding filament model, which proposes that muscle shortening occurs due to movement of the actin filament over the myosin filament.
The steps in muscular contraction are:
The nerve impulse travels down the transverse tubules and reaches the sarcoplasmic reticulum, and Ca+2 is released.
Ca+2 binds to the protein troponin.
Ca+2 binding to troponin causes a position change in tropomyosin away from the “active sites” on the actin molecule and permits a strong binding state between actin and myosin.
Muscular contraction occurs by multiple cycles of cross-bridge activity. Shortening will continue as long as energy is available and Ca+2 is free to bind to troponin.

29. Muscular Contraction
In Summary
When neural activity ceases at the neuromuscular junction, Ca+2 is removed from the sarcoplasmic reticulum by the Ca+2 pump. This results in tropomyosin moving to cover the active site on actin, and the muscle relaxes.

30. Characteristics of Muscle Fiber Types
Biochemical properties
Oxidative capacity
Number of capillaries, mitochondria, and amount of myoglobin
Type of myosin ATPase
Speed of ATP degradation
Contractile properties
Maximal force production
Force per unit of cross-sectional area
Speed of contraction (Vmax)
Myosin ATPase activity
Muscle fiber efficiency

31. How Are Muscle Fibers Typed?
Fiber Types
How Are Muscle Fibers Typed?
Muscle biopsy
Small piece of muscle removed
May not be representative of entire body
Staining for type of myosin ATPase
Type I fibers appear darkest
IIa fibers lightest
IIx fibers in between
Immunohistochemical staining
Selective antibody binds to unique myosin proteins
Fiber types differentiated by color difference
Gel electrophoresis
Identify myosin isoforms specific to different fiber types

32. Immunohistochemical Staining of Skeletal Muscle
Fiber Types
Immunohistochemical Staining of Skeletal Muscle
Blue = Type I fibers
Green = Type IIa fibers
Black = Type IIx fibers
Red = dystrophin (protein in sarcolemma)
Figure 8.11

33. Characteristics of Individual Fiber Types
Type IIx fibers
Fast-twitch fibers
Fast-glycolytic fibers
Type IIa fibers
Intermediate fibers
Fast-oxidative glycolytic fibers
Type I fibers
Slow-twitch fibers
Slow-oxidative fibers

34. Characteristics of Muscle Fiber Types

35. Comparison of Maximal Shortening Velocities Between Fiber Types
Figure 8.12

36. Do Fast Fibers Exert More Force Than Slow Fibers?
Fiber Types
Do Fast Fibers Exert More Force Than Slow Fibers?
Maximal force per cross-sectional area
10–20% higher in fast fibers (IIa and IIx) compared to slow (Type I) fibers
Force production related to number of myosin cross-bridges in strong binding state
Fast fibers contain more cross-bridges per cross-sectional area

37. Fiber Types
In Summary
Human skeletal muscle fiber types can be divided into three general classes of fibers based on their biochemical and contractile properties properties. Two categories of fast fibers exist, type IIx and type IIa. One type of slow slow fiber exists, type I fibers.
The biochemical and contractile properties characteristic of all muscle fiber types are summarized in table 8.1.

38. Fiber Types
In Summary
Although classifying skeletal muscle fibers into three general groups is a convenient system to study the properties of muscle fibers, it is important to appreciate that human skeletal muscle fibers exhibit a wide range of contractile and biochemical properties. That is, the biochemical and contractile properties of type IIx, type IIa, and type I fibers represent a continuum instead of three neat packages.

39. Fiber Types and Performance
Nonathletes
Have approximately 50% slow and 50% fast fibers
Power athletes
Sprinters
Higher percentage of fast fibers
Endurance athletes
Distance runners
Higher percentage of slow fibers
Fiber type is not the only variable that determines success in an athletic event

40. Distribution of Fiber Type in Athletes
Fiber Types
Distribution of Fiber Type in Athletes

41. Fiber Types
In Summary
Successful power athletes (e.g., sprinters) generally possess a large percentage of fast muscle fibers and, therefore, a low percentage of slow, type I fibers.
In contrast to power athletes, endurance athletes (e.g., marathoners) typically possess a high percentage of slow muscle fibers and a low percentage of fast fibers.

42. Exercise-Induced Changes in Skeletal Muscles
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
Exercise-Induced Changes in Skeletal Muscles
Strength training
Increase in muscle fiber size (hypertrophy)
Increase in muscle fiber number (hyperplasia)
Limited evidence in humans
Endurance training
Increase in oxidative capacity
Alteration in fiber type with training
Fast-to-slow shift
Type IIx  IIa
Type IIa  I with further training
Seen with endurance and resistance training

43. Effects of Endurance Training on Fiber Type
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
Effects of Endurance Training on Fiber Type
Figure 8.13

44. Muscle Atrophy Due to Inactivity
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
Muscle Atrophy Due to Inactivity
Loss of muscle mass and strength
Due to prolonged bed rest, limb immobilization, reduced loading during space flight
Initial atrophy (2 days)
Due to decreased protein synthesis
Further atrophy
Due to reduced protein synthesis
Atrophy is not permanent
Can be reversed by resistance training
During spaceflight, atrophy can be prevented by resistance exercise

45. Age-Related Changes in Skeletal Muscle
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
Age-Related Changes in Skeletal Muscle
Aging is associated with a loss of muscle mass
10% muscle mass lost between age 25–50 years
Additional 40% lost between age 50–80 years
Also a loss of fast fibers and gain in slow fibers
Also due to reduced physical activity
Regular exercise training can improve strength and endurance
Cannot completely eliminate the age-related loss in muscle mass

46. Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
In Summary
Both endurance and resistance exercise training have been shown to promote a fast-to-slow shift in skeletal muscle fiber types. However, this exercise-induced shift in fiber type is typically small and does not result in a complete transformation of all fast fibers (type II) into slow fibers (type I).
Prolonged periods of muscle disuse (bed rest, limb immobilization, etc.) result in muscle atrophy. This inactivity-induced atrophy results in a loss of muscle protein due to a reduction in protein synthesis and an increase in the rate of muscle protein breakdown.

47. Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
In Summary
Aging is associated with a loss of muscle mass. This age-related loss of muscle mass is low from age 25 to 50 years but increases rapidly after 50 years of age.
Regular exercise training can improve skeletal muscle strength and endurance in the elderly but cannot completely eliminate the age-related loss of muscle mass.

48. Types of Muscle Action Isometric Isotonic (dynamic)
Muscle Actions
Types of Muscle Action
Isometric
Muscle exerts force without changing length
Pulling against immovable object
Postural muscles
Isotonic (dynamic)
Concentric
Muscle shortens during force production
Eccentric
Muscle produces force but length increases

49. Muscle Actions
Muscle Actions

50. Isometric and Isotonic Muscle Actions
Figure 8.14

51. Speed of Muscle Action and Relaxation
Muscle twitch
Contraction as the result of a single stimulus
Latent period
Lasting ~5 ms
Contraction
Tension is developed
40 ms
Relaxation
50 ms
Speed of shortening is greater in fast fibers
SR releases Ca+2 at a faster rate
Higher ATPase activity

52. Speed of Muscle Action and Relaxation
Muscle Twitch
Figure 8.15

53. Force Regulation in Muscle
Force generation depends on:
Types and number of motor units recruited
More motor units = greater force
Fast motor units = greater force
Initial muscle length
“Ideal” length for force generation
Increased cross-bridge formation
Nature of the neural stimulation of motor units
Frequency of stimulation
Simple twitch
Summation
Tetanus

54. Relationship Between Stimulus Strength and Force of Contraction
Force Regulation in Muscle
Relationship Between Stimulus Strength and Force of Contraction
Figure 8.16

55. Length-Tension Relationships in Skeletal Muscle
Force Regulation in Muscle
Length-Tension Relationships in Skeletal Muscle
Figure 8.17

56. Simple Twitch, Summation, and Tetanus
Force Regulation in Muscle
Simple Twitch, Summation, and Tetanus
Figure 8.18

57. Force-Velocity Relationship
Force-Velocity / Power-Velocity Relationships
Force-Velocity Relationship
At any absolute force the speed of movement is greater in muscle with higher percent of fast-twitch fibers
The maximum velocity of shortening is greatest at the lowest force
True for both slow- and fast-twitch fibers

58. Muscle Force-Velocity Relationships
Force-Velocity / Power-Velocity Relationships
Muscle Force-Velocity Relationships
Figure 8.19

59. Force-Power Relationship
Force-Velocity / Power-Velocity Relationships
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 200–300 degrees•second–1
Power decreases beyond this velocity because force decreases with increasing movement speed

60. Muscle Force-Power Relationships
Force-Velocity / Power-Velocity Relationships
Muscle Force-Power Relationships
Figure 8.20

61. Force-Velocity / Power-Velocity Relationships
In Summary
The amount of force generated during muscular contraction is dependent on the following factors: (1) types and number of motor units recruited, (2) the initial muscle length, and (3) the nature of the motor units’ neural stimulation.
The addition of muscle twitches is termed summation. When the frequency of neural stimulation to a motor unit is increased, individual contractions are fused together in a sustained contraction called tetanus.
The peak force generated by muscle decreases as the speed of movement increases. However, in general, the amount of power generated by a muscle group increases as a function of movement velocity.