chapter 8 skeletal muscle: structure and function
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Chapter 8 Skeletal Muscle: Structure and Function. EXERCISE PHYSIOLOGY Theory and Application to Fitness and Performance, 6th edition Scott K. Powers & Edward T. Howley. Skeletal Muscle. Human body contains over 400 skeletal muscles 40-50% of total body weight Functions of skeletal muscle - PowerPoint PPT PresentationTRANSCRIPT
Chapter 8Skeletal Muscle: Structure
and Function
EXERCISE PHYSIOLOGYTheory and Application to Fitness and Performance,
6th edition
Scott K. Powers & Edward T. Howley
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
Connective Tissue Covering Skeletal Muscle
• Epimysium– Surrounds entire muscle
• Perimysium– Surrounds bundles of muscle fibers
• Fascicles
• Endomysium– Surrounds individual muscle fibers
Connective Tissue Surrounding Skeletal Muscle
Figure 8.1
Microstructure of Skeletal Muscle
• Sarcolemma – Muscle cell membrane
• Myofibrils – Threadlike strands within muscle fibers
• Actin (thin filament)• Myosin (thick filament)
• Sarcomere– Includes Z-line, M-line, H-zone, A-band, I-band
• Sarcoplasmic reticulum– Storage sites for calcium
• Transverse tubules
Microstructure of Skeletal Muscle
Figure 8.2
The Sarcoplasmic Reticulum and Transverse Tubules
Figure 8.3
The Neuromuscular Junction
• Junction between motor neuron and muscle fiber• 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
The Neuromuscular Junction
Figure 8.4
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
The Sliding
Filament Theory
Figure 8.5
The Relationships Among Troponin, Tropomyosin, Myosin, and Calcium
Figure 8.6
Energy for Muscle Contraction
• ATP is required for muscle contraction– Myosin ATPase breaks down ATP as fiber
contracts
• Sources of ATP– Phosphocreatine (PC)– Glycolysis– Oxidative phosphorylation
Sources of ATP for Muscle Contraction
Figure 8.7
Excitation-Contraction Coupling
• Depolarization of motor end plate (excitation) is coupled to muscular contraction– Action potential travels down T-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
Muscle Excitation,
Contraction, and
Relaxation
Figure 8.9
Figure 8.10
Steps Leading to Muscular Contraction
Muscle Fatigue
• Decrease in muscle force production– Reduced ability to perform work
• Contributing factors:– High-intensity exercise (~60 sec)
• 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
Muscle Fatigue
Figure 8.8
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
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
Characteristics of Muscle Fiber Types
Table 8.1
Comparison of Maximal Shortening Velocities Between Fiber Types
Figure 8.12
Type I
Histochemical Staining of Fiber Type
Figure 8.11
Type IIa Type IIx
Fiber Types and Performance
• Nonathletes – Have about 50% slow and 50% fast fibers
• Power athletes – Sprinters– Higher percentage of fast fibers
• Endurance athletes – Distance runners– Higher percentage of slow fibers
Exercise-Induced Changes in Skeletal Muscles
• Strength training– Increase in muscle fiber size (hypertrophy)– Increase in muscle fiber number (hyperplasia)
• 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
Effects of Endurance Training on Fiber Type
Figure 8.13
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
Age-Related Changes in Skeletal Muscle
• Aging is associated with a loss of muscle mass– 10% muscle mass lost between age 25–50 y– Additional 40% lost between age 50–80 y– 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
Types of Muscle Contraction
• 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
Muscle Actions
Table 8.3
Type of exercise Muscle Muscle Action Length
Change
_________________________________________________
Dynamic Concentric Decreases
Eccentric Increases
Static Isometric No Change
Isometric and Isotonic Contractions
Figure 8.14
Speed of Muscle Contraction 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
Muscle Twitch
Figure 8.15
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
Relationship Between Stimulus Strength and Force of Contraction
Figure 8.16
Length-Tension
Relationships in Skeletal
Muscle
Figure 8.17
Simple Twitch, Summation, and Tetanus
Figure 8.18
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
Muscle Force-Velocity Relationships
Figure 8.19
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
Muscle Force-Power Relationships
Figure 8.20
Receptors in Muscle
• Provide sensory feedback to nervous system– Tension development by muscle– Account of muscle length
• Muscle spindle
• Golgi tendon organ
Muscle Spindle
• Responds to changes in muscle length• Consists of:
– Intrafusal fibers • Run parallel to normal muscle fibers (extrafusal fibers)
– Gamma motor neuron • Stimulate intrafusal fibers to contract with extrafusal
fibers (by alpha motor neuron)
• Stretch reflex– Stretch on muscle causes reflex contraction
• Knee-jerk reflex
Muscle Spindles
Figure 8.21
Golgi Tendon Organ (GTO)
• Monitor tension developed in muscle– Prevents muscle damage during excessive force
generation
• Stimulation results in reflex relaxation of muscle– Inhibitory neurons send IPSPs to muscle fibers
Golgi Tendon Organ
Figure 8.22