structure and function of skeletal muscle
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Structure and Function 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. - PowerPoint PPT PresentationTRANSCRIPT
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Structure and Function of Skeletal Muscle
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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
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Structure of Skeletal Muscle:Connective Tissue Covering Epimysium
Surrounds entire muscle Perimysium
Surrounds bundles of muscle fibers Fascicles
Endomysium Surrounds individual muscle fibers
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Structure of Skeletal Muscle:Microstructure Sarcolemma
Muscle cell membrane Myofibrils
Threadlike strands within muscle fibers
Actin (thin filament) Troponin Tropomyosin
Myosin (thick filament)
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Structure of Skeletal Muscle:The Sarcomere Further divisions of myofibrils
Z-line A-band I-band
Within the sarcoplasm Sarcoplasmic reticulum
Storage sites for calcium Transverse tubules Terminal cisternae
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The Neuromuscular Junction Site where motor neuron meets the
muscle fiber Separated by gap called the neuromuscular
cleft
Acetylcholine is released from the motor neuron Causes an end-plate potential (EPP)
Depolarization of muscle fiber
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Illustration of the Neuromuscular Junction
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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
Reduction in the distance between Z-lines of the sarcomere
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The Sliding Filament Model of Muscle Contraction
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Cross-Bridge Formation in Muscle Contraction
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Sliding Filament Theory Rest – uncharged ATP cross-bridge
complex Excitation-– charged ATP cross-bridge
complex, “turned on” Contraction – actomyosin – ATP > ADP &
Pi + energy Recharging – reload cross-bridge with
ATP Relaxation – cross-bridges “turned off”
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Muscle Function All or none law – fiber contracts
completely or not at all Muscle strength gradation
Multiple motor unit summation – more motor units per unit of time
Wave summation – vary frequency of contraction of individual motor units
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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
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Sources of ATP for Muscle Contraction
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Properties of Muscle Fibers Biochemical properties
Oxidative capacity Type of ATPase
Contractile properties Maximal force production Speed of contraction Muscle fiber efficiency
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Individual Fiber TypesFast fibers Type IIb fibers
Fast-twitch fibers Fast-glycolytic fibers
Type IIa fibers Intermediate fibers Fast-oxidative glycolytic fibers
Slow fibers Type I fibers
Slow-twitch fibers Slow-oxidative fibers
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Comparison of Maximal Shortening Velocities Between Fiber Types
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Histochemical Staining of Fiber Type
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Fiber Types and Performance Power athletes
Sprinters Possess high percentage of fast fibers
Endurance athletes Distance runners Have high percentage of slow fibers
Others Weight lifters and nonathletes Have about 50% slow and 50% fast fibers
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Alteration of Fiber Type by Training Endurance and resistance training
Cannot change fast fibers to slow fibers
Can result in shift from Type IIb to IIa fibers
Toward more oxidative properties
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Training-Induced Changes in Muscle Fiber Type
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Hypertrophy and Hyperplasia Increase in size Increase in number
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Age-Related Changes in Skeletal Muscle Aging is associated with a loss of
muscle mass Rate increases after 50 years of age
Regular exercise training can improve strength and endurance Cannot completely eliminate the age-
related loss in muscle mass
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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
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Isotonic and Isometric Contractions
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Illustration of a Simple Twitch
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Force Regulation in Muscle 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
Nature of the motor units neural stimulation Frequency of stimulation
Simple twitch, summation, and tetanus
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Relationship Between Stimulus Frequency and Force Generation
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Length-Tension Relationship in Skeletal Muscle
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Simple Twitch, Summation, and Tetanus
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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
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Force-Velocity Relationship
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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 Force decreases with increasing
movement speed beyond this velocity
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Force-Power Relationship
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Receptors in Muscle Muscle spindle
Detect dynamic and static changes in muscle length
Stretch reflex Stretch on muscle causes reflex contraction
Golgi tendon organ (GTO) Monitor tension developed in muscle Prevents damage during excessive force
generation Stimulation results in reflex relaxation of muscle
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Muscle Spindle
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Golgi Tendon Organ
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