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Skeletal Muscle
Structure and Function
Chapter 8
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 of Muscle
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
Microstructure of Skeletal Muscle
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• Storage sites for calcium
Within the Sarcoplasm
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
Illustration of the Neuromuscular Junction
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
The Sliding Filament Model of Muscle Contraction
Cross-Bridge Formation in Muscle Contraction
Energy for Muscle Contraction
ATP is required for muscle contraction– Myosin ATPase breaks down ATP as fiber
contracts
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
Steps Leading to Muscular Contraction
CD ROM
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
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
Muscle Fiber Types
Fast fibers Slow fibers
Characteristic Type IIb Type IIa Type I
Number of mitochondria Low High/moderate High
Resistance to fatigue Low High/moderate High
Predominant energy system Anaerobic Combination Aerobic
ATPase activity Highest High Low
Vmax (speed of shortening) Highest Intermediate Low
Efficiency Low Moderate High
Specific tension High High Moderate
(x)
Comparison of Maximal Shortening Velocities Between Fiber Types
(x)
Histochemical Staining of Fiber Type
Type IIb (x)
Type IIa
Type I
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
How Is Fiber Type Distributed In a Population?
100% FT100% ST
50% Each
How do I get more FT muscle fibers?
Grow more? Change my ST to FT?
Pick the right parents?
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
So how do I gain strength?
Protein Arrangement and Strength
Increase sarcomeres (myofibrils) in parallel– more cross bridges to pull– requires more room in diameter– requires more fluid– hypertrophy
Increase in sarcomeres in series?– Only with growth in height / limb length– Sarcomeres would crowd each other– Would result in decreased force production
Protein Arrangement and Strength
Protein Arrangement and Strength
Hyperplasia?– more cells– stem cells – for injury repair
Protein Arrangement and Strength
Fiber arrangement– Fusiform
– penniform
More sarcomeres in parallel, more tension produced
More cross bridges generating tension
Protein Arrangement and Strength
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
Types of Muscle Contraction
Isometric– Muscle exerts force without changing length– Pulling against immovable object– Stabilization– Postural muscles
Types of Muscle Contraction
Isotonic (dynamic)– Concentric
• Muscle shortens during force production
– Eccentric• Muscle produces force but length increases
Types of Muscle Contraction
Isokinetic (dynamic)– Constant speed of contraction– Machine assisted
Isotonic and Isometric Contractions
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”
Histochemical Staining of Fiber Type
Type IIb
Type IIa
Type I
Force Regulation in Muscle
Initial muscle length– “Ideal” length for force generation– Maximum vs. minimum cross bridging
Length-Tension Relationship in Skeletal Muscle
Force Regulation in Muscle
Nature of the motor units’ neural stimulation– Frequency of stimulation
• Simple twitch, summation, and tetanus
Illustration of a Simple Twitch
Simple Twitch, Summation, and Tetanus
Relationship Between Stimulus Frequency and Force Generation
Determinants of Contraction Velocity- Single Fiber
Nerve conduction velocity
Myofibrillar ATPase activity
Determinants of Contraction Velocity- Whole Muscle
Same factors as for single fiber– Nerve conduction velocity– ATPase activity
Number of FT fibers recruited
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
Force-Velocity RelationshipF
orce
(N
ewto
ns)
Velocity (deg./sec)
FT
ST
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
Force-Power Relationship
Is there a safety mechanism?
Receptors in Muscle
Muscle spindle– Detect dynamic and static changes in muscle length– Stretch reflex
• Stretch on muscle causes reflex contraction
Muscle Spindle
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
Golgi Tendon Organ
Questions?
End
Force-Velocity Relationship
Structure of Skeletal Muscle:Connective Tissue Covering Epimysium
– Surrounds entire muscle Perimysium
– Surrounds bundles of muscle fibers• Fascicles
Endomysium– Surrounds individual muscle fibers
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
Illustration of a Simple Twitch
Illustration of the Steps of Excitation-Contraction Coupling
Training-Induced Changes in Muscle Fiber Type
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