skeletal muscle: structure and function

Scott K. Powers • Edward T. HowleyScott K. Powers • Edward T. Howley

Theory and Application to Fitness and PerformanceTheory and Application to Fitness and PerformanceSEVENTH EDITION

Chapter

Presentation prepared by:

Brian B. Parr, Ph.D.University of South Carolina Aiken

Copyright ©2009 The McGraw-Hill Companies, Inc. Permission required for reproduction or display outside of classroom use.

Skeletal Muscle: Skeletal Muscle: Structure and FunctionStructure and Function

Chapter 8

Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.

ObjectivesObjectives

1. Draw and label the microstructure of skeletal muscle.

2. Define satellite cells. How do these cells differ from the nuclei located within skeletal muscle fibers?

3. List the chain of events that occur during muscular contraction.

4. Define both dynamic and static exercise. What types of muscle action occur during each form of exercise?

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ObjectivesObjectives

5. What three factors determine the amount of force produced during muscular contraction?

6. List the three human skeletal muscle fiber types. Compare and contrast the major biochemical and mechanical properties of each.

7. How does skeletal muscle fiber type influence athletic performance?

8. Graph and describe the relationship between movement velocity and the amount of force exerted during muscular contraction.

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OutlineOutline Structure of Skeletal

Muscle Neuromuscular Junction Muscular Contraction

Overview of the Sliding Filament Model

Energy for ContractionRegulation of Excitation-Contraction Coupling

Fiber TypesBiochemical and Contractile Characteristics of Skeletal Muscle

Characteristics of Individual Fiber Types

Fiber Types and Performance

Alterations in Skeletal Muscle Due to Exercise, Inactivity, and AgingExercise-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

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Skeletal MuscleSkeletal 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

Structure of Skeletal Muscle

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Connective Tissue Covering Skeletal MuscleConnective 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


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Connective Tissue Surrounding Connective Tissue Surrounding Skeletal MuscleSkeletal Muscle


Figure 8.1

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Satellite CellsSatellite 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


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Microstructure of Muscle FibersMicrostructure 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


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Microstructure of Skeletal MuscleMicrostructure of Skeletal Muscle


Figure 8.2

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The Sarcoplasmic Reticulum and The Sarcoplasmic Reticulum and Transverse TubulesTransverse Tubules


Figure 8.3

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Neuromuscular JunctionNeuromuscular 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

Neuromuscular Junction

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The Neuromuscular JunctionThe Neuromuscular JunctionNeuromuscular Junction

Figure 8.4

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In SummaryIn 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).


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In SummaryIn 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.


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The Sliding Filament ModelThe 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

Muscular Contraction

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Copyright ©2009 The McGraw-Hill Companies, Inc. All Rights Reserved.Figure 8.5

The Sliding The Sliding Filament Filament Theory of Theory of

ContractionContraction


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The Relationships Among Troponin, The Relationships Among Troponin, Tropomyosin, Myosin, and CalciumTropomyosin, Myosin, and Calcium

Figure 8.6


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Energy for Muscle ContractionEnergy 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


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Sources of ATP for Muscle ContractionSources of ATP for Muscle Contraction

Figure 8.7


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A Closer Look 8.1A Closer Look 8.1Muscle FatigueMuscle 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


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Muscular FatigueMuscular Fatigue

Figure 8.8


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Excitation-Contraction CouplingExcitation-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


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Step-by-Step Summary of Excitation-Step-by-Step Summary of Excitation-Contraction CouplingContraction Coupling

• Excitation1. Action potential in motor neuron causes

release of acetylcholine into synaptic cleft.2. 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).


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Step-by-Step Summary of Excitation-Step-by-Step Summary of Excitation-Contraction CouplingContraction Coupling

• Contraction1. At rest, myosin cross-bridges in weak binding state.2. Ca+2 binds to troponin, causes shift in tropomyosin to

uncover active sites, and cross-bridge forms strong binding state.

3. Pi released from myosin, cross-bridge movement occurs.

4. ADP released from myosin.5. 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.


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Muscle Muscle Excitation, Excitation,

Contraction, Contraction, and Relaxationand Relaxation

Figure 8.9


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Steps Leading to Muscular Steps Leading to Muscular ContractionContraction

Figure 8.10


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In SummaryIn 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.


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In SummaryIn 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.


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Characteristics of Muscle Fiber TypesCharacteristics 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

Fiber Types

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Fiber Types

How Are Muscle Fibers Typed?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

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Immunohistochemical Staining of Skeletal MuscleImmunohistochemical Staining of Skeletal Muscle

Fiber Types

Figure 8.11

Blue = Type I fibersGreen = Type IIa fibersBlack = Type IIx fibersRed = dystrophin (protein in sarcolemma)

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Characteristics of Individual Fiber TypesCharacteristics 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

Fiber Types

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Characteristics of Muscle Fiber Characteristics of Muscle Fiber TypesTypes

Fiber Types

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Comparison of Maximal Comparison of Maximal Shortening Velocities Between Shortening Velocities Between

Fiber TypesFiber Types

Fiber Types

Figure 8.12

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Do Fast Fibers Exert More Force Than Slow Do Fast Fibers Exert More Force Than Slow Fibers?Fibers?

Fiber Types

• 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

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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.

Fiber Types

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In SummaryIn 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.

Fiber Types

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Fiber Types and PerformanceFiber 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

Fiber Types

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Distribution of Fiber Type in Distribution of Fiber Type in AthletesAthletes

Fiber Types

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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.

Fiber Types

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Exercise-Induced Changes in Exercise-Induced Changes in Skeletal MusclesSkeletal 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

Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging

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Effects of Endurance Training on Effects of Endurance Training on Fiber TypeFiber Type


Figure 8.13

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Muscle Atrophy Due to InactivityMuscle 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


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Age-Related Changes in Skeletal Age-Related Changes in Skeletal MuscleMuscle

• 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


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In SummaryIn 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.


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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.


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Types of Muscle Action 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

Muscle Actions

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Muscle ActionsMuscle ActionsMuscle Actions

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Isometric and Isotonic Muscle ActionsIsometric and Isotonic Muscle Actions

Muscle Actions

Figure 8.14

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Speed of Muscle Action and RelaxationSpeed 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

Speed of Muscle Action and Relaxation

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Muscle TwitchMuscle Twitch

Figure 8.15

Speed of Muscle Action and Relaxation

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Force Regulation in Muscle

Force Regulation in MuscleForce 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

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Relationship Between Stimulus Strength and Relationship Between Stimulus Strength and Force of ContractionForce of Contraction

Figure 8.16


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Length-Tension Length-Tension Relationships in Relationships in Skeletal MuscleSkeletal Muscle


Figure 8.17

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Simple Twitch, Summation, and TetanusSimple Twitch, Summation, and Tetanus


Figure 8.18

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Force-Velocity RelationshipForce-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

Force-Velocity / Power-Velocity Relationships

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Muscle Force-Velocity RelationshipsMuscle Force-Velocity Relationships

Figure 8.19


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Force-Power RelationshipForce-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


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Muscle Force-Power RelationshipsMuscle Force-Power Relationships


Figure 8.20

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In SummaryIn 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.


skeletal muscle: structure and function

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