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Chapter 8
Skeletal Muscle
Structure and Function
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
Fastest fibers
 Type IIb(x) fibers
– Slow-twitch fibers
– Slow-oxidative 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 (x)
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
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?
50% Each
100% ST
100% FT
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
Protein Arrangement and Strength

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

Hyperplasia?
– more cells
– stem cells – for injury repair
Protein Arrangement and Strength

Fiber arrangement
– Fusiform
– penniform
Protein Arrangement and Strength


More sarcomeres in parallel, more
tension produced
More cross bridges generating tension
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 agerelated 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 (Newtons)
Force-Velocity Relationship
FT
ST
Velocity (deg./sec)
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