Structure and Function of
Exercising Muscle
CHAPTER 1 Overview
• Functional anatomy of skeletal muscle
– Muscle fiber
– Myofibril
– Muscle fiber contraction
• Skeletal muscle and exercise
– Muscle fiber types
– Muscle fiber (motor unit) recruitment
– Orderly recruitment of muscle fibers (size principle)
– Fiber type and athletic success
Three Types of Muscle Tissue
• Smooth muscle: involuntary, hollow organs
• Cardiac muscle: involuntary, heart
• Skeletal muscle: voluntary, skeleton
Figure 1.1
Skeletal Muscle Anatomy
• Entire muscle
– Surrounded by epimysium
– Consists of many bundles (fasciculi)
• Fasciculi
– Surrounded by perimysium
– Consists of individual muscle cells (muscle fibers)
• Muscle fiber
– Surrounded by endomysium
– Consists of myofibrils divided into sarcomeres
Figure 1.2
Structure of Muscle Fibers
• Plasmalemma (cell membrane)
– Fuses with tendon
– Conducts action potential
– Maintains pH, transports nutrients
• Satellite cells
– Muscle growth, development
– Response to injury, immobilization, training
Structure of Muscle Fibers
• Sarcoplasm
– Cytoplasm of muscle cell
– Unique features: glycogen storage, myoglobin
• Transverse tubules (T-tubules)
– Extensions of plasmalemma
– Carry action potential deep into muscle fiber
• Sarcoplasmic reticulum (SR): Ca2+ storage
Figure 1.3
Myofibrils and Sarcomeres
• Myofibrils
– Muscle  fasciculi  muscle fiber  myofibril
– Hundreds to thousands per muscle fiber
• Sarcomeres
– Basic contractile element of skeletal muscle
– End to end for full myofibril length
Figure 1.4
Sarcomeres
• Distinctive striped appearance (striations)
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–
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A-bands: dark stripes
I-bands: light stripes
H-zone: middle of A-band
M-line: middle of H-zone
• Common boundary structure: Z-disk
Sarcomere: Protein Filaments
• Used for muscle contraction
• Actin (thin filaments)
– Show up lighter under microscope
– I-band contains only actin filaments
• Myosin (thick filaments)
– Show up darker under microscope
– A-band contains both actin and myosin filaments
– H-zone contains only myosin filaments
Myosin (Thick Filaments)
• Two intertwined filaments with globular
heads
• Globular heads
– Protrude 360° from thick filament axis
– Will interact with actin filaments for contraction
• Stabilized by titin
Actin (Thin Filaments)
• Actually composed of three proteins
– Actin: contains myosin-binding site
– Tropomyosin: covers active site at rest
– Troponin: anchored to actin, moves tropomyosin
• Anchored at Z-disk
• Equally spaced out by titin
Figure 1.5
Motor Units
• a-Motor neurons innervate muscle fibers
• Motor unit
– Single a-motor neuron + all fibers it innervates
– More operating motor units = more contractile force
• Neuromuscular junction
– Site of communication between neuron and muscle
– Consists of synapse between a-motor neuron and
muscle fiber
Figure 1.6
Skeletal Muscle Contraction
(Excitation-Contraction Coupling)
1. Action potential (AP) starts in brain
2. AP arrives at axon terminal, releases
acetylcholine (ACh)
3. ACh crosses synapse, binds to ACh receptors
on plasmalemma
4. AP travels down plasmalemma, T-tubules
5. Triggers Ca2+ release from sarcoplasmic
reticulum (SR)
6. Ca2+ enables actin-myosin contraction
Figure 1.7
Role of Ca2+ in Muscle Contraction
• AP arrives at SR from T-tubule
– SR sensitive to electrical charge
– Causes mass release of Ca2+ into sarcoplasm
• Ca2+ binds to troponin on thin filament
– At rest, tropomyosin covers myosin-binding site,
blocking actin-myosin attraction
– Troponin-Ca2+ complex moves tropomyosin
– Myosin binds to actin, contraction can occur
Sliding Filament Theory:
How Muscles Create Movement
• Process of actin-myosin contraction
• Relaxed state
– No actin-myosin interaction at binding site
– Myofilaments overlap a little
• Contracted state
– Myosin head pulls actin toward sarcomere center
(power stroke)
– Filaments slide past each other
– Sarcomeres, myofibrils, muscle fiber all shorten
Figure 1.8
Sliding Filament Theory:
How Muscles Create Movement
• After power stroke ends
– Myosin detaches from active site
– Myosin head rotates back to original position
– Myosin attaches to another active site farther down
• Process continues until
– Z-disk reaches myosin filaments or
– AP stops, Ca2+ gets pumped back into SR
Figure 1.9
Energy for Muscle Contraction
• Adenosine triphosphate (ATP)
• Binds to myosin head
– ATPase on myosin head
– ATP  ADP + Pi + energy
• Necessary for muscle contraction
Muscle Relaxation
• AP ends, electrical stimulation of SR stops
• Ca2+ pumped back into SR
– Stored until next AP arrives
– Requires ATP
• Without Ca2+, troponin and tropomyosin
return to resting conformation
– Covers myosin-binding site
– Prevents actin-myosin cross-bridging
Muscle Fiber Types
• Type I
– ~50% of fibers in an average muscle
– Peak tension in 110 ms (slow twitch)
• Type II
– Peak tension in 50 ms (fast twitch)
– Type IIa (~25% of fibers in an average muscle)
– Type IIx (~25% of fibers in an average muscle)
Figure 1.10
Type I Versus Type II Differentiation
• Speed of myosin ATPase varies
– Fast myosin ATPase = fast contraction cycling
– Slower myosin ATPase = slower contraction cycling
• Muscle biopsy
– Small (10-100 g) piece of muscle removed
– Frozen, sliced, examined under microscope
• Gel electrophoresis
– Type I versus II fibers have different types of myosin
– Separates different types of myosin by size
Muscle Biopsy
Figure 1.11
Type I Versus Type II
• Sarcoplasmic reticulum
– Type II fibers have a more highly developed SR
– Faster Ca2+ release, 3 to 5 times faster Vo
• Motor units
– Type I motor unit: smaller neuron, <300 fibers
– Type II motor unit: larger neuron, >300 fibers
Type I Versus Type II: Peak Power
• Peak power: type IIx > type IIa > type I
– Effects of different SR, motor units, etc.
– Single muscle fiber recording
• Regardless of fiber type, all muscle fibers
reach peak power at ~20% peak force
Single Muscle Fiber Peak Power
Table 1.1
Distribution of Fiber Types:
Type I:Type II Ratios
• Each person has different ratios
• Arm and leg ratios are similar in one person
– Endurance athlete: type I predominates
– Power athlete: type II predominates
• Soleus: type I in everyone
Type I Fibers During Exercise
• High aerobic endurance
– Can maintain exercise for prolonged periods
– Require oxygen for ATP production
– Low-intensity aerobic exercise, daily activities
• Efficiently produce ATP from fat,
carbohydrate
Type II Fibers During Exercise
• Type II fibers in general
– Poor aerobic endurance, fatigue quickly
– Produce ATP anaerobically
• Type IIa
– More force, faster fatigue than type I
– Short, high-intensity endurance events (1,600 m run)
• Type IIx
– Seldom used for everyday activities
– Short, explosive sprints (100 m)
Table 1.2
Fiber Type Determinants
• Genetic factors
– Determine which a-motor neurons innervate fibers
– Fibers differentiate based on a-motor neuron
• Training factors
– Endurance versus strength training, detraining
– Can induce small (10%) change in fiber type
• Aging: muscles lose type II motor units
Muscle Fiber Recruitment
• Also called motor unit recruitment
• Method for altering force production
– Less force production: fewer or smaller motor units
– More force production: more or larger motor units
– Type I motor units smaller than type II
• Recruitment order: type I, type IIa, type IIx
Orderly Recruitment
and the Size Principle
• Recruit minimum number of motor units
needed
– Smallest (type I) motor units recruited first
– Midsized (type IIa) motor units recruited next
– Largest (type IIx) motor units recruited last
• Recruited in same order each time
• Size principle: order of recruitment of motor
units directly related to size of a-motor
neuron
Fiber Type and Athletic Success
• Endurance athletes—type I predominates
• Sprinters—type II predominates
• Fiber type not sole predictor of success
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Cardiovascular function
Motivation
Training habits
Muscle size
Types of Muscle Contraction
• Static (isometric) contraction
– Muscle produces force but does not change length
– Joint angle does not change
– Myosin cross-bridges form and recycle, no sliding
• Dynamic contraction
– Muscle produces force and changes length
– Joint movement produced
Dynamic Contraction Subtypes
• Concentric contraction
– Muscle shortens while producing force
– Most familiar type of contraction
– Sarcomere shortens, filaments slide toward center
• Eccentric contraction
– Muscle lengthens while producing force
– Cross-bridges form but sarcomere lengthens
– Example: lowering heavy weight
Generation of Force
• Motor unit recruitment
– Type II motor units = more force
– Type I motor units = less force
– Fewer small fibers versus more large fibers
• Frequency of stimulation (rate coding)
– Twitch
– Summation
– Tetanus
Generation of Force
• Length-tension relationship
– Optimal sarcomere length = optimal overlap
– Too short or too stretched = little or no force
develops
• Speed-force relationship
– Concentric: maximal force development decreases
at higher speeds
– Eccentric: maximal force development increases at
higher speeds
Figure 1.12
Figure 1.13
Figure 1.14