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Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 32
Structural Support
and Movement
(Sections 32.5 - 32.7)
Albia Dugger • Miami Dade College
32.5 How Skeletal Muscle Contracts
• A skeletal muscle consists of bundles of muscle fibers
covered by an outer sheath
• Bones of a human in motion move when skeletal muscles
attached to them shorten
• The internal organization of a skeletal muscle promotes a
strong, directional contraction
• A muscle shortens when muscle fibers, and the contractile
units inside the fibers, shorten
Motion and Skeletal Muscles
Motion and Skeletal Muscles
outer
sheath
of one
skeletal
muscle
one bundle of many
muscle fibers in parallel
inside the sheath
Fig. 32.11a, p. 528
Structure of Skeletal Muscle
• Many myofibrils run the length of a muscle fiber
• skeletal muscle fiber
• Multinucleated skeletal muscle cell
• myofibrils
• Threadlike, cross-banded skeletal muscle components
that consist of sarcomeres arranged end to end
Structure of Skeletal Muscle (cont.)
• Each myofibril has light-to-dark cross-bands which are units
of muscle contraction (sarcomeres)
• The sarcomere has parallel arrays of thin and thick filaments
•
sarcomere
• Contractile unit of skeletal and cardiac muscle
Structure of Skeletal Muscle (cont.)
• Each thin filament consists of two chains of a globular protein
(actin)
• Thicker filaments consist of myosin
• actin
• Protein that is the main component of thin filaments of
muscle fibers
• myosin
• Protein in thick filaments of muscle fibers
Sarcomere Structure
Sarcomere Structure
B one myofibril, made up of sarcomeres arranged end to end
sarcomere
Z line
sarcomere
Z line
Z line
Fig. 32.11b, p. 528
Sarcomere
Structure
Z line
Z line
C one sarcomere, with
parallel actin and
myosin filaments
actin
Z line
myosin
actin
Z line
Fig. 32.11c, p. 528
Animation 6.7: Structure of skeletal muscle
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Sliding-Filament Model
• The sliding-filament model describes how ATP-driven
sliding of actin filaments past myosin filaments shortens the
sarcomere
• Shortening of all sarcomeres in all myofibrils of all muscle
fibers brings about muscle contraction
• sliding-filament model
• How interactions among actin and myosin filaments
shorten a sarcomere and bring about muscle contraction
Steps in the Sliding-Filament Model
1. In a muscle at rest, actin and myosin filaments lie next to one
another, but do not interact
2. Myosin heads in the thick filaments are activated by a
phosphate-group transfer from ATP
3. Release of calcium from intracellular storage allows myosin
heads to bind to sites on adjacent actin filaments, forming
cross-bridges
Steps in the Sliding-Filament Model
4. A myosin head releases bound ADP and phosphate as it tilts
toward the sarcomere center with actin filaments attached
5. New ATP binds to myosin heads, causing them to release
actin and return to their original orientation
6. Myosin heads repeatedly binding to and pulling on adjacent
actin filaments cause the sarcomere to contract
The Sliding-Filament Model
The Sliding-Filament Model
Fig. 32.12.1, p. 529
The Sliding-Filament Model
1
actin
myosin
actin
Sarcomere between contractions
Fig. 32.12.1, p. 529
The Sliding-Filament Model
Fig. 32.12.2,3, p. 529
The Sliding-Filament Model
2
myosin head with bound ADP and P
one of many myosin-binding sites on actin
3
cross-bridge
cross-bridge
Fig. 32.12.2,3, p. 529
The Sliding-Filament Model
Fig. 32.12.4, p. 529
The Sliding-Filament Model
ADP and P released
4
Fig. 32.12.4, p. 529
The Sliding-Filament Model
Fig. 32.12.5,6, p. 529
The Sliding-Filament Model
5
cross-bridge broken
6
cross-bridge broken
Same sarcomere, contracted
Fig. 32.12.5,6, p. 529
ANIMATION: Sliding filament model
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Animation: Structure of a Sarcomere
Animation: Muscle Contraction Overview
32.6 From Signal to Response
• Release of ACh at a neuromuscular junction causes an action
potential which is propagated along the plasma membrane of
the muscle cell, and along T tubules to the sarcoplasmic
reticulum
• Calcium ions released by this organelle allow actin and
myosin heads to interact so muscle contraction occurs
• sarcoplasmic reticulum
• Specialized endoplasmic reticulum in muscle cells
• Stores and releases calcium ions
Nervous Control of Contraction (1)
• A signal travels along
the axon of a motor
neuron from the spinal
cord to a skeletal
muscle
Nervous Control of Contraction (1)
motor neuron
section from spinal cord
Fig. 32.13a, p. 530
Animation 6.9: Nervous system and muscle
contraction
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Nervous Control of Contraction (2)
• Signal transfers from
motor neuron to muscle
at neuromuscular
junctions
• ACh from axon
terminals diffuses into
muscle fiber and causes
action potentials
Nervous Control of Contraction (2)
neuromuscular junction
section from skeletal muscle
Fig. 32.13b, p. 530
Nervous Control of Contraction (3)
• Action potentials
propagate along the
plasma membrane to T
tubules, to sarcoplasmic
reticulum, which
releases calcium ions
• Ions promote
interactions of myosin
and actin
Nervous Control of Contraction (3)
sarcoplasmic
T
reticulum
tubule
one
myofibril
in muscle
fiber
muscle
fiber’s
plasma
membrane
Fig. 32.13c, p. 530
1 A signal travels
motor neuron
along the axon of a
motor neuron, from
the spinal cord to a
skeletal muscle.
section from spinal cord
2 The signal is
transferred from the motor
neuron to the muscle at
neuromuscular junctions.
Here, ACh released by the
neuron’s axon terminals
diffuses into the muscle
fiber and causes action
potentials.
Nervous
Control of
Contraction
neuromuscular junction
section from skeletal muscle
T
sarcoplasmic
tubule reticulum
3 Action potentials
propagate along a muscle
fiber’s plasma membrane
down toT tubules, then to
the sarcoplasmic
reticulum, which releases
calciumions. The ions
promote interactions of
myosin and actin that
result in contraction.
one
myofibril
in muscle
fiber
muscle
fiber’s
plasma
membrane
Stepped Art
Fig. 32.13, p. 530
Motor Units and Muscle Tension
• A motor neuron and all of the muscle fibers it synapses with
constitute one motor unit
• Brief stimulation of a motor unit causes a muscle twitch
• Repeated stimulation causes a sustained contraction that
generates more force (muscle tension), depending on the
number of muscle fibers that contract
Key Terms
• motor unit
• One motor neuron and the muscle fibers it controls
• muscle twitch
• Brief muscle contraction
• muscle tension
• Force exerted by a contracting muscle
Muscle Tension
Muscle Tension
Force
relaxation starts
stimulus contraction
A Brief stimulation causes a twitch.
Fig. 32.14b, p. 530
Force
Muscle Tension
twitch
sustained
contraction
repeated stimulation
Time
B Repeated stimulation within a short interval
causes a sustained contraction with greater force.
Fig. 32.14b, p. 530
Animation: Types of contractions
Energy for Contraction
• Muscle fibers produce ATP needed for contraction by three
pathways: dephosphorylation of creatine phosphate, aerobic
respiration, and lactate fermentation
• ATP
• The first energy source a muscle uses, but muscle has a
limited amount of ATP
Three Pathways of ATP Production
• Muscle has a large store of creatine phosphate, which can
transfer a phosphate to ADP and form ATP – fuels muscle
contraction until other pathways increase ATP output
• Aerobic respiration (oxygen-requiring) yields most ATP used
by a muscle during prolonged, moderate activity
• Lactate fermentation produces less ATP than aerobic
respiration, but operates even when oxygen level is low
Three Pathways of ATP Production
Three Pathways of ATP Production
1
dephosphorylation
of creatine
phosphate
ADP + Pi
creatine
2
aerobic
respiration
oxygen
3
lactate
fermentation
glucose from bloodstream and
from glycogen breakdown in cells
Fig. 32.15, p. 531
Animation: Energy sources for contraction
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Types of Muscle Fibers
• Muscles have a mix of red and white fibers, which differ in the
way they produce ATP
• Muscle fibers can also be subdivided into fast fibers or slow
fibers based on the ATPase activity of their myosin
• Fast fibers split ATP more efficiently and contract more
quickly than slow fibers when stimulated
Red Fibers and White Fibers
• Red fibers make ATP mainly by aerobic respiration
• Have many mitochondria and oxygen-storing myoglobin
• Can be either fast or slow
• White fibers make ATP mainly by lactate fermentation
• Have no myoglobin and few mitochondria
• All white fibers are fast fibers
Key Concepts
• How Skeletal Muscle Contracts
• A muscle fiber contains many myofibrils, each divided
crosswise into sarcomeres
• Sarcomeres contain parallel filaments of the proteins actin
and myosin
• Muscle contracts when ATP-driven interactions between
these proteins shortens sarcomeres
BBC Video: How Muscles Hold Tension
32.7 Muscles and Health
• In humans, all muscle fibers form before birth
• Exercise can’t add muscle fibers, but it can increase muscle
strength and endurance
• Muscle function can be impaired by genetic disorders,
infectious disease, and some toxins
Effects of Exercise
• Aerobic exercise, which is low in intensity and long in
duration, makes muscles more resistant to fatigue
• Strength training (intense, short-duration exercise such as
weight lifting) stimulates formation of more actin and myosin,
as well more enzymes of glycolysis
Aerobic Exercise
• Aerobic exercise
increases the number of
mitochondria in
muscles, which
increases endurance
Strength Training
• Strength training involves two types of muscle contractions:
• Isotonically contracting muscles shorten and move some
load, as when you lift an object
• Isometrically contracting muscles tense but do not shorten, as
when you try to lift an object but fail because its weight
exceeds the muscle’s capacity
Isotonic and Isometric Contraction
Isotonic and Isometric Contraction
A Isotonic contraction.
Muscle tension is greater
than the opposing force
and the muscle shortens,
as when you lift a light
weight.
B Isometric contraction.
Muscle tension is less
than the opposing force
and the muscle remains
at the same length, rather
than shortening.
Fig. 32.17, p. 532
Isotonic and Isometric Contraction
A Isotonic contraction.
Muscle tension is greater
than the opposing force
and the muscle shortens,
as when you lift a light
weight.
Fig. 32.17a, p. 532
Isotonic and Isometric Contraction
B Isometric contraction.
Muscle tension is less
than the opposing force
and the muscle remains
at the same length, rather
than shortening.
Fig. 32.17b, p. 532
Muscles and Aging
• As people age, muscles shrink:
• Number of muscle fibers declines
• Fibers grow more slowly in response to exercise
• Muscle injuries take longer to heal
• Exercise can be helpful at any age:
• Strength training slows loss of muscle tissue
• Aerobic exercise improves circulation, helps lift
depression, and improves brain functions
Muscular Dystrophy
• Muscular dystrophies are a class of genetic disorders that
cause muscles to break down
• In Duchenne muscular dystrophy, the affected gene encodes,
a protein (dystrophin) in muscle fibers’ plasma membranes
• Mutated dystrophin allows foreign material to enter a muscle
fiber, which causes the fiber to break down, resulting in death
by respiratory failure
Effects of Muscular Dystrophy
Motor Neuron Disorders
• Muscular weakness or paralysis occurs when motor neurons
cannot signal muscles to contract
• Poliovirus infects and kills motor neurons, causing death or
paralysis
• Amyotrophic lateral sclerosis (ALS or Lou Gehrig’s disease)
also kills motor neurons, causing death by respiratory failure
Botulism and Tetanus
• Some bacteria make deadly toxins that disrupt the flow of
signals from nerves to muscles
• Botulinum toxin prevents motor neurons from releasing
acetylcholine (ACh) – muscles can’t contract without this
neurotransmitter
• Tetanus toxin in the spinal cord blocks release of
neurotransmitters that inhibit motor neurons – muscles stiffen
and cannot be released from contraction
Tetanus
Key Concepts
• Factors Affecting Contraction
• Muscle fibers in a muscle are organized into motor units
that contract in response to signals from a motor neuron
• Diseases or disorders can interfere with muscle function
• Exercise improves muscle strength and endurance
Muscles and Myostatin (revisited)
• Drugs that inhibit
myostatin production or
prevent myostatin
activity may help to slow
the muscle loss that
results from muscular
dystrophy, ALS, and
even normal aging