actin

Structural Support and Movement
Chapter 36 Part 2
36.7 How Does Skeletal Muscle Contract?
 Myofibrils (bundles of contractile filaments) run
the length of the muscle fiber
 Myofibrils are divided into bands (striations) that
define units of contraction (sarcomeres)
• Z-bands attach sarcomeres to each other
 Sarcomeres contain two types of filaments
• Thin, globular protein filaments (actin)
• Thick, motor protein filaments (myosin)
Fine Structure of Skeletal Muscle
Fig. 36-17a, p. 628
outer
sheath
of one
skeletal
muscle
one bundle of many
muscle fibers in
parallel inside the
sheath
one myofibril
in one fiber
Fig. 36-17a, p. 628
Fig. 36-17b, p. 628
one myofibril
inside fiber
b Skeletal muscle fiber, longitudinal
section. All bands of its myofibrils
line up in rows and give the fiber a
striped appearance.
sarcomere
Z band
sarcomere
Z band
H zone
Z band
Fig. 36-17b, p. 628
Fig. 36-17c, p. 628
Z band
H zone
Z band
c Sarcomeres. Many thick
and thin filaments overlap
in an A band. Only thick
filaments extend across
the H zone. Only thin
filaments extend across I
bands to the Z bands.
Different proteins organize
and stabilize the array.
I band
A band
I band
Fig. 36-17c, p. 628
Fig. 36-17 (d-e), p. 628
one actin
molecule
d
Arrangement of actin molecules in the thin filaments
part of a
myosin
molecule
e
part of
a thin
filament
part of a
thick
filament
Arrangement of myosin molecules in the thick filaments
Fig. 36-17 (d-e), p. 628
Animation: Structure of skeletal muscle
The Sliding Filament Model
 Sliding filament model
• Interactions among protein filaments within a
muscle fiber’s individual contractile units
(sarcomeres) bring about muscle contraction
• A sarcomere shortens when actin filaments are
pulled toward the center of the sarcomere by
ATP-fueled interactions with myosin filaments
The Sliding Filament Model
Fig. 36-18a, p. 629
A Relative positions
of actin and myosin
filaments inside a
sarcomere between
contractions
actin
Z band
myosin
actin
Z band
Fig. 36-18a, p. 629
Fig. 36-18b, p. 629
B Relative positions
of actin and myosin
filaments in the same
sarcomere,
contracted
Fig. 36-18b, p. 629
Fig. 36-18c, p. 629
myosin head
one of many myosin-binding sites on actin
C Myosin in a muscle at rest. Earlier, all myosin heads were energized by
binding ATP, which they hydrolyzed to ADP and inorganic phosphate.
Fig. 36-18c, p. 629
Fig. 36-18d, p. 629
cross-bridge
cross-bridge
D A rise in the local concentration of
calcium exposes binding sites for myosin
on actin filaments, so cross-bridges form.
Fig. 36-18d, p. 629
Fig. 36-18e, p. 629
E Binding makes each myosin head tilt toward the
sarcomere’s center and slide the bound actin along with it.
ADP and phosphate are released as the myosin heads drag
the actin filaments inward, which pulls the Z bands closer.
Fig. 36-18e, p. 629
Fig. 36-18f, p. 629
F New ATP binds to myosin heads, which detach from actin.
ATP is hydrolyzed, which returns myosin heads to their
original positions.
Fig. 36-18f, p. 629
A Relative positions of actin and myosin filaments
inside a sarcomere between contractions
actin
myosin
actin
Z band
Z band
B Relative positions of actin and myosin filaments in the
same sarcomere, contracted
myosin head
one of many myosin-binding sites on actin
cross-bridge
cross-bridge
C Myosin in a muscle at rest. Earlier, all myosin heads
were energized by binding ATP, which they hydrolyzed
to ADP and inorganic phosphate.
D A rise in the local concentration of calcium exposes
binding sites for myosin on actin filaments, so crossbridges form.
E Binding makes each myosin head tilt toward the
sarcomere’s center and slide the bound actin along with
it. ADP and phosphate are released as the myosin heads
drag the actin filaments inward, which pulls the Z bands
closer.
F New ATP binds to myosin heads, which detach from
actin. ATP is hydrolyzed, which returns myosin heads to
their original positions.
Stepped Art
Fig. 36-18, p. 629
Animation: Sliding filament model
36.8 From Signal to Response:
A Closer Look at Contraction
 Like neurons, muscle cells are excitable
• Skeletal muscle contracts in response to a signal
from a motor neuron
• Release of ACh at a neuromuscular junction
causes an action potential in the muscle cell
Nervous Control of Contraction
 Action potentials travel along muscle plasma
membrane, down T tubules, to the sarcoplasmic
reticulum (a smooth endoplasmic reticulum)
 Action potentials open voltage-gated channels in
sarcoplasmic reticulum, triggering calcium
release that allows contraction in myofibrils
Nervous Control of Contraction
Fig. 36-19a, p. 630
motor neuron
A A signal travels
along the axon of a
motor neuron, from
the spinal cord to a
skeletal muscle.
section from spinal cord
Fig. 36-19a, p. 630
Fig. 36-19b, p. 630
B 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.
neuromuscular junction
section from skeletal muscle
Fig. 36-19b, p. 630
Fig. 36-19c, p. 630
C Action potentials
propagate along a
muscle fiber’s plasma
membrane down to
T tubules, then to
the sarcoplasmic
reticulum, which
releases calcium ions.
The ions promote
interactions of myosin
and actin that result
in contraction.
sarcoplasmic
T
reticulum
tubule
one
myofibril
in muscle
fiber
muscle
fiber’s
plasma
membrane
Fig. 36-19c, p. 630
Animation: Nervous system and muscle
contraction
The Roles of Troponin and Tropomyosin
 Two proteins regulate bonding of actin to myosin
• Tropomyosin prevents actin from binding to myosin
• Troponin has calcium binding sites
 Calcium binds to troponin, which pulls tropomyosin
away from myosin-binding sites on actin
 Cross-bridges form, sarcomeres shorten, and
muscle contracts
Interactions of Actin,
Tropomyosin, and Troponin
A Actin (tan) with troponin (teal) and
tropomyosin (green) in a thin filament
myosin-binding of muscle at rest.
site blocked by
B View of a section through the filament
tropomyosin
shown above.
C Some calcium ions (orange) released
by the sarcoplasmic reticulum bind to
troponin.
D Troponin changes shape and pulls tropomyosin
away from the myosin-binding
site.
myosin head
E The myosin head binds to the nowexposed binding site.
F A cross-bridge forms
between actin and myosin.
Fig. 36-20, p. 631
Animation: Troponin and tropomyosin
36.9 Energy for Contraction
 Multiple metabolic pathways can supply the ATP
required for muscle contraction
 Muscles use any stored ATP, then transfer
phosphate from creatine phosphate to ADP to
form ATP
 With ongoing exercise, aerobic respiration and
lactic acid fermentation supply ATP
Three Metabolic Pathways Supply ATP
pathway 1
dephosphorylation of
creatine phosphate
ADP + Pi
creatine
pathway 2
aerobic respiration
oxygen
pathway 3
lactate fermentation
glucose from bloodstream and
from glycogen breakdown in cells
Fig. 36-21, p. 631
pathway 1
dephosphorylation of
creatine phosphate
ADP + Pi
creatine
pathway 2
aerobic respiration
oxygen
pathway 3
lactate fermentation
glucose from bloodstream and
from glycogen breakdown in cells
Stepped Art
Fig. 36-21, p. 631
Animation: Energy sources for
contraction
36.10 Properties of Whole Muscles
 Motor unit
• One motor neuron and all of the muscle fibers its
axons synapse with
 Muscle twitch
• Contraction produced by brief stimulation of a
motor unit
 Tetanus
• A sustained contraction caused by repeated
stimulation of a motor unit in a short interval
Muscle Twitch and Tetanus
B Repeated stimuli over a
short time have an additive
effect; they increase the
force of contraction.
Force
relaxation starts
stimulus
contraction
Force
A A single, brief stimulus
causes a twitch, a rapid
contraction followed by
immediate relaxation.
C Sustained stimulation
causes tetanus, a sustained
contraction with several
times the force of a twitch.
Force
six stimulations per second
twitch
tetanic contraction
repeated stimulation
Time
Fig. 36-22, p. 632
B Repeated stimuli over a
short time have an additive
effect; they increase the
force of contraction.
Force
relaxation starts
stimulus
contraction
Force
A A single, brief stimulus
causes a twitch, a rapid
contraction followed by
immediate relaxation.
C Sustained stimulation
causes tetanus, a sustained
contraction with several
times the force of a twitch.
Force
six stimulations per second
twitch
tetanic contraction
repeated stimulation
Time
Stepped Art
Fig. 36-22, p. 632
Animation: Types of contractions
Motor Units and Muscle Tension
 Muscle tension
• The mechanical force exerted by a muscle
• The more motor units stimulated, the greater the
muscle tension
 A load opposes muscle tension
• Isotonic contraction: muscle shorten and move
the load
• Isometric contraction: muscles tense but do not
shorten or move the load
Isotonic and Isometric Contraction
contracted
muscle can
shorten
contracted
muscle can’t
shorten
Fig. 36-23, p. 632
Fatigue, Exercise, and Aging
 Muscle fatigue
• Decrease in capacity to generate force; muscle
tension declines despite repeated stimulation
• Aerobic exercise makes muscles more resistant
to fatigue (increases blood supply, mitochondria)
• Intense exercise increases actin and myosin
 All muscle fibers form before birth; number and
size of muscle fibers decline as people age
36.11 Disruption of Muscle Contraction
 Some genetic disorders, diseases, or toxins can
cause muscles to contract too little or too much
• Muscular dystrophy (X-linked disorder)
• Motor neuron disorders (polio, ALS)
• Botulism (Clostridium botulinum toxin) and
tetanus (C. tetani toxin)
Muscular Dystrophy
 Muscle fibers break down, muscles fail – death
results from respiratory failure
Fig. 36-24a, p. 633
Fig. 36-24b, p. 633
Tetanus
 C. tetani infection, preventable by tetanus vaccine
36.7-36.11 Key Concepts
Skeletal Muscle Function
 Muscle fibers contract in response to signals
from a motor neuron
 A muscle fiber contains many myofibrils, each
divided crosswise into sarcomeres
 ATP-driven interactions between protein
filaments shorten sarcomeres, causing muscle
contraction
Animation: Muscle contraction overview
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Video: Pumping up muscles
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Animation: Structure of a sarcomere