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Muscle Tissue
• Nearly half of body's mass
• Transforms chemical energy (ATP) to
directed mechanical energy  exerts
force
• Three types
– Skeletal
– Cardiac
– Smooth
• Myo, mys, and sarco - prefixes for muscle
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Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (1 of 4)
© 2013 Pearson Education, Inc.
Special Characteristics of Muscle Tissue
• Excitability (responsiveness or irritability):
ability to receive and respond to stimuli
• Contractility: ability to shorten forcibly
when stimulated
• Extensibility: ability to be stretched
• Elasticity: ability to recoil to resting length
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Muscle Functions
• Four important functions
– Movement of bones or fluids (e.g., blood)
– Maintaining posture and body position
– Stabilizing joints
– Heat generation
Additional functions
– Protects organs, forms valves, controls pupil
size, causes "goosebumps"
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Skeletal Muscle
• Each muscle served by one artery, one
nerve, and one or more veins
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Skeletal Muscle
• Connective tissue sheaths of skeletal
muscle
– Reinforce whole muscle
– External to internal
• Epimysium: surrounding entire muscle; may blend
with fascia
• Perimysium: surrounding fascicles (groups of
muscle fibers)
• Endomysium: surrounding each muscle fiber
© 2013 Pearson Education, Inc.
Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium.
Bone
Epimysium
Epimysium
Perimysium
Tendon
Endomysium
Muscle fiber
in middle of
a fascicle
Blood vessel
Perimysium
wrapping a fascicle
Endomysium
(between individual
muscle fibers)
Muscle
fiber
Fascicle
Perimysium
© 2013 Pearson Education, Inc.
What is compartment syndrome?
Leg muscles are wrapped with dense leathery tissue
called fascia that divides them into groups called
compartments.
This dense, inelastic cover prevents muscles from bulging
during normal walking. Unfortunately, this fascial
envelope is unable to stretch to accommodate swollen
muscles.
Severe fractures, trauma, vascular injuries and electrical
injuries can produce muscle damage. As injured muscle
swells, pressure rises within the constricting
compartment.
Eventually, internal pressure rises so high that local
circulation is cut off and the affected muscle dies. The
local increased pressure can also damage associated
nerves resulting in a loss of both power and sensation.
Chilling picture – look at your own risk!
How is compartment syndrome diagnosed?
Surgeons who frequently treat lower extremity trauma
are always on the look out for the signs and symptoms
of compartment syndrome.
The development of a tense, swollen leg where
stretching the affected muscle produces a
disproportionate amount of discomfort is highly
suspicious for the presence of compartment syndrome.
Weakness and diminished sensation may also develop,
however, pulses are usually felt past the point of
irreversible damage.
Clinical suspicion may be verified with a special
pressure gauge device.
How is compartment syndrome treated?
By releasing local pressure to restore
circulation to local nerves and muscles.
Through two leg incisions the leathery
tissue that envelopes the four groups of
muscles, nerve and vessels is opened to
relieve pressure.
When the muscle swelling resolves the leg
incisions may be closed or covered with a
skin graft to achieve a healed wound.
How is compartment syndrome treated?
Compartment syndrome is treated by
releasing local pressure to restore
circulation to local nerves and muscles.
Through two leg incisions the leathery
tissue that envelopes the four groups of
muscles, nerve and vessels is opened to
relieve pressure.
When the muscle swelling resolves the leg
incisions may be closed or covered with a
skin graft to achieve a healed wound.
Skeletal Muscle: Attachments
• Attach in at least two places
– Insertion – movable bone
– Origin – immovable (less movable) bone
• Attachments direct or indirect
– Direct—epimysium fused to periosteum of
bone or perichondrium of cartilage
– Indirect—connective tissue wrappings extend
beyond muscle as ropelike tendon or
sheetlike aponeurosis
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Table 9.1 Structure and Organizational Levels of Skeletal Muscle (1 of 3)
© 2013 Pearson Education, Inc.
Table 9.1 Structure and Organizational Levels of Skeletal Muscle (2 of 3)
© 2013 Pearson Education, Inc.
Table 9.1 Structure and Organizational Levels of Skeletal Muscle (3 of 3)
© 2013 Pearson Education, Inc.
Micro Anatomy of A Skeletal Muscle Fiber
• Long, cylindrical cell
– 10 to 100 µm in diameter; up to 30 cm long
• Multiple peripheral nuclei
• Sarcolemma = plasma membrane
• Sarcoplasm = cytoplasm
– Glycosomes for glycogen storage,
myoglobin for O2 storage
• Modified structures: myofibrils,
sarcoplasmic reticulum, and T tubules
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Myofibrils
• ~80% of cell volume
• Contain sarcomeres - contractile units
– Sarcomeres contain myofilaments
• Exhibit striations - perfectly aligned
repeating dark A bands and light I bands
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Figure 9.2b Microscopic anatomy of a skeletal muscle fiber.
Diagram of part
of a muscle
fiber showing
the myofibrils.
One myofibril
extends from the
cut end of the
fiber.
Sarcolemma
Mitochondrion
Myofibril
Dark
A band
© 2013 Pearson Education, Inc.
Light Nucleus
I band
Figure 9.2c Microscopic anatomy of a skeletal muscle fiber.
Thin (actin)
filament
Small part of one
myofibril
enlarged to show
the myofilaments
responsible for the
banding pattern.
Thick
Each sarcomere
extends from one Z (myosin)
filament
disc to the next.
© 2013 Pearson Education, Inc.
Z disc
I band
H zone
Z disc
I band
A band
Sarcomere
M line
Figure 9.2d Microscopic anatomy of a skeletal muscle fiber.
Z disc
Enlargement of
one sarcomere
(sectioned lengthwise). Notice the
myosin heads on
the thick filaments.
© 2013 Pearson Education, Inc.
Sarcomere
M line
Z disc
Thin
(actin)
filament
Elastic
(titin)
filaments
Thick
(myosin)
filament
Myofibril Banding Pattern
• Orderly arrangement of actin and myosin
myofilaments within sarcomere
– Actin myofilaments = thin filaments
• Anchored to Z discs
– Myosin myofilaments = thick filaments
© 2013 Pearson Education, Inc.
Thick Filament
• Composed of protein myosin
– Myosin heads contain 2 light polypeptide
chains that act as cross bridges during
contraction
• Binding sites for actin of thin filaments
• Binding sites for ATP
• ATPase enzymes
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Thin Filament
• Twisted double strand of protein F actin
consists of G (globular) actin
• G actin bears active sites for myosin head
attachment during contraction
• Tropomyosin and troponin - regulatory
proteins bound to actin
© 2013 Pearson Education, Inc.
Figure 9.3 Composition of thick and thin filaments.
Longitudinal section of filaments within one
sarcomere of a myofibril
Thick filament
Thin filament
In the center of the sarcomere, the thick filaments
lack myosin heads. Myosin heads are present only
in areas of myosin-actin overlap.
Thick filament.
Thin filament
Each thick filament consists of many myosin
molecules whose heads protrude at opposite ends
of the filament.
Portion of a thick filament
Myosin head
A thin filament consists of two strands of actin
subunits twisted into a helix plus two types of
regulatory proteins (troponin and tropomyosin).
Portion of a thin filament
Tropomyosin
Troponin Actin
Actin-binding sites
Heads
ATPbinding
site Flexible hinge region
Myosin molecule
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Tail
Active sites
for myosin
attachment
Actin subunits
Actin subunits
Structure of Myofibril
• Elastic filament
– Composed of protein titin
– Holds thick filaments in place; helps recoil
after stretch; resists excessive stretching
• Dystrophin-Links thin filaments to sarcolemma
– Skeletal and cardiac muscle without enough
dystrophin become damaged as muscles
contract and relax. Damaged cells weaken
and die over time, causing muscle weakness
and heart problems seen in Duchenne
muscular dystrophy.
© 2013 Pearson Education, Inc.
Sarcoplasmic Reticulum (SR)
• Network of smooth ER surround myofibrils
• Terminal cisternae form perpendicular
cross channels
• Functions in regulation of intracellular Ca2+
levels
– Stores and releases Ca2+
© 2013 Pearson Education, Inc.
Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle.
Part of a skeletal
muscle fiber (cell)
I band
Z disc
Myofibril
Sarcolemma
A band
H zone
M
line
I band
Z disc
Sarcolemma
Triad:
• T tubule
• Terminal
cisterns of
the SR (2)
Tubules of
the SR
Myofibrils
Mitochondria
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Sliding Filament Model of Contraction
• Generation of force
• Does not necessarily cause shortening of
fiber
• Shortening occurs when tension
generated by cross bridges on thin
filaments exceeds forces opposing
shortening
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Sliding Filament Model of Contraction
• In relaxed state, thin and thick filaments
overlap only at ends of A band
• Sliding filament model of contraction
– During contraction, thin filaments slide past
thick filaments  actin and myosin overlap
more
– Occurs when myosin heads bind to actin 
cross bridges
© 2013 Pearson Education, Inc.
Figure 9.6 Sliding filament model of contraction.
Slide 2
1 Fully relaxed sarcomere of a muscle fiber
Z
I
© 2013 Pearson Education, Inc.
H
A
Z
I
Figure 9.6 Sliding filament model of contraction.
Slide 3
2 Fully contracted sarcomere of a muscle fiber
Z
© 2013 Pearson Education, Inc.
I
Z
A
I
Physiology of Skeletal Muscle Fibers
• For skeletal muscle to contract
– Activation (at neuromuscular
junction)
• Nervous system stimulation must generate
action potential (AP) in sarcolemma
– Excitation-contraction coupling
• AP propagated along sarcolemma
• Intracellular Ca2+ levels must rise briefly
© 2013 Pearson Education, Inc.
Figure 9.7 The phases leading to muscle fiber contraction.
Action potential (AP) arrives at axon
terminal at neuromuscular junction
ACh released; binds to receptors
on sarcolemma
Phase 1
Motor neuron
stimulates
muscle fiber
(see Figure 9.8).
Ion permeability of sarcolemma changes
Local change in membrane voltage
(depolarization) occurs
Local depolarization (end plate
potential) ignites AP in sarcolemma
AP travels across the entire sarcolemma
AP travels along T tubules
Phase 2:
Excitation-contraction
coupling occurs (see
Figures 9.9 and 9.11).
SR releases Ca2+; Ca2+ binds to
troponin; myosin-binding sites
(active sites) on actin exposed
Myosin heads bind to actin;
contraction begins
© 2013 Pearson Education, Inc.
Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
Slide 2
1 Action potential arrives at axon
terminal of motor neuron.
Synaptic vesicle
containing ACh
Axon terminal
of motor neuron
Synaptic
cleft
Fusing synaptic
vesiclesa
ACh
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
© 2013 Pearson Education, Inc.
Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
Slide 3
1 Action potential arrives at axon
terminal of motor neuron.
2 Voltage-gated Ca2+ channels
open. Ca2+ enters the axon terminal
moving down its electochemical
gradient.
Synaptic vesicle
containing ACh
Axon terminal
of motor neuron
Synaptic
cleft
Fusing synaptic
vesiclesa
ACh
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
© 2013 Pearson Education, Inc.
Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
Slide 4
1 Action potential arrives at axon
terminal of motor neuron.
2 Voltage-gated Ca2+ channels
open. Ca2+ enters the axon terminal
moving down its electochemical
gradient.
3 Ca2+ entry causes ACh (a
neurotransmitter) to be released
by exocytosis.
Synaptic vesicle
containing ACh
Axon terminal
of motor neuron
Synaptic
cleft
Fusing synaptic
vesiclesa
ACh
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
© 2013 Pearson Education, Inc.
Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
Slide 5
1 Action potential arrives at axon
terminal of motor neuron.
2 Voltage-gated Ca2+ channels
open. Ca2+ enters the axon terminal
moving down its electochemical
gradient.
3 Ca2+ entry causes ACh (a
neurotransmitter) to be released
by exocytosis.
4 ACh diffuses across the synaptic
cleft and binds to its receptors on
the sarcolemma.
© 2013 Pearson Education, Inc.
Synaptic vesicle
containing ACh
Axon terminal
of motor neuron
Synaptic
cleft
Fusing synaptic
vesiclesa
ACh
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
5 ACh binding opens ion
channels in the receptors that
allow simultaneous passage of
Na+ into the muscle fiber and K+
out of the muscle fiber. More Na+
ions enter than K+ ions exit,
which produces a local change
in the membrane potential called
the end plate potential.
© 2013 Pearson Education, Inc.
Postsynaptic membrane
ion channel opens;
ions pass.
Slide 6
Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
6 ACh effects are terminated by
its breakdown in the synaptic
cleft by acetylcholinesterase and
diffusion away from the junction.
ACh
Degraded ACh
Acetylcholinesterase
© 2013 Pearson Education, Inc.
Ion channel closes;
ions cannot pass.
Slide 7
Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
Myelinated axon
of motor neuron
Action
potential (AP)
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
1 Action potential arrives at axon
terminal of motor neuron.
2 Voltage-gated Ca2+ channels
open. Ca2+ enters the axon terminal
moving down its electochemical
gradient.
Synaptic vesicle
containing ACh
Axon terminal
of motor neuron
Fusing synaptic
vesicles
3 Ca2+ entry causes ACh (a
neurotransmitter) to be released
by exocytosis.
ACh
4 ACh diffuses across the synaptic
cleft and binds to its receptors on
the sarcolemma.
5 ACh binding opens ion
channels in the receptors that
allow simultaneous passage of
Na+ into the muscle fiber and K+
out of the muscle fiber. More Na+
ions enter than K+ ions exit,
which produces a local change
in the membrane potential called
the end plate potential.
© 2013 Pearson Education, Inc.
6 ACh effects are terminated by
its breakdown in the synaptic
cleft by acetylcholinesterase and
diffusion away from the junction.
Synaptic
cleft
Junctional
folds of
sarcolemma
Sarcoplasm of
muscle fiber
Postsynaptic
membrane
ion channel opens;
ions pass.
ACh
Acetylcholinesterase
Degraded ACh
Ion channel closes;
ions cannot pass.
Slide 8
Destruction of Acetylcholine
• ACh effects quickly terminated by enzyme
acetylcholinesterase in synaptic cleft
– Breaks down ACh to acetic acid and choline
– Prevents continued muscle fiber contraction in
absence of additional stimulation
© 2013 Pearson Education, Inc.
Figure 9.9 Summary of events in the generation and propagation of an action potential in a skeletal
muscle fiber.
ACh-containing
synaptic vesicle
Ca2+
Ca2+
Axon terminal of
neuromuscular
junction
Synaptic
cleft
Wave of
depolarization
1 An end plate potential is generated at the
neuromuscular junction (see Figure 9.8).
© 2013 Pearson Education, Inc.
Slide 2
Figure 9.9 Summary of events in the generation and propagation of an action potential in a skeletal
muscle fiber.
Open Na+
Closed K+
channel
Slide 3
channel
Na+
+ + + +
+ + + ++ ++ +
ACh-containing
synaptic vesicle
Ca2+
Ca2+
Axon terminal of
neuromuscular
junction
Synaptic
cleft
K+
Action potential
2 Depolarization: Generating and propagating an action
potential (AP). The local depolarization current spreads to adjacent
areas of the sarcolemma. This opens voltage-gated sodium channels
there, so Na+ enters following its electrochemical gradient and initiates
the AP. The AP is propagated as its local depolarization wave spreads to
adjacent areas of the sarcolemma, opening voltage-gated channels there.
Again Na+ diffuses into the cell following its electrochemical gradient.
Wave of
depolarization
1 An end plate potential is generated at the
neuromuscular junction (see Figure 9.8).
© 2013 Pearson Education, Inc.
+ + + +
+ + + +
Figure 9.9 Summary of events in the generation and propagation of an action potential in a skeletal
muscle fiber.
Open Na+
Closed K+
channel
Slide 4
channel
Na+
+ + + +
+ + + ++ ++ +
ACh-containing
synaptic vesicle
Ca2+
Ca2+
K+
Axon terminal of
neuromuscular
junction
Synaptic
cleft
+ + + +
+ + + +
Action potential
2 Depolarization: Generating and propagating an action
potential (AP). The local depolarization current spreads to adjacent
areas of the sarcolemma. This opens voltage-gated sodium channels
there, so Na+ enters following its electrochemical gradient and initiates
the AP. The AP is propagated as its local depolarization wave spreads to
adjacent areas of the sarcolemma, opening voltage-gated channels there.
Again Na+ diffuses into the cell following its electrochemical gradient.
Wave of
depolarization
Closed Na+
channel
1 An end plate potential is generated at the
neuromuscular junction (see Figure 9.8).
Open K+
channel
Na+
+ + + + + + + +
+ + + +
+ + + + + + + ++ +
K+
© 2013 Pearson Education, Inc.
3 Repolarization: Restoring the sarcolemma to its initial
polarized state (negative inside, positive outside). Repolarization
occurs as Na+ channels close (inactivate) and voltage-gated K+ channels
open. Because K+ concentration is substantially higher inside the cell
than in the extracellular fluid, K+ diffuses rapidly out of the muscle fiber.
Membrane potential (mV)
Figure 9.10 Action potential tracing indicates changes in Na+ and K+ ion channels.
+30
0
Na+ channels
close, K+ channels
open
Depolarization
due to Na+ entry
Repolarization
due to K+ exit
Na+
channels
open
K+ channels
closed
–95
0
© 2013 Pearson Education, Inc.
5
10
Time (ms)
15
20
Excitation-Contraction (E-C) Coupling
• Events that transmit AP along sarcolemma
lead to sliding of myofilaments
• AP brief; ends before contraction
– Causes rise in intracellular Ca2+ which 
contraction
• Latent period
– Time when E-C coupling events occur
– Time between AP initiation and beginning of
contraction
© 2013 Pearson Education, Inc.
Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an
action potential along the sarcolemma leads to the sliding of myofilaments.
Setting the stage
The events at the neuromuscular junction (NMJ)
set the stage for E-C coupling by providing
excitation. Released acetylcholine binds to
receptor proteins on the sarcolemma and triggers
an action potential in a muscle fiber.
Axon terminal of
motor neuron at NMJ
Action potential is
generated
Synaptic
cleft
ACh
Muscle
fiber
Sarcolemma
T tubule
Terminal
cistern
of SR
Triad
One sarcomere
One myofibril
© 2013 Pearson Education, Inc.
Slide 2
Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an
action potential along the sarcolemma leads to the sliding of myofilaments.
Steps in E-C Coupling:
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
1 The action potential (AP)
propagates along the
sarcolemma and down the
T tubules.
2 Calcium ions are released.
Transmission of the AP along the
T tubules of the triads causes the
voltage-sensitive tubule proteins to
change shape. This shape change
opens the Ca2+ release channels in
the terminal cisterns of the
sarcoplasmic reticulum (SR),
allowing Ca2+ to flow into the
cytosol.
Ca2+
release
channel
Terminal
cistern
of SR
Actin
Troponin
Tropomyosin
blocking active sites
Myosin
Active sites exposed and
ready for myosin binding
Myosin
cross
bridge
© 2013 Pearson Education, Inc.
3 Calcium binds to
troponin and removes
the blocking action of
tropomyosin. When Ca2+
binds, troponin changes
shape, exposing binding
sites for myosin (active
sites) on the thin filaments.
4 Contraction begins:
Myosin binding to actin
forms cross bridges and
contraction (cross bridge
cycling) begins. At this
point, E-C coupling is over.
The aftermath
When the muscle AP ceases, the voltage-sensitive tubule proteins return to
their original shape, closing the Ca2+ release channels of the SR. Ca2+
levels in the sarcoplasm fall as Ca2+ is continually pumped back into the
SR by active transport. Without Ca2+, the blocking action of tropomyosin is
restored, myosin-actin interaction is inhibited, and relaxation occurs. Each
time an AP arrives at the neuromuscular junction, the sequence of
E-C coupling is repeated.
Slide 3
Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an
action potential along the sarcolemma leads to the sliding of myofilaments.
Slide 4
Steps in E-C Coupling:
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
Ca2+
release
channel
Terminal
cistern
of SR
© 2013 Pearson Education, Inc.
1 The action potential (AP)
propagates along the
sarcolemma and down the
T tubules.
Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an
action potential along the sarcolemma leads to the sliding of myofilaments.
Slide 5
Steps in E-C Coupling:
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
Ca2+
release
channel
Terminal
cistern
of SR
© 2013 Pearson Education, Inc.
1 The action potential (AP)
propagates along the
sarcolemma and down the
T tubules.
2 Calcium ions are released.
Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an
action potential along the sarcolemma leads to the sliding of myofilaments.
Actin
Troponin
The aftermath
© 2013 Pearson Education, Inc.
Tropomyosin
blocking active sites
Myosin
Slide 6
Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an
action potential along the sarcolemma leads to the sliding of myofilaments.
Slide 7
Actin
Troponin
Tropomyosin
blocking active sites
Myosin
Active sites exposed and
ready for myosin binding
The aftermath
© 2013 Pearson Education, Inc.
3 Calcium binds to
troponin and removes
the blocking action of
tropomyosin. When Ca2+
binds, troponin changes
shape, exposing binding
sites for myosin (active
sites) on the thin filaments.
Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an
action potential along the sarcolemma leads to the sliding of myofilaments.
Slide 8
Actin
Troponin
Tropomyosin
blocking active sites
Myosin
Active sites exposed and
ready for myosin binding
Myosin
cross
bridge
The aftermath
© 2013 Pearson Education, Inc.
3 Calcium binds to
troponin and removes
the blocking action of
tropomyosin. When Ca2+
binds, troponin changes
shape, exposing binding
sites for myosin (active
sites) on the thin filaments.
4 Contraction begins:
Myosin binding to actin
forms cross bridges and
contraction (cross bridge
cycling) begins. At this
point, E-C coupling is over.
Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an
action potential along the sarcolemma leads to the sliding of myofilaments.
Steps in E-C Coupling:
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
2 Calcium ions are released.
Transmission of the AP along the
T tubules of the triads causes the
voltage-sensitive tubule proteins to
change shape. This shape change
opens the Ca2+ release channels in
the terminal cisterns of the
sarcoplasmic reticulum (SR),
allowing Ca2+ to flow into the
cytosol.
Ca2+
release
channel
PLAY
Terminal
cistern
of SR
A&P Flix™:
Excitationcontraction
coupling.
Actin
Troponin
Tropomyosin
blocking active sites
Myosin
Active sites exposed and
ready for myosin binding
Myosin
cross
bridge
© 2013 Pearson Education, Inc.
1 The action potential (AP)
propagates along the
sarcolemma and down the
T tubules.
3 Calcium binds to
troponin and removes
the blocking action of
tropomyosin. When Ca2+
binds, troponin changes
shape, exposing binding
sites for myosin (active
sites) on the thin filaments.
4 Contraction begins:
Myosin binding to actin
forms cross bridges and
contraction (cross bridge
cycling) begins. At this
point, E-C coupling is over.
The aftermath
When the muscle AP ceases, the voltage-sensitive tubule proteins return to
their original shape, closing the Ca2+ release channels of the SR. Ca2+
levels in the sarcoplasm fall as Ca2+ is continually pumped back into the
SR by active transport. Without Ca2+, the blocking action of tropomyosin is
restored, myosin-actin interaction is inhibited, and relaxation occurs. Each
time an AP arrives at the neuromuscular junction, the sequence of
E-C coupling is repeated.
Slide 9
Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an
action potential along the sarcolemma leads to the sliding of myofilaments.
Slide 10
Steps in E-C Coupling:
Setting the stage
The events at the neuromuscular
junction (NMJ) set the stage for
E-C coupling by providing
excitation. Released acetylcholine
binds to receptor proteins on the
sarcolemma and triggers an action
potential in a muscle fiber.
Synaptic
cleft
Voltage-sensitive
tubule protein
Sarcolemma
T tubule
1 The action potential (AP)
propagates along the
sarcolemma and down the
T tubules.
2 Calcium ions are released.
Transmission of the AP along the
T tubules of the triads causes the
voltage-sensitive tubule proteins to
change shape. This shape change
opens the Ca2+ release channels in
the terminal cisterns of the
sarcoplasmic reticulum (SR),
allowing Ca2+ to flow into the
cytosol.
Ca2+
release
channel
Terminal
cistern
of SR
Axon terminal of
motor neuron at NMJ
Action potential
is generated
ACh
Actin
Sarcolemma
Troponin
T tubule
Terminal
cistern
of SR
Muscle fiber
Tropomyosin
blocking active sites
Myosin
Triad
Active sites exposed and
ready for myosin binding
One sarcomere
One myofibril
Myosin
cross
bridge
3 Calcium binds to
troponin and removes
the blocking action of
tropomyosin. When Ca2+
binds, troponin changes
shape, exposing binding
sites for myosin (active
sites) on the thin filaments.
4 Contraction begins:
Myosin binding to actin
forms cross bridges and
contraction (cross bridge
cycling) begins. At this
point, E-C coupling is over.
The aftermath
When the muscle AP ceases, the voltage-sensitive tubule proteins return
to their original shape, closing the Ca2+ release channels of the SR. Ca2+
levels in the sarcoplasm fall as Ca2+ is continually pumped back into the
SR by active transport. Without Ca2+, the blocking action of tropomyosin
is restored, myosin-actin interaction is inhibited, and relaxation occurs.
Each time an AP arrives at the neuromuscular junction, the sequence of
E-C coupling is repeated.
© 2013 Pearson Education, Inc.
Role of Calcium (Ca2+) in Contraction
• At low intracellular Ca2+ conc
– Tropomyosin blocks active sites on actin
– Myosin heads cannot attach to actin
– Muscle fiber relaxed
© 2013 Pearson Education, Inc.
Role of Calcium (Ca2+) in Contraction
• At higher intracellular Ca2+ conc
– Ca2+ binds to troponin
• Troponin changes shape and moves tropomyosin
away from myosin-binding sites
• Myosin heads bind to actin, causing sarcomere
shortening and muscle contraction
– When nerve stimulation ceases, Ca2+ pumped
back into SR and contraction ends
© 2013 Pearson Education, Inc.
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Actin
Myosin
cross bridge
Slide 2
Thin filament
Thick
filament
Myosin
1 Cross bridge formation.
Energized myosin head attaches
to an actin myofilament, forming
a cross bridge.
© 2013 Pearson Education, Inc.
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Slide 3
2 The power (working) stroke. ADP
and Pi are released and the myosin head
pivots and bends, changing to its bent
low-energy state. As a result it pulls the
actin filament toward the M line.
© 2013 Pearson Education, Inc.
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Slide 4
3 Cross bridge detachment. After ATP
attaches to myosin, the link between myosin
and actin weakens, and the myosin head
detaches (the cross bridge “breaks”).
© 2013 Pearson Education, Inc.
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Slide 5
ATP
hydrolysis
4 Cocking of the myosin head.
*This cycle will continue as long
as ATP is available and Ca2+ is
bound to troponin.
© 2013 Pearson Education, Inc.
As ATP is hydrolyzed to ADP and Pi,
the myosin head returns to its
prestroke high-energy, or “cocked,”
position. *
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Actin
Ca2+ Thin filament
Myosin
cross bridge
PLAY
A&P Flix™: The
Cross Bridge
Cycle
Slide 6
Thick
filament
Myosin
1 Cross bridge formation.
Energized myosin head attaches
to an actin myofilament, forming
a cross bridge.
ATP
hydrolysis
4 Cocking of the myosin head.
As ATP is hydrolyzed to ADP and Pi,
the myosin head returns to its
prestroke high-energy, or “cocked,”
position. *
*This cycle will continue as long
as ATP is available and Ca2+ is
bound to troponin.
© 2013 Pearson Education, Inc.
2 The power (working) stroke. ADP
and Pi are released and the myosin head
pivots and bends, changing to its bent
low-energy state. As a result it pulls the
actin filament toward the M line.
In the absence
of ATP, myosin
heads will not
detach, causing
rigor mortis.
3 Cross bridge detachment. After ATP
attaches to myosin, the link between myosin
and actin weakens, and the myosin head
detaches (the cross bridge “breaks”).
Homeostatic Imbalance
• Rigor mortis
– Cross bridge detachment requires ATP
– 3–4 hours after death muscles begin to stiffen
with weak rigidity at 12 hours post mortem
• Dying cells take in calcium  cross bridge
formation
• No ATP generated to break cross bridges
© 2013 Pearson Education, Inc.