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Barbara Heard,
Atlantic Cape Community
College
CHAPTER
9
Muscles and
Muscle
Tissue: Part A
© Annie Leibovitz/Contact Press Images
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Types of Muscle Tissue
• Skeletal muscles
– Organs attached to bones and skin
– Elongated cells called muscle fibers
– Striated (striped)
– Voluntary (i.e., conscious control)
– Contract rapidly; tire easily; powerful
– Require nervous system stimulation
© 2013 Pearson Education, Inc.
Types of Muscle Tissue
• Cardiac muscle
– Only in heart; bulk of heart walls
– Striated
– Can contract without nervous system
stimulation
– Involuntary
© 2013 Pearson Education, Inc.
Types of Muscle Tissue
• Smooth muscle
– In walls of hollow organs, e.g., stomach,
urinary bladder, and airways
– Not striated
– Can contract without nervous system
stimulation
– Involuntary
© 2013 Pearson Education, Inc.
Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (1 of 4)
© 2013 Pearson Education, Inc.
Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (2 of 4)
© 2013 Pearson Education, Inc.
Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (3 of 4)
© 2013 Pearson Education, Inc.
Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (4 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
© 2013 Pearson Education, Inc.
Muscle Functions
• Four important functions
– Movement of bones or fluids (e.g., blood)
– Maintaining posture and body position
– Stabilizing joints
– Heat generation (especially skeletal muscle)
• Additional functions
– Protects organs, forms valves, controls pupil
size, causes "goosebumps"
© 2013 Pearson Education, Inc.
Skeletal Muscle
• Each muscle served by one artery, one
nerve, and one or more veins
– Huge nutrient and oxygen need; generates
large amount of waste
© 2013 Pearson Education, Inc.
Skeletal Muscle
• Connective tissue sheaths of skeletal
muscle
– Support cells; reinforce whole muscle
– External to internal
• Epimysium: dense irregular connective tissue
surrounding entire muscle; may blend with fascia
• Perimysium: fibrous connective tissue
surrounding fascicles (groups of muscle fibers)
• Endomysium: fine areolar connective tissue
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.
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
© 2013 Pearson Education, Inc.
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.
Microscopic 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
© 2013 Pearson Education, Inc.
Myofibrils
• Densely packed, rodlike elements
• ~80% of cell volume
• Contain sarcomeres - contractile units
– Sarcomeres contain myofilaments
• Exhibit striations - perfectly aligned
repeating series of dark A bands and light I
bands
© 2013 Pearson Education, Inc.
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
Striations
• Thick filaments: run entire length of an A band
• Thin filaments: run length of I band and
partway into A band
• Sarcomere: smallest contractile unit
© 2013 Pearson Education, Inc.
Sarcomere
• Smallest contractile unit (functional unit) of
muscle fiber
• Align along myofibril like boxcars of train
• Composed of thick and thin myofilaments
made of contractile proteins
© 2013 Pearson Education, Inc.
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
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
© 2013 Pearson Education, Inc.
Tail
Active sites
for myosin
attachment
Actin subunits
Actin subunits
Myofibril Banding Pattern
• Orderly arrangement of actin and myosin
myofilaments within sarcomere
– Actin myofilaments = thin filaments
– Myosin myofilaments = thick filaments
© 2013 Pearson Education, Inc.
Ultrastructure of Thick Filament
• Composed of protein myosin
– Myosin tails
– Myosin heads
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Ultrastructure of Thin Filament
• Twisted double strand of fibrous protein
F actin
• F actin consists of G (globular) actin
subunits
• 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
© 2013 Pearson Education, Inc.
Tail
Active sites
for myosin
attachment
Actin subunits
Actin subunits
Sarcoplasmic Reticulum (SR)
• Network of smooth endoplasmic reticulum
surrounding each myofibril
• 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
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Sliding Filament Model of Contraction
• 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.
Sliding Filament Model of Contraction
• Myosin heads bind to actin; sliding begins
• Cross bridges form and break several
times, ratcheting thin filaments toward
center of sarcomere
– Causes shortening of muscle fiber
© 2013 Pearson Education, Inc.
Figure 9.6 Sliding filament model of contraction.
Slide 1
1 Fully relaxed sarcomere of a muscle fiber
H
A
Z
I
Z
I
2 Fully contracted sarcomere of a muscle fiber
Z
Z
© 2013 Pearson Education, Inc.
I
A
I
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
Figure 9.6 Sliding filament model of contraction.
Slide 4
1 Fully relaxed sarcomere of a muscle fiber
H
A
Z
I
Z
I
2 Fully contracted sarcomere of a muscle fiber
Z
Z
© 2013 Pearson Education, Inc.
I
A
I
Physiology of Skeletal Muscle Fibers
• For skeletal muscle to contract
– Activation (at neuromuscular
junction)
• Must be nervous system stimulation
• Must generate action potential in
sarcolemma
– Excitation-contraction coupling
• Action potential 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.
The Nerve Stimulus and Events at the
Neuromuscular Junction
• Skeletal muscles stimulated by somatic
motor neurons
• Axons of motor neurons travel from central
nervous system via nerves to skeletal
muscle
• Each axon forms several branches as it
enters muscle
• Each axon ending forms neuromuscular
junction with single muscle fiber
– Usually only one per muscle fiber
© 2013 Pearson Education, Inc.
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 1
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
Neuromuscular Junction (NMJ)
• Situated midway along length of muscle fiber
• Axon terminal and muscle fiber separated by
gel-filled space called synaptic cleft
© 2013 Pearson Education, Inc.
Events at the Neuromuscular Junction
• Nerve impulse arrives at axon terminal 
ACh released into synaptic cleft
• ACh diffuses across cleft and binds with
receptors on sarcolemma 
• Electrical events  generation of action
potential
PLAY
A&P Flix™: Events at the Neuromuscular Junction
© 2013 Pearson Education, Inc.
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.
Generation of an Action Potential
• Resting sarcolemma polarized
– Voltage across membrane
• Action potential caused by changes in
electrical charges
• Occurs in three steps
– End plate potential
– Depolarization
– Repolarization
© 2013 Pearson Education, Inc.
Generation of an Action Potential Across
the Sarcolemma
• End plate potential (local depolarization)
– More Na+ diffuses in, so interior of
sarcolemma becomes less negative
© 2013 Pearson Education, Inc.
Events in Generation of an Action Potential
• Depolarization –
• Na+ influx decreases membrane voltage
toward critical voltage called threshold
– If threshold reached, AP initiated
– Once initiated, is unstoppable  muscle fiber
contraction
© 2013 Pearson Education, Inc.
Events in Generation of an Action Potential
• Repolarization – restoring electrical
conditions of RMP
– Na+ channels close and voltage-gated K+
channels open
– K+ efflux rapidly restores resting polarity
– Fiber cannot be stimulated - in refractory
period until repolarization complete
– Ionic conditions of resting state restored by
Na+-K+ pump
© 2013 Pearson Education, Inc.
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
© 2013 Pearson Education, Inc.
Events of Excitation-Contraction (E-C)
Coupling
• AP propagated along sarcomere
• Voltage-sensitive proteins stimulate Ca2+
release from SR
– Ca2+ necessary for 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.
Slide 1
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.
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.
Channels Involved in Initiating Muscle
Contraction
• Nerve impulse reaches axon terminal  voltagegated calcium channels open 
ACh released to synaptic cleft
• ACh binds to its receptors on sarcolemma 
opens ligand-gated Na+ and K+ channels  end
plate potential 
• Opens voltage-gated Na+ channels  AP
propagation 
• Voltage-sensitive proteins in T tubules change
shape  SR releases Ca2+ to cytosol
© 2013 Pearson Education, Inc.
Role of Calcium (Ca2+) in Contraction
• At low intracellular Ca2+ concentration
– 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+ concentrations
– 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 nervous stimulation ceases, Ca2+
pumped back into SR and contraction ends
© 2013 Pearson Education, Inc.
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.