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PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College
C H A P T E R
Copyright © 2010 Pearson Education, Inc.
9 Muscles and Muscle Tissue: Part A
Copyright © 2010 Pearson Education, Inc.
Warm Up 12/12/16
•Describe the major differences between cardiac, skeletal and smooth muscle in terms of appearance, control, function, and location!
Copyright © 2010 Pearson Education, Inc.
Three Types of Muscle Tissue
1. Skeletal muscle tissue:• Attached to bones and skin
• Striated
• Voluntary (i.e., conscious control)
• Powerful
• Primary topic of this chapter
Copyright © 2010 Pearson Education, Inc.
Three Types of Muscle Tissue
2. Cardiac muscle tissue:• Only in the heart
• Striated
• Involuntary
• More details in Chapter 18
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Three Types of Muscle Tissue
3. Smooth muscle tissue:• In the walls of hollow organs, e.g., stomach,
urinary bladder, and airways
• Not striated
• Involuntary
• More details later in this chapter
Copyright © 2010 Pearson Education, Inc. Table 9.3
Copyright © 2010 Pearson Education, Inc.
Special Characteristics of Muscle Tissue
•Excitability (responsiveness or irritability): ability to receive and respond to stimuli
•Contractility: ability to shorten when stimulated
•Extensibility: ability to be stretched
•Elasticity: ability to recoil to resting length
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Muscle Functions
1. Movement of bones or fluids (e.g., blood)
2. Maintaining posture and body position
3. Stabilizing joints
4. Heat generation (especially skeletal muscle)
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Skeletal Muscle
•Each muscle is served by one artery, one nerve, and one or more veins
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Skeletal Muscle
•Connective tissue sheaths of skeletal muscle:• Epimysium: dense regular connective tissue
surrounding entire muscle • Perimysium: fibrous connective tissue surrounding
fascicles (groups of muscle fibers)• Endomysium: fine areolar connective tissue
surrounding each muscle fiber
Copyright © 2010 Pearson Education, Inc. Figure 9.1
Bone
Perimysium
Endomysium(between individualmuscle fibers)
Muscle fiber
Fascicle(wrapped by perimysium)
Epimysium
Tendon
Epimysium
Muscle fiberin middle ofa fascicleBlood
vessel
PerimysiumEndomysium
Fascicle(a
)
(b)
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Skeletal Muscle: Attachments
•Muscles attach:• Directly—epimysium of muscle is fused to the
periosteum of bone or perichondrium of cartilage
• Indirectly—connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheetlike aponeurosis
Copyright © 2010 Pearson Education, Inc. Table 9.1
Copyright © 2010 Pearson Education, Inc.
Microscopic Anatomy of a Skeletal Muscle Fiber•Cylindrical cell 10 to 100 μm in diameter, up to 30 cm long
•Multiple peripheral nuclei
•Many mitochondria
•Glycosomes for glycogen storage, myoglobin for O2 storage
•Also contain myofibrils, sarcoplasmic reticulum, and T tubules
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Myofibrils
•Densely packed, rodlike elements
• ~80% of cell volume
•Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands
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NucleusLight I band
Dark A band
Sarcolemma
Mitochondrion
(b) Diagram of part of a muscle fiber showing the myofibrils. Onemyofibril is extended afrom the cut end of the fiber.
Myofibril
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Sarcomere
•Smallest contractile unit (functional unit) of a muscle fiber
•The region of a myofibril between two successive Z discs
•Composed of thick and thin myofilaments made of contractile proteins
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Features of a Sarcomere
• Thick filaments: run the entire length of an A band• Thin filaments: run the length of the I band and partway into the A band• Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another•H zone: lighter midregion where filaments do not overlap •M line: line of protein myomesin that holds adjacent thick filaments together
Copyright © 2010 Pearson Education, Inc. Figure 9.2c, d
I band I bandA bandSarcomere
H zoneThin (actin)filament
Thick (myosin)filament
Z disc Z disc
M line
(c)
Small part of one myofibril enlarged to show the myofilamentsresponsible for the banding pattern. Each sarcomere extends fromone Z disc to the next.
Z disc Z discM lineSarcomere
Thin (actin)filament
Thick(myosin)filament
Elastic (titin)filaments
(d)
Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments.
Copyright © 2010 Pearson Education, Inc.
Ultrastructure of Thick Filament
•Composed of the protein myosin• Myosin tails contain:
• 2 interwoven, heavy polypeptide chains
• Myosin heads contain:
• 2 smaller, 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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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
Copyright © 2010 Pearson Education, Inc. Figure 9.3
Flexible hinge region
Tail
Tropomyosin
Troponin
Actin
Myosin head
ATP-bindingsite
Heads
Active sitesfor myosinattachment
Actinsubunits
Actin-binding sites
Thick filamentEach thick filament consists of manymyosin molecules whose heads protrude at opposite ends of the filament.
Thin filamentA thin filament consists of two strandsof actin subunits twisted into a helix plus two types of regulatory proteins(troponin and tropomyosin).
Thin filament
Thick filament
In the center of the sarcomere, the thickfilaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap.
Longitudinal section of filamentswithin one sarcomere of a myofibril
Portion of a thick filament Portion of a thin
filament
Myosin molecule
Actin subunits
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Sarcoplasmic Reticulum (SR)
•Network of smooth endoplasmic reticulum surrounding each myofibril
•Pairs of terminal cisternae form perpendicular cross channels
•Functions in the regulation of intracellular Ca2+ levels
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T Tubules
•Continuous with the sarcolemma
•Penetrate the cell’s interior at each A band–I band junction
•Associate with the paired terminal cisternae to form triads that encircle each sarcomere
Copyright © 2010 Pearson Education, Inc. Figure 9.5
Myofibril
Myofibrils
Triad:
Tubules ofthe SR
Sarcolemma
Sarcolemma
Mitochondria
I band I bandA bandH zone
Z disc
Z disc
Part of a skeletalmuscle fiber (cell)
• T tubule• Terminalcisternaeof the SR (2)
M line
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Triad Relationships
•T tubules conduct impulses deep into muscle fiber
• Integral proteins protrude into the intermembrane space from T tubule and SR cisternae membranes
•T tubule proteins: voltage sensors
•SR foot proteins: gated channels that regulate Ca2+ release from the SR cisternae
Copyright © 2010 Pearson Education, Inc.
Copyright © 2010 Pearson Education, Inc.
9A-2 Notes:
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Warm Up! Explain the role and function of the T tubules and sarcoplasmic reticulum in relation to muscle contraction.
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Contraction
•The generation of force
•Does not necessarily cause shortening of the fiber
•Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening
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Sliding Filament Model of Contraction
• In the relaxed state, thin and thick filaments overlap only slightly
•During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line
•As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens
Copyright © 2010 Pearson Education, Inc. Figure 9.6
I
Fully relaxed sarcomere of a muscle fiber
Fully contracted sarcomere of a muscle fiber
IAZ ZH
I IAZ Z
1
2
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Requirements for Skeletal Muscle Contraction1. Activation: neural stimulation at a
neuromuscular junction
2. Excitation-contraction coupling: • Generation and propagation of an action
potential along the sarcolemma
• Final trigger: a brief rise in intracellular Ca2+ levels
Copyright © 2010 Pearson Education, Inc.
Events at the Neuromuscular Junction
•Skeletal muscles are stimulated by somatic motor neurons •Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles•Each axon forms several branches as it enters a muscle •Each axon ending forms a neuromuscular junction with a single muscle fiber
Copyright © 2010 Pearson Education, Inc.
Figure 9.8
Nucleus
Actionpotential (AP)
Myelinated axonof motor neuron
Axon terminal ofneuromuscular junction
Sarcolemma ofthe muscle fiber
Ca2
+
Ca2+
Axon terminalof motor neuron
Synaptic vesicle containing AChMitochondrionSynapticcleft
Fusing synaptic vesicles
1 Action potential arrives ataxon terminal of motor neuron.
2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.
Figure 9.8
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Neuromuscular Junction
•Situated midway along the length of a muscle fiber
•Axon terminal and muscle fiber are separated by a gel-filled space called the synaptic cleft
•Synaptic vesicles of axon terminal contain the neurotransmitter acetylcholine (ACh)
•Junctional folds of the sarcolemma contain ACh receptors
Copyright © 2010 Pearson Education, Inc.
Events at the Neuromuscular Junction
•Nerve impulse arrives at axon terminal
•ACh is released and binds with receptors on the sarcolemma
•Electrical events lead to the generation of an action potential
PLAY A&P Flix™: Events at the Neuromuscular Junction
Copyright © 2010 Pearson Education, Inc. Figure 9.8
Nucleus
Actionpotential (AP)
Myelinated axonof motor neuronAxon terminal of
neuromuscular junction
Sarcolemma ofthe muscle fiber
Ca2+ Ca2+
Axon terminalof motor neuron
Synaptic vesiclecontaining AChMitochondrionSynapticcleft
Junctionalfolds ofsarcolemma
Fusing synaptic vesicles
ACh
Sarcoplasm ofmuscle fiber
Postsynaptic membraneion channel opens;ions pass.
Na+ K+
Ach–
Na+
K+
Degraded ACh
Acetyl-cholinesterase
Postsynaptic membraneion channel closed;ions cannot pass.
1 Action potential arrives ataxon terminal of motor neuron.
2 Voltage-gated Ca2+ channels open and Ca2+ enters the axon terminal.
3 Ca2+ entry causes some synaptic vesicles to release their contents (acetylcholine)by exocytosis.4 Acetylcholine, a
neurotransmitter, diffuses across the synaptic cleft and binds to receptors in the sarcolemma.
5 ACh binding opens ionchannels that allow simultaneous passage of Na+ into the musclefiber and K+ out of the muscle fiber.
6 ACh effects are terminated by its enzymatic breakdown in the synaptic cleft by acetylcholinesterase.
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Destruction of Acetylcholine
•ACh effects are quickly terminated by the enzyme acetylcholinesterase
•Prevents continued muscle fiber contraction in the absence of additional stimulation
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Events in Generation of an Action Potential
1. Local depolarization (end plate potential):• ACh binding opens chemically (ligand) gated
ion channels
• Simultaneous diffusion of Na+ (inward) and K+ (outward)
• More Na+ diffuses, so the interior of the sarcolemma becomes less negative
• Local depolarization – end plate potential
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Events in Generation of an Action Potential
2. Generation and propagation of an action potential:• End plate potential spreads to adjacent
membrane areas• Voltage-gated Na+ channels open• Na+ influx decreases the membrane voltage
toward a critical threshold• If threshold is reached, an action potential is
generated
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Events in Generation of an Action Potential
•Local depolarization wave continues to spread, changing the permeability of the sarcolemma
•Voltage-regulated Na+ channels open in the adjacent patch, causing it to depolarize to threshold
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Events in Generation of an Action Potential
3. Repolarization:• Na+ channels close and voltage-gated K+
channels open• K+ efflux rapidly restores the resting polarity• Fiber cannot be stimulated and is in a
refractory period until repolarization is complete
• Ionic conditions of the resting state are restored by the Na+-K+ pump
Copyright © 2010 Pearson Education, Inc. Figure 9.9
Na+
Na+
Open Na+
Channel
Closed Na+
Channel
Closed K+
Channel
Open K+
Channel
Action potential++++++
++++++
Axon terminal
Synapticcleft
ACh
ACh
Sarcoplasm of muscle fiber
K+
2 Generation and propagation ofthe action potential (AP)
3 Repolarization
1 Local depolarization: generation of the end plate potential on the sarcolemma
K+
K+
Na+
K+
Na+
Wave of dep
olar
izat
io
n
Copyright © 2010 Pearson Education, Inc. Figure 9.9, step 1
Na+
Na+
Open Na+
Channel
Closed K+
Channel
K+
Na+
K+ Action potential
+++++++++++
+
Axon terminal
Synaptic
cleft
ACh
ACh
Sarcoplasm of muscle fiber
K+
1 Local depolarization: generation of the end plate potential on the sarcolemma1
Wave ofde
pola
rizat
ion
Copyright © 2010 Pearson Education, Inc. Figure 9.9, step 2
Na+
Na+
Open Na+
Channel
Closed K+
Channel
K+
Na+
K+ Action potential
+++++++++++
+
Axon terminal
Synaptic
cleft
ACh
ACh
Sarcoplasm of muscle fiber
K+
Generation and propagation of the action potential (AP)
1 Local depolarization: generation of the end plate potential on the sarcolemma
2
1
Wave ofde
pola
rizat
ion
Copyright © 2010 Pearson Education, Inc. Figure 9.9, step 3
Na+
Closed Na+
Channel
Open K+
Channel
K+
Repolarization3
Copyright © 2010 Pearson Education, Inc. Figure 9.9
Na+
Na+
Open Na+
Channel
Closed K+
Channel
Action potential++++++
++++++
Axon terminal
Synaptic
cleft
ACh
ACh
Sarcoplasm of muscle fiber
K+
2 Generation and propagation ofthe action potential (AP)
3 Repolarization
1 Local depolarization: generation of the end plate potential on the sarcolemma
K+
K+
Na+
K+
Na+
Wave of dep
olar
izat
io
n
Closed Na+
Channel
Open K+
Channel
Copyright © 2010 Pearson Education, Inc. Figure 9.10
Na+ channelsclose, K+ channelsopen
K+ channelsclose
Repolarizationdue to K+ exit
Threshold
Na+
channelsopen
Depolarizationdue to Na+ entry
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Excitation-Contraction (E-C) Coupling
•Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments
•Latent period:• Time when E-C coupling events occur
• Time between AP initiation and the beginning of contraction
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Events of Excitation-Contraction (E-C) Coupling •AP is propagated along sarcolemma to T tubules
•Voltage-sensitive proteins stimulate Ca2+ release from SR • Ca2+ is necessary for contraction
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 1
Axon terminalof motor neuron
Muscle fiberTriad
One sarcomere
Synaptic cleft
Setting the stage
Sarcolemma
Action potentialis generated
Terminal cisterna of SR ACh
Ca2+
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 2
Action potential is propagated alongthe sarcolemma and down the T tubules.
Steps in E-C Coupling:
Troponin
Tropomyosinblocking active sites
Myosin
Actin
Active sites exposed and ready for myosin binding
Ca2+
Terminal cisterna of SR
Voltage-sensitivetubule protein
T tubule
Ca2+
releasechannel
Myosincross bridge
Ca2+
Sarcolemma
Calcium ions are released.
Calcium binds to troponin andremoves the blocking action oftropomyosin.
Contraction begins
The aftermath
1
2
3
4
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 3
Steps inE-C Coupling:
Terminal cisterna of SR
Voltage-sensitivetubule protein
T tubule
Ca2+
releasechannel
Ca2+
Sarcolemma
Action potential ispropagated along thesarcolemma and downthe T tubules.
1
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 4
Steps inE-C Coupling:
Terminal cisterna of SR
Voltage-sensitivetubule protein
T tubule
Ca2+
releasechannel
Ca2+
Sarcolemma
Action potential ispropagated along thesarcolemma and downthe T tubules.
Calciumions arereleased.
1
2
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 5
Troponin
Tropomyosinblocking active sitesMyosin
Actin
Ca2+
The aftermath
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 6
Troponin
Tropomyosinblocking active sitesMyosin
Actin
Active sites exposed and ready for myosin binding
Ca2+
Calcium binds totroponin and removesthe blocking action of tropomyosin.
The aftermath
3
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 7
Troponin
Tropomyosinblocking active sitesMyosin
Actin
Active sites exposed and ready for myosin binding
Ca2+
Myosincross bridge
Calcium binds totroponin and removesthe blocking action oftropomyosin.
Contraction begins
The aftermath
3
4
Copyright © 2010 Pearson Education, Inc. Figure 9.11, step 8
Action potential is propagated alongthe sarcolemma and down the T tubules.
Steps in E-C Coupling:
Troponin
Tropomyosinblocking active sites
Myosin
Actin
Active sites exposed and ready for myosin binding
Ca2+
Terminal cisterna of SR
Voltage-sensitivetubule protein
T tubule
Ca2+
releasechannel
Myosincross bridge
Ca2+
Sarcolemma
Calcium ions are released.
Calcium binds to troponin andremoves the blocking action oftropomyosin.
Contraction begins
The aftermath
1
2
3
4
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Role of Calcium (Ca2+) in Contraction
•At low intracellular Ca2+ concentration:• Tropomyosin blocks the active sites on actin
• Myosin heads cannot attach to actin
• Muscle fiber relaxes
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Role of Calcium (Ca2+) in Contraction
•At higher intracellular Ca2+ concentrations:• Ca2+ binds to troponin
• Troponin changes shape and moves tropomyosin away from active sites
• Events of the cross bridge cycle occur
• When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends
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Cross Bridge Cycle
•Continues as long as the Ca2+ signal and adequate ATP are present
•Cross bridge formation—high-energy myosin head attaches to thin filament
•Working (power) stroke—myosin head pivots and pulls thin filament toward M line
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Cross Bridge Cycle
•Cross bridge detachment—ATP attaches to myosin head and the cross bridge detaches
• “Cocking” of the myosin head—energy from hydrolysis of ATP cocks the myosin head into the high-energy state
Copyright © 2010 Pearson Education, Inc. Figure 9.12
1
Actin
Cross bridge formation.
Cocking of myosin head.
The power (working) stroke.
Cross bridge detachment.
Ca2+
Myosincross bridge
Thick filament
Thin filament
ADP
Myosin
Pi
ATPhydrolysi
s
ATP
ATP
24
3
ADP
Pi
ADPPi
Copyright © 2010 Pearson Education, Inc. Figure 9.12, step 1
Actin
Cross bridge formation.
Ca2
+
Myosincross bridge Thick
filament
Thin filament
ADP
Myosin
Pi
1
Copyright © 2010 Pearson Education, Inc. Figure 9.12, step 3
The power (working) stroke.
ADPPi
2
Copyright © 2010 Pearson Education, Inc. Figure 9.12, step 4
Cross bridge detachment.
ATP
3
Copyright © 2010 Pearson Education, Inc. Figure 9.12, step 5
Cocking of myosin head.
ATPhydrolysi
s
ADPPi
4
Copyright © 2010 Pearson Education, Inc. Figure 9.12
1
Actin
Cross bridge formation.
Cocking of myosin head.
The power (working) stroke.
Cross bridge detachment.
Ca2+
Myosincross bridge
Thick filament
Thin filament
ADP
Myosin
Pi
ATPhydrolysi
s
ATP
ATP
24
3
ADP
Pi
ADPPi
Copyright © 2010 Pearson Education, Inc.
Steps in E-C Coupling - Storytelling!
1. Fill in the blanks to the steps of the E-C “story” that I wrote out for you!
2. Define the terms in the word bank provided.
3. Retell the story by creating your own captions on the pictures provided. Be sure to label all * structures as well!
Copyright © 2010 Pearson Education, Inc.
To instigate the process… at the Neuromuscular Junction.
Neurotransmitters are released from
the axon terminal. As they diffuse across
the synaptic cleft, they attach to
Acetylcholine (ACh) receptors on the
sarcolemma.
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Step 1
Net entry of Na+ initiates an Action
Potential which is propagated along the
sarcolemma and down the T Tubules.
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Step 2
The action potential in the T tubule
activates voltage-sensitive receptors
which in turn trigger Ca2+ release from
the terminal cisternae of the
Sarcoplasmic Reticulum in the cytosol.
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Step 3
Calcium ions bind to troponin; this
changes shape, removing the blocking
action of tropomyosin → therefore
leave the actin active sites exposed.
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Step 4
Contraction! (The Cross Bridge
Cycle***) Myosin heads alternately
attach to actin and detach, pulling the
Actin filaments toward the center of the
sarcomere. Powered by ATP hydrolysis.
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Step 4 broken down… 4.1 ***
Myosin head attached to the actin
myofilament, forming the cross bridge.
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4.2*** Inorganic Phosphate (Pi) generated in the
previous contraction cycle is released,
initiating the power (working) stroke. The
myosin head pivots and bends as it pulls on
the actin filament, sliding it toward the
M-Line. Then ADP is released.
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4.3***
A new ATP attached to the myosin
head, the link between myosin and actin
weakens, and the cross bridge
detaches.
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4.4***
As ATP split into ADP and Pi, the
myosin head is energized (cocked into
the high-energy conformation) Ready to
start over!
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Step 5
Removal of Ca2+ by active transport into
the sarcoplasmic reticulum (SR) after the
action potential ends.
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Step 6
Tropomyosin blockage is restored,
blocking the myosin binding sites on
actin. Contraction ends and the muscle
fiber relaxes!!
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Warm Up 1/9/17
→ Through what process is ATP made?!
• With oxygen =
• Without oxygen =
→ (Where does this process occur?)
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What are the 6 Steps of EC Coupling?!
Remember that step 4 is our dance! (the cross bridge cycle!) - broken down into 4.1, 2, 3 and 4!
Copyright © 2010 Pearson Education, Inc.
mitochondria
Synaptic vesicle
Acetylcholine (ACh)
Axon term
inal
Sodium rushes into the cell, sending an action potential down the length of the cell membrane (sarcolemma) and into the cell via T Tubules.
1
ACh Receptor
Sarcolemma
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T Tubule
Sarcoplasmic Reticulum (SR) Voltage gated
receptor
Calcium 2 T tubules stimulate calcium release from the SR
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3Calcium binds to troponin, unlocking the tropomyosin gate and exposing the myosin binding sites
Troponin
Tropomyosin
Myosin binding site
Actin
Myosin
Myosinhead
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4Contraction begins! Actin is pulled towards the center of the sarcomere by myosin. (Also known as the Cross Bridge Cycle)
Let’s practice our dance!
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Sarcoplasmic Reticulum (SR)
Calcium
5Calcium is removed from the cytosol and stored back in the Sarcoplasmic Reticulum for future use!
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6Contraction ends! Tropomyosin gate and troponin lock are restored to their original positions, blocking the myosin sites on actin.
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