1. 9
Muscles and Muscle Physiology

2. Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (1 of 4)
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3. Special Characteristics of Muscle Tissue
Excitability:
Contractility:
Extensibility:
Elasticity:
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4. Four important functions
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"
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5. Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium.
Bone
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
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6. 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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7. Table 9.1 Structure and Organizational Levels of Skeletal Muscle (1 of 3)
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8. Table 9.1 Structure and Organizational Levels of Skeletal Muscle (2 of 3)
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9. Sarcolemma Mitochondrion Dark A band Light I band Nucleus
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
Light
I band
Nucleus
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10. Densely packed, rodlike elements ~80% of cell volume
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
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11. Z disc H zone Z disc I band A band I band M line Sarcomere
Figure 9.2c Microscopic anatomy of a skeletal muscle fiber.
Thin (actin)
filament
Z disc
H zone
Z disc
Small part of one
myofibril
enlarged to show
the myofilaments
responsible for the
banding pattern.
Each sarcomere
extends from one Z
disc to the next.
Thick
(myosin)
filament
I band
A band
I band
M line
Sarcomere
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12. Striations H zone: M line: Z disc (line): Thick filaments:
Thin filaments:
Sarcomere:
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13. Z disc H zone Z disc I band A band I band M line Sarcomere
Figure 9.2c Microscopic anatomy of a skeletal muscle fiber.
Thin (actin)
filament
Z disc
H zone
Z disc
Small part of one
myofibril
enlarged to show
the myofilaments
responsible for the
banding pattern.
Each sarcomere
extends from one Z
disc to the next.
Thick
(myosin)
filament
I band
A band
I band
M line
Sarcomere
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14. one sarcomere (sectioned length- wise). Notice the myosin heads on
Figure 9.2d Microscopic anatomy of a skeletal muscle fiber.
Sarcomere
Thin
(actin)
filament
Z disc
M line
Z disc
Enlargement of
one sarcomere (sectioned length-
wise). Notice the
myosin heads on
the thick filaments.
Elastic
(titin)
filaments
Thick
(myosin)
filament
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15. Portion of a thick filament Portion of a thin 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 oppositeends of the filament.
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 thick filament
Portion of a thin filament
Myosin head
Tropomyosin
Troponin
Actin
Actin-binding sites
Heads
Tail
ATP-
binding
site
Active sites
for myosin
attachment
Actin subunits
Flexible hinge region
Myosin molecule
Actin subunits
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16. Part of a skeletal muscle fiber (cell) I band A band I band Z disc
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
A band
I band
Z disc
H zone
Z disc M
line
Sarcolemma
Myofibril
Triad:
• T tubule
• Terminal
cisterns of
the SR (2)
Sarcolemma
Tubules of
the SR
Myofibrils
Mitochondria
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17. 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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18. 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
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19. Fully relaxed sarcomere of a muscle fiber
Figure 9.6 Sliding filament model of contraction.
Slide 1
Slide 1 1
Fully relaxed sarcomere of a muscle fiber Z H Z I A I 2
Fully contracted sarcomere of a muscle fiber Z Z
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20. 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
Pulls Z discs toward M line
I bands shorten; Z discs closer; H zones disappear; A bands move closer (length stays same)
Review Sliding Filament Theory on IP
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21. Fully relaxed sarcomere of a muscle fiber
Figure 9.6 Sliding filament model of contraction.
Slide 4 1
Fully relaxed sarcomere of a muscle fiber Z H Z I A I 2
Fully contracted sarcomere of a muscle fiber Z Z
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22. 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
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23. Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released.
Slide 1
Myelinated axon
of motor neuron
Action
potential (AP)
Axon terminal of
neuromuscular
junction
Sarcolemma of
the muscle fiber
Action potential arrives at axon
terminal of motor neuron. 1
Voltage-gated Ca2+ channels open. Ca2+ enters the axon terminal moving down its electochemical gradient.
Synaptic vesicle
containing ACh 2
Axon terminal
of motor neuron
Synaptic
cleft
Fusing synaptic
vesicles
Ca2+ entry causes ACh (a
neurotransmitter) to be released
by exocytosis. 3
ACh
Junctional
folds of
sarcolemma
ACh diffuses across the synaptic cleft and binds to its receptors on
the sarcolemma. 4
Sarcoplasm of
muscle fiber
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. 5
Postsynaptic
membrane
ion channel opens;
ions pass.
Degraded ACh
ACh
Ion channel closes;
ions cannot pass.
ACh effects are terminated by
its breakdown in the synaptic
cleft by acetylcholinesterase and
diffusion away from the junction. 6
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Acetylcho-
linesterase
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24. Action potential (AP) arrives at axon
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
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25. Figure 9.9 Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber.
Slide 1
Open Na+
channel
Closed K+
channel
Na+
− − − − − − − −
− − − − −
− − − − − −
ACh-containing
synaptic vesicle
− − − − K+
Axon terminal of
neuromuscular
junction
Action potential
Ca2+
Ca2+
Synaptic
cleft
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. 2
Wave of
depolarization
Closed Na+
channel
Open K+
channel
An end plate potential is generated at the
neuromuscular junction (see Figure 9.8). 1
Na+  
− − − − − − − −
− − − −
− − − −− − − − K+
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. 3
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26. Membrane potential (mV)
Figure Action potential tracing indicates changes in Na+ and K+ ion channels.
+30
Na+ channels
close, K+ channels
open
Depolarization
due to Na+ entry
Membrane potential (mV)
Repolarization
due to K+ exit
Na+
channels
open
K+ channels
closed
–95 5 10 15 20
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Time (ms)
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27. 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
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28. The events at the neuromuscular junction (NMJ)
Figure 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 2
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
Axon terminal of
motor neuron at NMJ
Action poten-
tial is
generated
ACh
Sarcolemma
T tubule
Terminal
cistern
of SR
Muscle
fiber
Triad
One sarcomere
One myofibril
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29. A&P Flix™: Excitation-contraction coupling.
Figure 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 9
Steps in E-C Coupling:
Sarcolemma
The action potential (AP) propagates along the sarcolemma and down the
T tubules. 1
Voltage-sensitive
tubule protein
T tubule
PLAY
Ca2+
release
channel
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. 2
Terminal
cistern
of SR
A&P Flix™: Excitation-contraction coupling.
Actin
Troponin
Tropomyosin
blocking active sites
Myosin
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. 3
Active sites exposed and
ready for myosin binding
Contraction begins: Myosin binding to actin forms cross bridges and contraction (cross bridge cycling) begins. At this point, E-C coupling is over. 4
Myosin
cross
bridge
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.
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30. 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:
Sarcolemma
Voltage-sensitive
tubule protein
T tubule
The action potential (AP) propagates along the sarcolemma and down the
T tubules. 1
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.
Ca2+
release
channel
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. 2
Terminal
cistern
of SR
Synaptic
cleft
Axon terminal of
motor neuron at NMJ
Action potential
is generated
ACh
Sarcolemma
Actin
T tubule
Troponin
Tropomyosin
blocking active sites
Terminal
cistern
of SR
Myosin
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. 3
Muscle fiber
Triad
Active sites exposed and
ready for myosin binding
One sarcomere
Contraction begins: Myosin binding to actin forms cross bridges and contraction (cross bridge cycling) begins. At this point, E-C coupling is over. 4
Myosin
cross
bridge
One myofibril
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.
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31. Continues as long as Ca2+ signal and adequate ATP present
Cross Bridge Cycle
Continues as long as Ca2+ signal and adequate ATP 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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32. Cross Bridge Cycle
Cross bridge detachment—ATP attaches to myosin head and cross bridge detaches
"Cocking" of myosin head—energy from hydrolysis of ATP cocks myosin head into high-energy state
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33. A&P Flix™: The Cross Bridge Cycle
Figure The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere.
Slide 6
Actin
Ca2+
Thin filament
PLAY
Myosin
cross bridge
Thick
filament
A&P Flix™: The Cross Bridge Cycle
Myosin
Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming
a cross bridge. 1
ATP
hydrolysis
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. * 4
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. 2
In the absence of ATP, myosin heads will not detach, causing rigor mortis.
*This cycle will continue as long
as ATP is available and Ca2+ is
bound to troponin.
Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (the cross bridge “breaks”). 3
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34. Role of Calcium (Ca2+) in Contraction
At low intracellular Ca2+ concentration? -
At high intracellular Ca2+ concentration?
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35. To re-establish RMP at sarcolemma and synaptic knob
ATP is needed ……
To re-establish RMP at sarcolemma and synaptic knob
For detachment and “re-cocking” of myosin heads
For sarcoplasmic reticulum to reabsorb Ca++ ( by ATP dependant calcium pump)
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36. Review Principles of Muscle Mechanics
Contraction may/may not shorten muscle
Isometric contraction: no shortening; muscle tension increases but does not exceed load
Isotonic contraction: muscle shortens because muscle tension exceeds load
Force and duration of contraction vary in response to stimuli of different frequencies and intensities
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37. Ach were not removed from synaptic cleft.
What if??????
Ach were not removed from synaptic cleft.
Little or no ATP could be produced
The CNS sends volleys of high frequency impulses to various muscles
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