1. 9
Muscles and Muscle Tissue: Part A

2. Nearly half of body's mass
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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3. 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
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4. Types of Muscle Tissue Cardiac muscle
Only in heart; bulk of heart walls
Striated
Can contract without nervous system stimulation
Involuntary
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5. 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
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6. Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (1 of 4)
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7. Special Characteristics of Muscle Tissue
Excitability (responsiveness): 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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8. 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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9. 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
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10. 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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11. Table 9.1 Structure and Organizational Levels of Skeletal Muscle (1 of 3)
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12. Table 9.1 Structure and Organizational Levels of Skeletal Muscle (2 of 3)
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13. Table 9.1 Structure and Organizational Levels of Skeletal Muscle (3 of 3)
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14. 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
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15. 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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16. 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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17. Striations
H zone: lighter region in midsection of dark A band where filaments do not overlap
M line: line of protein myomesin bisects H zone
Z disc (line): coin-shaped sheet of proteins on midline of light I band that anchors thin filaments and connects myofibrils to one another
Thick filaments: run entire length of an A band
Thin filaments: run length of I band and partway into A band
Sarcomere: region between two successive Z discs
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18. Smallest contractile unit (functional unit) of muscle fiber
Sarcomere
Smallest contractile unit (functional unit) of muscle fiber
Align along myofibril like boxcars of train
Contains A band with ½ I band at each end
Composed of thick and thin myofilaments made of contractile proteins
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19. 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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20. 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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21. Myofibril Banding Pattern
Orderly arrangement of actin and myosin myofilaments within sarcomere
Actin myofilaments = thin filaments
Extend across I band and partway in A band
Anchored to Z discs
Myosin myofilaments = thick filaments
Extend length of A band
Connected at M line
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22. Ultrastructure of Thick Filament
Composed of protein myosin
Each composed of 2 heavy and four light polypeptide chains
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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23. 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
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24. 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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25. 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 proteins of sarcolemma
Nebulin, myomesin, C proteins bind filaments or sarcomeres together; maintain alignment
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26. Sarcoplasmic Reticulum (SR)
Network of smooth endoplasmic reticulum surrounding each myofibril
Most run longitudinally
Pairs of terminal cisterns form perpendicular cross channels
Functions in regulation of intracellular Ca2+ levels
Stores and releases Ca2+
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27. Continuations of sarcolemma Lumen continuous with extracellular space
T Tubules
Continuations of sarcolemma
Lumen continuous with extracellular space
Increase muscle fiber's surface area
Penetrate cell's interior at each A band–I band junction
Associate with paired terminal cisterns to form triads that encircle each sarcomere
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28. 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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29. Triad Relationships
T tubules conduct impulses deep into muscle fiber; every sarcomere
Integral proteins protrude into intermembrane space from T tubule and SR cistern membranes and connect with each other
T tubule integral proteins act as voltage sensors and change shape in response to voltage changes
SR integral proteins are channels that release Ca2+ from SR cisterns when voltage sensors change shape
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30. 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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31. 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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32. 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)
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33. Fully relaxed sarcomere of a muscle fiber
Figure 9.6 Sliding filament model of contraction.
Slide 2 1
Fully relaxed sarcomere of a muscle fiber Z H Z I A I
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34. Fully contracted sarcomere of a muscle fiber
Figure 9.6 Sliding filament model of contraction.
Slide 3 2
Fully contracted sarcomere of a muscle fiber Z Z I A I
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35. 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
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36. Events of Excitation-Contraction (E-C) Coupling
AP propagated along sarcomere to T tubules
Voltage-sensitive proteins stimulate Ca2+ release from SR
Ca2+ necessary for contraction
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37. 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
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38. 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
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39. 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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40. 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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41. Figure The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere.
Slide 1
Actin
Ca2+
Thin filament
Myosin
cross bridge
Thick
filament
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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42. Actin Thin filament Myosin
Figure The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere.
Slide 2
Actin
Thin filament
Myosin
cross bridge
Thick
filament
Myosin
Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming a cross bridge. 1
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43. Figure The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere.
Slide 3
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
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44. Figure The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere.
Slide 4
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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45. Figure 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. 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.
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46. 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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47. 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
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