Transcript 9_muscle_tissue_

PowerPoint® Lecture Slides
prepared by
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
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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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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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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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Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (1 of 4)
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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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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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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
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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
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Table 9.1 Structure and Organizational Levels of Skeletal Muscle (1 of 3)
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Table 9.1 Structure and Organizational Levels of Skeletal Muscle (2 of 3)
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Table 9.1 Structure and Organizational Levels of Skeletal Muscle (3 of 3)
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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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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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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
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Light Nucleus
I band
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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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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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.
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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.
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Sarcomere
M line
Z disc
Thin
(actin)
filament
Elastic
(titin)
filaments
Thick
(myosin)
filament
Myofibril Banding Pattern
• Orderly arrangement of actin and myosin
myofilaments within sarcomere
– Actin myofilaments = thin filaments
• 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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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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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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Figure 9.3 Composition of thick and thin filaments.
Longitudinal section of filaments within one
sarcomere of a myofibril
Thick filament
Thin filament
In the center of the sarcomere, the thick filaments
lack myosin heads. Myosin heads are present only
in areas of myosin-actin overlap.
Thick filament.
Thin filament
Each thick filament consists of many myosin
molecules whose heads protrude at opposite ends
of the filament.
Portion of a thick filament
Myosin head
A thin filament consists of two strands of actin
subunits twisted into a helix plus two types of
regulatory proteins (troponin and tropomyosin).
Portion of a thin filament
Tropomyosin
Troponin Actin
Actin-binding sites
Heads
ATPbinding
site Flexible hinge region
Myosin molecule
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Tail
Active sites
for myosin
attachment
Actin subunits
Actin subunits
Structure of Myofibril
• Elastic filament
– Composed of protein titin
– Holds thick filaments in place; helps recoil
after stretch; resists excessive stretching
• Dystrophin
– Links thin filaments to proteins of sarcolemma
• Nebulin, myomesin, C proteins bind
filaments or sarcomeres together; maintain
alignment
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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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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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Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle.
Part of a skeletal
muscle fiber (cell)
I band
Z disc
Myofibril
Sarcolemma
A band
H zone
M
line
I band
Z disc
Sarcolemma
Triad:
• T tubule
• Terminal
cisterns of
the SR (2)
Tubules of
the SR
Myofibrils
Mitochondria
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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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Sliding Filament Model of Contraction
• Generation of force
• Does not necessarily cause shortening of
fiber
• Shortening occurs when tension
generated by cross bridges on thin
filaments exceeds forces opposing
shortening
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Sliding Filament Model of Contraction
• In relaxed state, thin and thick filaments
overlap only at ends of A band
• Sliding filament model of contraction
– During contraction, thin filaments slide past
thick filaments  actin and myosin overlap
more
– Occurs when myosin heads bind to actin 
cross bridges
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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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Figure 9.6 Sliding filament model of contraction.
Slide 2
1 Fully relaxed sarcomere of a muscle fiber
Z
I
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H
A
Z
I
Figure 9.6 Sliding filament model of contraction.
Slide 3
2 Fully contracted sarcomere of a muscle fiber
Z
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I
Z
A
I
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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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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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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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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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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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
© 2013 Pearson Education, Inc.
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Actin
Ca2+ Thin filament
Myosin
cross bridge
Thick
filament
Myosin
1 Cross bridge formation.
Energized myosin head attaches
to an actin myofilament, forming
a cross bridge.
ATP
hydrolysis
4 Cocking of the myosin head.
As ATP is hydrolyzed to ADP and Pi,
the myosin head returns to its
prestroke high-energy, or “cocked,”
position. *
*This cycle will continue as long
as ATP is available and Ca2+ is
bound to troponin.
© 2013 Pearson Education, Inc.
2 The power (working) stroke. ADP
and Pi are released and the myosin head
pivots and bends, changing to its bent
low-energy state. As a result it pulls the
actin filament toward the M line.
In the absence
of ATP, myosin
heads will not
detach, causing
rigor mortis.
3 Cross bridge detachment. After ATP
attaches to myosin, the link between myosin
and actin weakens, and the myosin head
detaches (the cross bridge “breaks”).
Slide 1
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Actin
Myosin
cross bridge
Slide 2
Thin filament
Thick
filament
Myosin
1 Cross bridge formation.
Energized myosin head attaches
to an actin myofilament, forming
a cross bridge.
© 2013 Pearson Education, Inc.
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Slide 3
2 The power (working) stroke. ADP
and Pi are released and the myosin head
pivots and bends, changing to its bent
low-energy state. As a result it pulls the
actin filament toward the M line.
© 2013 Pearson Education, Inc.
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Slide 4
3 Cross bridge detachment. After ATP
attaches to myosin, the link between myosin
and actin weakens, and the myosin head
detaches (the cross bridge “breaks”).
© 2013 Pearson Education, Inc.
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Slide 5
ATP
hydrolysis
4 Cocking of the myosin head.
*This cycle will continue as long
as ATP is available and Ca2+ is
bound to troponin.
© 2013 Pearson Education, Inc.
As ATP is hydrolyzed to ADP and Pi,
the myosin head returns to its
prestroke high-energy, or “cocked,”
position. *
Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments
toward the center of the sarcomere.
Actin
Ca2+ Thin filament
Myosin
cross bridge
PLAY
A&P Flix™: The
Cross Bridge
Cycle
Slide 6
Thick
filament
Myosin
1 Cross bridge formation.
Energized myosin head attaches
to an actin myofilament, forming
a cross bridge.
ATP
hydrolysis
4 Cocking of the myosin head.
As ATP is hydrolyzed to ADP and Pi,
the myosin head returns to its
prestroke high-energy, or “cocked,”
position. *
*This cycle will continue as long
as ATP is available and Ca2+ is
bound to troponin.
© 2013 Pearson Education, Inc.
2 The power (working) stroke. ADP
and Pi are released and the myosin head
pivots and bends, changing to its bent
low-energy state. As a result it pulls the
actin filament toward the M line.
In the absence
of ATP, myosin
heads will not
detach, causing
rigor mortis.
3 Cross bridge detachment. After ATP
attaches to myosin, the link between myosin
and actin weakens, and the myosin head
detaches (the cross bridge “breaks”).
Homeostatic Imbalance
• Rigor mortis
– Cross bridge detachment requires ATP
– 3–4 hours after death muscles begin to stiffen
with weak rigidity at 12 hours post mortem
• Dying cells take in calcium  cross bridge
formation
• No ATP generated to break cross bridges
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