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10
Muscle Tissue
PowerPoint® Lecture Presentations prepared by
Jason LaPres
Lone Star College—North Harris
© 2012 Pearson Education, Inc.
10-1 An Introduction to Muscle Tissue
• Learning Outcomes
• 10-1 Specify the functions of skeletal muscle tissue.
• 10-2 Describe the organization of muscle at the
tissue level.
• 10-3 Explain the characteristics of skeletal muscle
fibers, and identify the structural components of
a sarcomere.
• 10-4 Identify the components of the neuromuscular
junction, and summarize the events involved in
the neural control of skeletal muscle contraction
and relaxation.
© 2012 Pearson Education, Inc.
10-1 An Introduction to Muscle Tissue
• Learning Outcomes
• 10-5 Describe the mechanism responsible for tension
production in a muscle fiber, and compare the
different types of muscle contraction.
• 10-6 Describe the mechanisms by which muscle
fibers obtain the energy to power contractions.
• 10-7 Relate the types of muscle fibers to muscle
performance, and distinguish between
aerobic and anaerobic endurance.
© 2012 Pearson Education, Inc.
10-1 An Introduction to Muscle Tissue
• Learning Outcomes
• 10-8 Identify the structural and functional differences
between skeletal muscle fibers and cardiac
muscle cells.
• 10-9 Identify the structural and functional differences
between skeletal muscle fibers and smooth
muscle cells, and discuss the roles of smooth
muscle tissue in systems throughout the body.
© 2012 Pearson Education, Inc.
An Introduction to Muscle Tissue
• Muscle Tissue
• A primary tissue type, divided into:
• Skeletal muscle tissue
• Cardiac muscle tissue
• Smooth muscle tissue
© 2012 Pearson Education, Inc.
10-1 Functions of Skeletal Muscle Tissue
• Skeletal Muscles
• Are attached to the skeletal system
• Allow us to move
• The muscular system
• Includes only skeletal muscles
© 2012 Pearson Education, Inc.
10-1 Functions of Skeletal Muscle Tissue
• Six Functions of Skeletal Muscle Tissue
1. Produce skeletal movement
2. Maintain posture and body position
3. Support soft tissues
4. Guard entrances and exits
5. Maintain body temperature
6. Store nutrient reserves
© 2012 Pearson Education, Inc.
10-2 Organization of Muscle
• Skeletal Muscle
• Muscle tissue (muscle cells or fibers)
• Connective tissues
• Nerves
• Blood vessels
© 2012 Pearson Education, Inc.
10-2 Organization of Muscle
•
Organization of Connective Tissues
•
Muscles have three layers of connective tissues
1. Epimysium
2. Perimysium
3. Endomysium
© 2012 Pearson Education, Inc.
10-2 Organization of Muscle
• Epimysium
• Exterior collagen layer
• Connected to deep fascia
• Separates muscle from surrounding tissues
© 2012 Pearson Education, Inc.
10-2 Organization of Muscle
• Perimysium
• Surrounds muscle fiber bundles (fascicles)
• Contains blood vessel and nerve supply to
fascicles
© 2012 Pearson Education, Inc.
10-2 Organization of Muscle
• Endomysium
• Surrounds individual muscle cells (muscle fibers)
• Contains capillaries and nerve fibers contacting
muscle cells
• Contains myosatellite cells (stem cells) that repair
damage
© 2012 Pearson Education, Inc.
Figure 10-1 The Organization of Skeletal Muscles
Skeletal Muscle (organ)
Epimysium Perimysium
Endomysium
Nerve
Muscle
fascicle
Muscle Blood
fibers vessels
Epimysium
Blood vessels
and nerves
Tendon
Endomysium
Perimysium
© 2012 Pearson Education, Inc.
Figure 10-1 The Organization of Skeletal Muscles
Muscle Fascicle (bundle of fibers)
Perimysium
Muscle fiber
Epimysium
Blood vessels
and nerves
Endomysium
Tendon
Endomysium
Perimysium
© 2012 Pearson Education, Inc.
Figure 10-1 The Organization of Skeletal Muscles
Muscle Fiber (cell)
Capillary Myofibril
Endomysium
Sarcoplasm
Epimysium
Mitochondrion
Blood vessels
and nerves
Tendon
Myosatellite
cell
Sarcolemma
Nucleus
Axon of neuron
Endomysium
Perimysium
© 2012 Pearson Education, Inc.
10-2 Organization of Muscle
• Organization of Connective Tissues
• Muscle Attachments
• Endomysium, perimysium, and epimysium come
together:
• At ends of muscles
• To form connective tissue attachment to bone
matrix
• I.e., tendon (bundle) or aponeurosis (sheet)
© 2012 Pearson Education, Inc.
10-2 Organization of Muscle
• Blood Vessels and Nerves
• Muscles have extensive vascular systems that:
• Supply large amounts of oxygen
• Supply nutrients
• Carry away wastes
• Skeletal muscles are voluntary muscles, controlled
by nerves of the central nervous system (brain and
spinal cord)
© 2012 Pearson Education, Inc.
10-3 Characteristics of Skeletal Muscle Fibers
• Skeletal Muscle Cells
• Are very long
• Develop through fusion of mesodermal cells
(myoblasts)
• Become very large
• Contain hundreds of nuclei
© 2012 Pearson Education, Inc.
Figure 10-2 The Formation of a Multinucleate Skeletal Muscle Fiber
Muscle fibers develop
through the fusion of
mesodermal cells
called myoblasts.
Myoblasts
A muscle
fiber forms
by the
fusion of
myoblasts.
LM  612
Muscle fiber
Sarcolemma
Nuclei
Myofibrils
Myosatellite cell
Nuclei
Mitochondria
Immature
muscle fiber
Myosatellite cell
A diagrammatic view and a
micrograph of one muscle fiber.
Up to 30 cm
in length
Mature muscle fiber
© 2012 Pearson Education, Inc.
Figure 10-2a The Formation of a Multinucleate Skeletal Muscle Fiber
Muscle fibers develop
through the fusion of
mesodermal cells
called myoblasts.
Myoblasts
A muscle
fiber forms
by the
fusion of
myoblasts.
Myosatellite cell
Nuclei
Immature
muscle fiber
Myosatellite cell
Up to 30 cm
in length
Mature muscle fiber
© 2012 Pearson Education, Inc.
Figure 10-2b The Formation of a Multinucleate Skeletal Muscle Fiber
LM  612
Muscle fiber
Sarcolemma
Nuclei
Myofibrils
Mitochondria
A diagrammatic view and a
micrograph of one muscle fiber.
© 2012 Pearson Education, Inc.
10-3 Characteristics of Skeletal Muscle Fibers
• The Sarcolemma and Transverse Tubules
• The sarcolemma
• The cell membrane of a muscle fiber (cell)
• Surrounds the sarcoplasm (cytoplasm of muscle
fiber)
• A change in transmembrane potential begins
contractions
© 2012 Pearson Education, Inc.
10-3 Characteristics of Skeletal Muscle Fibers
• The Sarcolemma and Transverse Tubules
• Transverse tubules (T tubules)
• Transmit action potential through cell
• Allow entire muscle fiber to contract
simultaneously
• Have same properties as sarcolemma
© 2012 Pearson Education, Inc.
10-3 Characteristics of Skeletal Muscle Fibers
• Myofibrils
• Lengthwise subdivisions within muscle fiber
• Made up of bundles of protein filaments
(myofilaments)
• Myofilaments are responsible for muscle contraction
• Types of myofilaments:
• Thin filaments
• Made of the protein actin
• Thick filaments
• Made of the protein myosin
© 2012 Pearson Education, Inc.
10-3 Characteristics of Skeletal Muscle Fibers
• The Sarcoplasmic Reticulum (SR)
• A membranous structure surrounding each myofibril
• Helps transmit action potential to myofibril
• Similar in structure to smooth endoplasmic reticulum
• Forms chambers (terminal cisternae) attached to T
tubules
© 2012 Pearson Education, Inc.
10-3 Characteristics of Skeletal Muscle Fibers
• The Sarcoplasmic Reticulum (SR)
• Triad
• Is formed by one T tubule and two terminal
cisternae
• Cisternae
• Concentrate Ca2+ (via ion pumps)
• Release Ca2+ into sarcomeres to begin muscle
contraction
© 2012 Pearson Education, Inc.
Figure 10-3 The Structure of a Skeletal Muscle Fiber
Myofibril
Sarcolemma
Nuclei
Sarcoplasm
MUSCLE FIBER
Mitochondria
Terminal cisterna
Sarcolemma
Sarcolemma
Sarcoplasm
Myofibril
Myofibrils
Thin filament
Thick filament
Triad Sarcoplasmic T tubules
reticulum
© 2012 Pearson Education, Inc.
Figure 10-3 The Structure of a Skeletal Muscle Fiber
Myofibril
Nuclei
Sarcolemma
Sarcoplasm
© 2012 Pearson Education, Inc.
MUSCLE FIBER
Figure 10-3 The Structure of a Skeletal Muscle Fiber
Mitochondria
Terminal cisterna
Sarcolemma
Sarcolemma
Sarcoplasm
Myofibril
Myofibrils
Thin filament
Thick filament
Triad Sarcoplasmic T tubules
reticulum
© 2012 Pearson Education, Inc.
Figure 10-3 The Structure of a Skeletal Muscle Fiber
Mitochondria
Sarcolemma
Myofibril
Thin filament
Thick filament
© 2012 Pearson Education, Inc.
Figure 10-3 The Structure of a Skeletal Muscle Fiber
Terminal cisterna
Sarcolemma
Sarcoplasm
Myofibrils
Triad Sarcoplasmic T tubules
reticulum
© 2012 Pearson Education, Inc.
10-3 Structural Components of a Sarcomere
• Sarcomeres
• The contractile units of muscle
• Structural units of myofibrils
• Form visible patterns within myofibrils
• A striped or striated pattern within myofibrils
• Alternating dark, thick filaments (A bands) and light,
thin filaments (I bands)
© 2012 Pearson Education, Inc.
10-3 Structural Components of a Sarcomere
• Sarcomeres
• The A Band
• M line
• The center of the A band
• At midline of sarcomere
• The H Band
• The area around the M line
• Has thick filaments but no thin filaments
• Zone of overlap
• The densest, darkest area on a light micrograph
• Where thick and thin filaments overlap
© 2012 Pearson Education, Inc.
10-3 Structural Components of a Sarcomere
• Sarcomeres
• The I Band
• Z lines
• The centers of the I bands
• At two ends of sarcomere
• Titin
• Are strands of protein
• Reach from tips of thick filaments to the Z line
• Stabilize the filaments
© 2012 Pearson Education, Inc.
Figure 10-4a Sarcomere Structure, Part I
I band
A band
H band
Z line Titin
A longitudinal
section of a
sarcomere,
showing bands
Zone of overlap
M line
Sarcomere
© 2012 Pearson Education, Inc.
Thin
Thick
filament filament
Figure 10-4b Sarcomere Structure, Part I
I band
A band
H band
A corresponding
view of a
sarcomere in a
myofibril from a
muscle fiber in the Myofibril
gastrocnemius
Z line
muscle of the calf
TEM  64,000
Zone of overlap M line
Sarcomere
© 2012 Pearson Education, Inc.
Z line
Figure 10-5 Sarcomere Structure, Part II
Sarcomere
Myofibril
A superficial view
of a sarcomere
Thin
filament
Actinin
filaments
Thick
filament
Titin
filament
Attachment
of titin
Z line
Cross-sectional views of different
portions of a sarcomere
© 2012 Pearson Education, Inc.
I band
M line
H band
Zone of overlap
Figure 10-6 Levels of Functional Organization in a Skeletal Muscle
Skeletal Muscle
Myofibril
Surrounded by:
Sarcoplasmic
reticulum
Surrounded by:
Epimysium
Epimysium
Contains:
Muscle fascicles
Consists of:
Sarcomeres
(Z line to Z line)
Sarcomere
I band
A band
Muscle Fascicle
Contains:
Thick filaments
Surrounded by:
Perimysium
Perimysium
Thin filaments
Contains:
Muscle fibers
Z line
M line
H band
Muscle Fiber
Endomysium
Surrounded by:
Endomysium
Contains:
Myofibrils
© 2012 Pearson Education, Inc.
Titin Z line
Figure 10-6 Levels of Functional Organization in a Skeletal Muscle
Skeletal Muscle
Surrounded by:
Epimysium
Epimysium
© 2012 Pearson Education, Inc.
Contains:
Muscle fascicles
Figure 10-6 Levels of Functional Organization in a Skeletal Muscle
Muscle Fascicle
Perimysium
© 2012 Pearson Education, Inc.
Surrounded by:
Perimysium
Contains:
Muscle fibers
Figure 10-6 Levels of Functional Organization in a Skeletal Muscle
Muscle Fiber
Endomysium
Surrounded by:
Endomysium
Contains:
Myofibrils
© 2012 Pearson Education, Inc.
Figure 10-6 Levels of Functional Organization in a Skeletal Muscle
Myofibril
Surrounded by:
Sarcoplasmic
reticulum
Consists of:
Sarcomeres
(Z line to Z line)
© 2012 Pearson Education, Inc.
Figure 10-6 Levels of Functional Organization in a Skeletal Muscle
Sarcomere
I band
A band
Contains:
Thick filaments
Thin filaments
Z line
M line
H band
© 2012 Pearson Education, Inc.
Titin Z line
10-3 Structural Components of a Sarcomere
• Thin Filaments
• F-actin (filamentous actin)
• Is two twisted rows of globular G-actin
• The active sites on G-actin strands bind to myosin
• Nebulin
• Holds F-actin strands together
© 2012 Pearson Education, Inc.
10-3 Structural Components of a Sarcomere
• Thin Filaments
• Tropomyosin
• Is a double strand
• Prevents actin–myosin interaction
• Troponin
• A globular protein
• Binds tropomyosin to G-actin
• Controlled by Ca2+
© 2012 Pearson Education, Inc.
Figure 10-7ab Thick and Thin Filaments
Sarcomere
H band
Actinin Z line
Titin
Myofibril
The gross structure of a thin
filament, showing the
attachment at the Z line
Z line
M line
Troponin Active site
Nebulin
Tropomyosin G-actin
molecules
F-actin
strand
The organization of G-actin subunits
in an F-actin strand, and the position
of the troponin–tropomyosin complex
© 2012 Pearson Education, Inc.
Figure 10-7a Thick and Thin Filaments
Actinin Z line
Titin
The gross structure of a thin
filament, showing the
attachment at the Z line
© 2012 Pearson Education, Inc.
Figure 10-7b Thick and Thin Filaments
Troponin Active site
Nebulin
Tropomyosin G-actin
molecules
F-actin
strand
The organization of G-actin subunits
in an F-actin strand, and the position
of the troponin–tropomyosin complex
© 2012 Pearson Education, Inc.
10-3 Structural Components of a Sarcomere
• Initiating Contraction
• Ca2+ binds to receptor on troponin molecule
• Troponin–tropomyosin complex changes
• Exposes active site of F-actin
© 2012 Pearson Education, Inc.
10-3 Structural Components of a Sarcomere
• Thick Filaments
• Contain about 300 twisted myosin subunits
• Contain titin strands that recoil after stretching
• The mysosin molecule
• Tail
• Binds to other myosin molecules
• Head
• Made of two globular protein subunits
• Reaches the nearest thin filament
© 2012 Pearson Education, Inc.
Figure 10-7cd Thick and Thin Filaments
Titin
The structure of
thick filaments,
showing the
orientation of the
myosin molecules
© 2012 Pearson Education, Inc.
M line
Myosin
head
Myosin tail
Hinge
The structure of a myosin molecule
10-3 Structural Components of a Sarcomere
• Myosin Action
• During contraction, myosin heads:
• Interact with actin filaments, forming crossbridges
• Pivot, producing motion
© 2012 Pearson Education, Inc.
10-3 Structural Components of a Sarcomere
• Sliding Filaments and Muscle Contraction
• Sliding filament theory
• Thin filaments of sarcomere slide toward M line,
alongside thick filaments
• The width of A zone stays the same
• Z lines move closer together
© 2012 Pearson Education, Inc.
Figure 10-8a Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber
I band
Z line
A band
H band
Z line
A relaxed sarcomere showing
location of the A band, Z
lines, and I band.
© 2012 Pearson Education, Inc.
Figure 10-8b Changes in the Appearance of a Sarcomere during the Contraction of a Skeletal Muscle Fiber
I band
A band
Z line
H band
Z line
During a contraction, the A band stays the
same width, but the Z lines move closer
together and the I band gets smaller. When
the ends of a myofibril are free to move,
the sarcomeres shorten simultaneously
and the ends of the myofibril are pulled
toward its center.
© 2012 Pearson Education, Inc.
10-3 Structural Components of a Sarcomere
• Skeletal Muscle Contraction
• The process of contraction
• Neural stimulation of sarcolemma
• Causes excitation–contraction coupling
• Muscle fiber contraction
• Interaction of thick and thin filaments
• Tension production
© 2012 Pearson Education, Inc.
Figure 10-9 An Overview of Skeletal Muscle Contraction
Neural control
Excitation–contraction coupling
Excitation
Calcium
release
triggers
Thick-thin
filament interaction
Muscle fiber
contraction
leads to
Tension
production
© 2012 Pearson Education, Inc.
ATP
Figure 10-9 An Overview of Skeletal Muscle Contraction
Neural control
© 2012 Pearson Education, Inc.
Figure 10-9 An Overview of Skeletal Muscle Contraction
Excitation
Calcium
release
triggers
Thick-thin
filament interaction
© 2012 Pearson Education, Inc.
ATP
Figure 10-9 An Overview of Skeletal Muscle Contraction
Muscle fiber
contraction
leads to
Tension
production
© 2012 Pearson Education, Inc.
10-4 Components of the Neuromuscular Junction
• The Control of Skeletal Muscle Activity
• The neuromuscular junction (NMJ)
• Special intercellular connection between the
nervous system and skeletal muscle fiber
• Controls calcium ion release into the sarcoplasm
A&P FLIX Events at the Neuromuscular Junction
© 2012 Pearson Education, Inc.
Figure 10-11 Skeletal Muscle Innervation
Motor neuron
Path of electrical impulse
(action potential)
Axon
Neuromuscular
junction
Synaptic
terminal
SEE BELOW
Sarcoplasmic
reticulum
Motor
end plate
Myofibril
© 2012 Pearson Education, Inc.
Motor end plate
Figure 10-11 Skeletal Muscle Innervation
The cytoplasm of the synaptic
terminal contains vesicles
filled with molecules of
acetylcholine, or ACh.
Acetylcholine is a
neurotransmitter, a chemical
released by a neuron to change
the permeability or other
properties of another cell’s
plasma membrane. The
synaptic cleft and the
motor end plate contain
molecules of the enzyme
acetylcholinesterase (AChE),
which breaks down ACh.
Vesicles
The synaptic cleft, a
narrow space, separates
the synaptic terminal of
the neuron from the
opposing motor end
plate.
© 2012 Pearson Education, Inc.
Junctional AChE
fold of
motor end plate
ACh
Figure 10-11 Skeletal Muscle Innervation
The stimulus for ACh release
is the arrival of an electrical
impulse, or action potential,
at the synaptic terminal. An
action potential is a sudden
change in the transmembrane
potential that travels along
the length of the axon.
Arriving action
potential
© 2012 Pearson Education, Inc.
Figure 10-11 Skeletal Muscle Innervation
When the action potential
reaches the neuron’s synaptic
terminal, permeability
changes in the membrane
trigger the exocytosis of ACh
into the synaptic cleft.
Exocytosis occurs as vesicles
fuse with the neuron’s plasma
membrane.
Motor
end plate
© 2012 Pearson Education, Inc.
Figure 10-11 Skeletal Muscle Innervation
ACh molecules diffuse
across the synatpic cleft and
bind to ACh receptors on the
surface of the motor end
plate. ACh binding alters the
membrane’s permeability to
sodium ions. Because the
extracellular fluid contains a
high concentration of
sodium ions, and sodium
ion concentration inside the
cell is very low, sodium ions
rush into the sarcoplasm.
ACh
receptor site
© 2012 Pearson Education, Inc.
Figure 10-11 Skeletal Muscle Innervation
The sudden inrush of
sodium ions results in the
generation of an action
potential in the
sarcolemma. AChE quickly
breaks down the ACh on
the motor end plate and in
the synaptic cleft, thus
inactivating the ACh
receptor sites.
Action
potential
AChE
© 2012 Pearson Education, Inc.
10-4 Components of the Neuromuscular Junction
• Excitation–Contraction Coupling
• Action potential reaches a triad
• Releasing Ca2+
• Triggering contraction
• Requires myosin heads to be in “cocked” position
• Loaded by ATP energy
A&P FLIX Excitation-Contraction Coupling
© 2012 Pearson Education, Inc.
Figure 10-10 The Exposure of Active Sites
SARCOPLASMIC RETICULUM
Calcium channels
open
Myosin tail
(thick filament)
Tropomyosin
strand
Troponin
G-actin
(thin filament)
Active site
Nebulin
In a resting sarcomere, the
tropomyosin strands cover
the active sites on the thin
filaments, preventing
cross-bridge formation.
© 2012 Pearson Education, Inc.
When calcium ions enter
the sarcomere, they bind
to troponin, which
rotates and swings the
tropomyosin away from
the active sites.
Cross-bridge
formation then occurs,
and the contraction
cycle begins.
10-4 Skeletal Muscle Contraction
• The Contraction Cycle
1. Contraction Cycle Begins
2. Active-Site Exposure
3. Cross-Bridge Formation
4. Myosin Head Pivoting
5. Cross-Bridge Detachment
6. Myosin Reactivation
A&P FLIX The Cross Bridge Cycle
© 2012 Pearson Education, Inc.
Figure 10-12 The Contraction Cycle
Contraction Cycle Begins
The contraction cycle, which
involves a series of interrelated
steps, begins with the arrival of
calcium ions within the zone of
overlap.
Myosin head
Troponin
Tropomyosin
© 2012 Pearson Education, Inc.
Actin
Figure 10-12 The Contraction Cycle
Active-Site Exposure
Calcium ions bind to troponin,
weakening the bond between
actin and the troponin–
tropomyosin complex. The
troponin molecule then changes
position, rolling the tropomyosin
molecule away from the active
sites on actin and allowing
interaction with the energized
myosin heads.
Sarcoplasm
Active
site
© 2012 Pearson Education, Inc.
Figure 10-12 The Contraction Cycle
Cross-Bridge Formation
Once the active sites are
exposed, the energized
myosin heads bind to them,
forming cross-bridges.
© 2012 Pearson Education, Inc.
Figure 10-12 The Contraction Cycle
Myosin Head Pivoting
After cross-bridge formation,
the energy that was stored in
the resting state is released
as the myosin head pivots
toward the M line. This action
is called the power stroke;
when it occurs, the bound
ADP and phosphate group
are released.
© 2012 Pearson Education, Inc.
Figure 10-12 The Contraction Cycle
Cross-Bridge Detachment
When another ATP binds to
the myosin head, the link
between the myosin head and
the active site on the actin
molecule is broken. The
active site is now exposed
and able to form another
cross-bridge.
© 2012 Pearson Education, Inc.
Figure 10-12 The Contraction Cycle
Myosin Reactivation
Myosin reactivation
occurs when the free
myosin head splits ATP
into ADP and P. The
energy released is used to
recock the myosin head.
© 2012 Pearson Education, Inc.
Figure 10-12 The Contraction Cycle
Resting Sarcomere
Zone of overlap
(shown in sequence above)
© 2012 Pearson Education, Inc.
Figure 10-12 The Contraction Cycle
Contracted Sarcomere
© 2012 Pearson Education, Inc.
10-4 Skeletal Muscle Contraction
• Fiber Shortening
• As sarcomeres shorten, muscle pulls together,
producing tension
• Muscle shortening can occur at both ends of the
muscle, or at only one end of the muscle
• This depends on the way the muscle is attached at
the ends
© 2012 Pearson Education, Inc.
Figure 10-13 Shortening during a Contraction
When both ends are free to move, the ends of a
contracting muscle fiber move toward the center of
the muscle fiber.
When one end of a myofibril is fixed in position, and
the other end free to move, the free end is pulled
toward the fixed end.
© 2012 Pearson Education, Inc.
10-4 Skeletal Muscle Relaxation
• Relaxation
• Contraction Duration
• Depends on:
• Duration of neural stimulus
• Number of free calcium ions in sarcoplasm
• Availability of ATP
© 2012 Pearson Education, Inc.
10-4 Skeletal Muscle Relaxation
• Relaxation
• Ca2+ concentrations fall
• Ca2+ detaches from troponin
• Active sites are re-covered by tropomyosin
• Rigor Mortis
• A fixed muscular contraction after death
• Caused when:
• Ion pumps cease to function; ran out of ATP
• Calcium builds up in the sarcoplasm
© 2012 Pearson Education, Inc.
10-4 Skeletal Muscle Contraction and Relaxation
• Summary
• Skeletal muscle fibers shorten as thin filaments
slide between thick filaments
• Free Ca2+ in the sarcoplasm triggers contraction
• SR releases Ca2+ when a motor neuron
stimulates the muscle fiber
• Contraction is an active process
• Relaxation and return to resting length are
passive
© 2012 Pearson Education, Inc.
Table 10-1 Steps Involved in Skeletal Muscle Contraction and Relaxation
Steps in Initiating Muscle Contraction
Motor
Synaptic
terminal end plate
Steps in Muscle Relaxation
T tubule Sarcolemma
Action
potential
reaches
T tubule
ACh released, binding
to receptors
Sarcoplasmic
reticulum
releases Ca2
Active site
exposure,
cross-bridge
formation
ACh broken down by AChE
Sarcoplasmic
reticulum
recaptures Ca2
Ca2
Actin
Myosin
Active sites
covered, no
cross-bridge
interaction
Contraction
ends
Contraction
begins
Relaxation occurs,
passive return to
resting length
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Tension Production by Muscles Fibers
• As a whole, a muscle fiber is either contracted or
relaxed
• Depends on:
• The number of pivoting cross-bridges
• The fiber’s resting length at the time of stimulation
• The frequency of stimulation
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Tension Production by Muscles Fibers
• Length–Tension Relationships
• Number of pivoting cross-bridges depends on:
• Amount of overlap between thick and thin fibers
• Optimum overlap produces greatest amount of
tension
• Too much or too little reduces efficiency
• Normal resting sarcomere length
• Is 75% to 130% of optimal length
© 2012 Pearson Education, Inc.
Tension (percent of maximum)
Figure 10-14 The Effect of Sarcomere Length on Active Tension
Normal
range
Decreased length
Increased sarcomere length
Optimal resting length:
The normal range of
sarcomere lengths in the
body is 75 to 130 percent of
the optimal length.
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Tension Production by Muscles Fibers
• The Frequency of Stimulation
• A single neural stimulation produces:
• A single contraction or twitch
• Which lasts about 7–100 msec.
• Sustained muscular contractions
• Require many repeated stimuli
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
•
Tension Production by Muscles Fibers
•
Twitches
1. Latent period
•
The action potential moves through sarcolemma
•
Causing Ca2+ release
2. Contraction phase
•
Calcium ions bind
•
Tension builds to peak
3. Relaxation phase
•
Ca2+ levels fall
•
Active sites are covered and tension falls to
resting levels
© 2012 Pearson Education, Inc.
Figure 10-15a The Development of Tension in a Twitch
Eye muscle
Gastrocnemius
Tension
Soleus
Stimulus
Time (msec)
A myogram showing differences in
tension over time for a twitch in
different skeletal muscles.
© 2012 Pearson Education, Inc.
Figure 10-15b The Development of Tension in a Twitch
Tension
Maximum tension
development
Stimulus
Resting Latent Contraction
phase period
phase
Relaxation
phase
The details of tension over time for a single
twitch in the gastrocnemius muscle. Notice the
presence of a latent period, which corresponds
to the time needed for the conduction of an
action potential and the subsequent release of
calcium ions by the sarcoplasmic reticulum.
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Tension Production by Muscles Fibers
• Treppe
• A stair-step increase in twitch tension
• Repeated stimulations immediately after relaxation
phase
• Stimulus frequency <50/second
• Causes a series of contractions with increasing
tension
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Tension Production by Muscles Fibers
• Wave summation
• Increasing tension or summation of twitches
• Repeated stimulations before the end of relaxation
phase
• Stimulus frequency >50/second
• Causes increasing tension or summation of
twitches
© 2012 Pearson Education, Inc.
Figure 10-16ab Effects of Repeated Stimulations
 Stimulus
Tension
Maximum tension (in tetanus)
Maximum tension (in treppe)
Time
Time
Treppe. Treppe is an increase in
Wave summation. Wave
peak tension with each
successive stimulus delivered
shortly after the completion of
the relaxation phase of the
preceding twitch.
summation occurs when
successive stimuli arrive
before the relaxation phase
has been completed.
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Tension Production by Muscles Fibers
• Incomplete tetanus
• Twitches reach maximum tension
• If rapid stimulation continues and muscle is not
allowed to relax, twitches reach maximum level of
tension
• Complete tetanus
• If stimulation frequency is high enough, muscle
never begins to relax, and is in continuous
contraction
© 2012 Pearson Education, Inc.
Figure 10-16cd Effects of Repeated Stimulations
Tension
Maximum tension (in tetanus)
Time
Time
Incomplete tetanus.
Complete tetanus. During
Incomplete tetanus occurs if the
stimulus frequency increases
further. Tension production rises
to a peak, and the periods of
relaxation are very brief.
complete tetanus, the stimulus
frequency is so high that the
relaxation phase is eliminated;
tension plateaus at maximal
levels.
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Tension Production by Skeletal Muscles
• Depends on:
• Internal tension produced by muscle fibers
• External tension exerted by muscle fibers on
elastic extracellular fibers
• Total number of muscle fibers stimulated
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Motor Units and Tension Production
• Motor units in a skeletal muscle:
• Contain hundreds of muscle fibers
• That contract at the same time
• Controlled by a single motor neuron
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Motor Units and Tension Production
• Recruitment (multiple motor unit summation)
• In a whole muscle or group of muscles, smooth
motion and increasing tension are produced by slowly
increasing the size or number of motor units
stimulated
• Maximum tension
• Achieved when all motor units reach tetanus
• Can be sustained only a very short time
© 2012 Pearson Education, Inc.
Figure 10-17a The Arrangement and Activity of Motor Units in a Skeletal Muscle
Axons of
motor neurons
Motor
nerve
KEY
SPINAL CORD
Muscle fibers
Motor unit 1
Motor unit 2
Motor unit 3
Muscle fibers of different motor units are
intermingled, so the forces applied to the
tendon remain roughly balanced regardless of
which motor units are stimulated.
© 2012 Pearson Education, Inc.
Figure 10-17b The Arrangement and Activity of Motor Units in a Skeletal Muscle
Tension
Tension in tendon
Motor Motor Motor
unit 1 unit 2 unit 3
Time
© 2012 Pearson Education, Inc.
The tension applied to the
tendon remains relatively
constant, even though
individual motor units cycle
between contraction and
relaxation.
10-5 Tension Production and Contraction Types
• Motor Units and Tension Production
• Sustained tension
• Less than maximum tension
• Allows motor units rest in rotation
• Muscle tone
• The normal tension and firmness of a muscle at
rest
• Muscle units actively maintain body position,
without motion
• Increasing muscle tone increases metabolic
energy used, even at rest
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Motor Units and Tension Production
• Contraction are classified based on pattern of
tension production
• Isotonic contraction
• Isometric contraction
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Isotonic Contraction
• Skeletal muscle changes length
• Resulting in motion
• If muscle tension > load (resistance):
• Muscle shortens (concentric contraction)
• If muscle tension < load (resistance):
• Muscle lengthens (eccentric contraction)
© 2012 Pearson Education, Inc.
Figure 10-18a Concentric, Eccentric, and Isometric Contractions
Tendon
Muscle
contracts
(concentric
contraction)
2 kg
2 kg
Muscle
tension
(kg)
Amount of
load
Muscle
relaxes
Peak tension
production
Contraction
begins
Resting length
Muscle
length
(percent
of resting
length)
Time
© 2012 Pearson Education, Inc.
Figure 10-18b Concentric, Eccentric, and Isometric Contractions
Support removed
when contraction
begins
(eccentric contraction)
Muscle
tension
(kg)
Peak tension
production
Support removed,
contraction begins
6 kg
Resting length
6 kg
Time
© 2012 Pearson Education, Inc.
Muscle
length
(percent
of resting
length)
10-5 Tension Production and Contraction Types
• Isometric Contraction
• Skeletal muscle develops tension, but is
prevented from changing length
• iso- = same, metric = measure
© 2012 Pearson Education, Inc.
Figure 10-18c Concentric, Eccentric, and Isometric Contractions
Amount of load
Muscle
tension
(kg)
Muscle
contracts
(isometric
contraction)
Muscle
relaxes
Peak tension
production
Contraction
begins
6 kg
Length unchanged
Muscle
length
(percent
of resting
length)
6 kg
Time
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Load and Speed of Contraction
• Are inversely related
• The heavier the load (resistance) on a muscle
• The longer it takes for shortening to begin
• And the less the muscle will shorten
© 2012 Pearson Education, Inc.
Distance shortened
Figure 10-19 Load and Speed of Contraction
Small load
Intermediate load
Large load
Time (msec)
Stimulus
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Muscle Relaxation and the Return to Resting
Length
• Elastic Forces
• The pull of elastic elements (tendons and
ligaments)
• Expands the sarcomeres to resting length
• Opposing Muscle Contractions
• Reverse the direction of the original motion
• Are the work of opposing skeletal muscle pairs
© 2012 Pearson Education, Inc.
10-5 Tension Production and Contraction Types
• Muscle Relaxation and the Return to Resting
Length
• Gravity
• Can take the place of opposing muscle contraction
to return a muscle to its resting state
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• ATP Provides Energy For Muscle Contraction
• Sustained muscle contraction uses a lot of ATP
energy
• Muscles store enough energy to start contraction
• Muscle fibers must manufacture more ATP as
needed
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• ATP and CP Reserves
• Adenosine triphosphate (ATP)
• The active energy molecule
• Creatine phosphate (CP)
• The storage molecule for excess ATP energy in resting
muscle
• Energy recharges ADP to ATP
• Using the enzyme creatine kinase (CK)
• When CP is used up, other mechanisms generate ATP
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
•
ATP Generation
•
Cells produce ATP in two ways
1. Aerobic metabolism of fatty acids in the
mitochondria
2. Anaerobic glycolysis in the cytoplasm
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• Aerobic Metabolism
• Is the primary energy source of resting muscles
• Breaks down fatty acids
• Produces 34 ATP molecules per glucose molecule
• Glycolysis
• Is the primary energy source for peak muscular activity
• Produces two ATP molecules per molecule of glucose
• Breaks down glucose from glycogen stored in skeletal
muscles
© 2012 Pearson Education, Inc.
Table 10-2 Sources of Energy in a Typical Muscle Fiber
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• Energy Use and the Level of Muscular Activity
• Skeletal muscles at rest metabolize fatty acids
and store glycogen
• During light activity, muscles generate ATP
through anaerobic breakdown of carbohydrates,
lipids, or amino acids
• At peak activity, energy is provided by anaerobic
reactions that generate lactic acid as a
byproduct
© 2012 Pearson Education, Inc.
Figure 10-20 Muscle Metabolism
Fatty acids
Fatty acids
Blood vessels
Glucose
Glucose
Glycogen
Glycogen
Pyruvate
Mitochondria
Creatine
To myofibrils to support
muscle contraction
Resting muscle: Fatty acids are catabolized; the
Moderate activity: Glucose and fatty acids are
ATP produced is used to build energy reserves of ATP,
CP, and glycogen.
catabolized; the ATP produced is used to power
contraction.
Lactate
Glucose
Pyruvate
Glycogen
Creatine
Lactate
To myofibrils to support
muscle contraction
Peak activity: Most ATP is produced through glycolysis,
with lactate as a by-product. Mitochondrial activity
(not shown) now provides only about one-third of the
ATP consumed.
© 2012 Pearson Education, Inc.
Figure 10-20a Muscle Metabolism
Fatty acids
Blood vessels
Glucose
Mitochondria
Glycogen
Creatine
Resting muscle: Fatty acids are catabolized; the
ATP produced is used to build energy reserves of ATP,
CP, and glycogen.
© 2012 Pearson Education, Inc.
Figure 10-20b Muscle Metabolism
Fatty acids
Glucose
Glycogen
Pyruvate
To myofibrils to support
muscle contraction
Moderate activity: Glucose and fatty acids are
catabolized; the ATP produced is used to power
contraction.
© 2012 Pearson Education, Inc.
Figure 10-20c Muscle Metabolism
Lactate
Glucose
Pyruvate
Glycogen
Creatine
Lactate
To myofibrils to support
muscle contraction
Peak activity: Most ATP is produced through glycolysis,
with lactate as a by-product. Mitochondrial activity
(not shown) now provides only about one-third of the
ATP consumed.
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• Muscle Fatigue
• When muscles can no longer perform a required
activity, they are fatigued
• Results of Muscle Fatigue
• Depletion of metabolic reserves
• Damage to sarcolemma and sarcoplasmic
reticulum
• Low pH (lactic acid)
• Muscle exhaustion and pain
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• The Recovery Period
• The time required after exertion for muscles to
return to normal
• Oxygen becomes available
• Mitochondrial activity resumes
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• Lactic Acid Removal and Recycling
• The Cori Cycle
• The removal and recycling of lactic acid by the liver
• Liver converts lactate to pyruvate
• Glucose is released to recharge muscle glycogen
reserves
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• The Oxygen Debt
• After exercise or other exertion:
• The body needs more oxygen than usual to normalize
metabolic activities
• Resulting in heavy breathing
• Also called excess postexercise oxygen
consumption (EPOC)
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• Heat Production and Loss
• Active muscles produce heat
• Up to 70% of muscle energy can be lost as heat,
raising body temperature
© 2012 Pearson Education, Inc.
10-6 Energy to Power Contractions
• Hormones and Muscle Metabolism
• Growth hormone
• Testosterone
• Thyroid hormones
• Epinephrine
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
• Muscle Performance
• Force
• The maximum amount of tension produced
• Endurance
• The amount of time an activity can be sustained
• Force and endurance depend on:
• The types of muscle fibers
• Physical conditioning
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
•
Three Major Types of Skeletal Muscle Fibers
1. Fast fibers
2. Slow fibers
3. Intermediate fibers
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
• Fast Fibers
• Contract very quickly
• Have large diameter, large glycogen reserves,
few mitochondria
• Have strong contractions, fatigue quickly
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
• Slow Fibers
• Are slow to contract, slow to fatigue
• Have small diameter, more mitochondria
• Have high oxygen supply
• Contain myoglobin (red pigment, binds oxygen)
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
• Intermediate Fibers
• Are mid-sized
• Have low myoglobin
• Have more capillaries than fast fibers, slower to
fatigue
© 2012 Pearson Education, Inc.
Figure 10-21 Fast versus Slow Fibers
Slow fibers
Smaller diameter,
darker color due to
myoglobin; fatigue
resistant
LM  170
Fast fibers
Larger diameter,
paler color;
easily fatigued
LM  170
© 2012 Pearson Education, Inc.
LM  783
Table 10-3 Properties of Skeletal Muscle Fiber Types
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
• Muscle Performance and the Distribution of
Muscle Fibers
• White muscles
• Mostly fast fibers
• Pale (e.g., chicken breast)
• Red muscles
• Mostly slow fibers
• Dark (e.g., chicken legs)
• Most human muscles
• Mixed fibers
• Pink
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
• Muscle Hypertrophy
• Muscle growth from heavy training
• Increases diameter of muscle fibers
• Increases number of myofibrils
• Increases mitochondria, glycogen reserves
• Muscle Atrophy
• Lack of muscle activity
• Reduces muscle size, tone, and power
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
• Physical Conditioning
• Improves both power and endurance
• Anaerobic activities (e.g., 50-meter dash,
weightlifting)
• Use fast fibers
• Fatigue quickly with strenuous activity
• Improved by:
• Frequent, brief, intensive workouts
• Causes hypertrophy
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
• Physical Conditioning
• Improves both power and endurance
• Aerobic activities (prolonged activity)
• Supported by mitochondria
• Require oxygen and nutrients
• Improves:
• Endurance by training fast fibers to be more like
intermediate fibers
• Cardiovascular performance
© 2012 Pearson Education, Inc.
10-7 Types of Muscles Fibers and Endurance
• Importance of Exercise
• What you don’t use, you lose
• Muscle tone indicates base activity in motor units
of skeletal muscles
• Muscles become flaccid when inactive for days or
weeks
• Muscle fibers break down proteins, become
smaller and weaker
• With prolonged inactivity, fibrous tissue may
replace muscle fibers
© 2012 Pearson Education, Inc.
10-8 Cardiac Muscle Tissue
• Cardiac Muscle Tissue
• Cardiac muscle cells are striated and found
only in the heart
• Striations are similar to that of skeletal muscle
because the internal arrangement of
myofilaments is similar
© 2012 Pearson Education, Inc.
10-8 Cardiac Muscle Tissue
• Structural Characteristics of Cardiac Muscle
Tissue
• Unlike skeletal muscle, cardiac muscle cells
(cardiocytes):
• Are small
• Have a single nucleus
• Have short, wide T tubules
• Have no triads
• Have SR with no terminal cisternae
• Are aerobic (high in myoglobin, mitochondria)
• Have intercalated discs
© 2012 Pearson Education, Inc.
10-8 Cardiac Muscle Tissue
• Intercalated Discs
• Are specialized contact points between
cardiocytes
• Join cell membranes of adjacent cardiocytes (gap
junctions, desmosomes)
• Functions of intercalated discs:
• Maintain structure
• Enhance molecular and electrical connections
• Conduct action potentials
© 2012 Pearson Education, Inc.
10-8 Cardiac Muscle Tissue
• Intercalated Discs
• Coordination of cardiocytes
• Because intercalated discs link heart cells
mechanically, chemically, and electrically, the
heart functions like a single, fused mass of cells
© 2012 Pearson Education, Inc.
Figure 10-22a Cardiac Muscle Tissue
Cardiac
muscle cell
Intercalated
discs
Nucleus
Cardiac muscle tissue
LM  575
A light micrograph of cardiac muscle tissue.
© 2012 Pearson Education, Inc.
Figure 10-22b Cardiac Muscle Tissue
Cardiac muscle
cell (intact)
Intercalated disc
(sectioned)
A diagrammatic view of
cardiac muscle. Note
the striations and
intercalated
discs.
Mitochondria
Nucleus
Myofibrils
Intercalated
disc
© 2012 Pearson Education, Inc.
Cardiac muscle cell
(sectioned)
Figure 10-22c Cardiac Muscle Tissue
Entrance to T tubule
Sarcolemma
Mitochondrion
Myofibrils
Contact of sarcoplasmic
reticulum with
T tubule
Sarcoplasmic
reticulum
Cardiac muscle tissue showing short, broad
T-tubules and SR that lacks terminal cisternae.
© 2012 Pearson Education, Inc.
10-8 Cardiac Muscle Tissue
• Functional Characteristics of Cardiac Muscle Tissue
• Automaticity
• Contraction without neural stimulation
• Controlled by pacemaker cells
• Variable contraction tension
• Controlled by nervous system
• Extended contraction time
• Ten times as long as skeletal muscle
• Prevention of wave summation and tetanic
contractions by cell membranes
• Long refractory period
© 2012 Pearson Education, Inc.
10-9 Smooth Muscle Tissue
• Smooth Muscle in Body Systems
• Forms around other tissues
• In integumentary system
• Arrector pili muscles cause “goose bumps”
• In blood vessels and airways
• Regulates blood pressure and airflow
• In reproductive and glandular systems
• Produces movements
• In digestive and urinary systems
• Forms sphincters
• Produces contractions
© 2012 Pearson Education, Inc.
10-9 Smooth Muscle Tissue
• Structural Characteristics of Smooth Muscle
Tissue
• Nonstriated tissue
• Different internal organization of actin and myosin
• Different functional characteristics
© 2012 Pearson Education, Inc.
Figure 10-23a Smooth Muscle Tissue
Circular
muscle layer
Longitudinal
muscle layer
Smooth muscle tissue
LM  100
Many visceral organs contain several layers of
smooth muscle tissue oriented in different
directions. Here, a single sectional view shows
smooth muscle cells in both longitudinal (L) and
transverse (T) sections.
© 2012 Pearson Education, Inc.
Figure 10-23b Smooth Muscle Tissue
Relaxed (sectional view)
Dense body
Myosin
Actin
Relaxed (superficial view)
Intermediate
filaments (desmin)
Adjacent smooth muscle cells are
bound together at dense bodies,
transmitting the contractile forces
from cell to cell throughout the tissue.
Contracted
(superficial
view)
A single relaxed smooth muscle cell is spindle
shaped and has no striations. Note the changes in
cell shape as contraction occurs.
© 2012 Pearson Education, Inc.
10-9 Smooth Muscle Tissue
• Characteristics of Smooth Muscle Cells
• Long, slender, and spindle shaped
• Have a single, central nucleus
• Have no T tubules, myofibrils, or sarcomeres
• Have no tendons or aponeuroses
• Have scattered myosin fibers
• Myosin fibers have more heads per thick filament
• Have thin filaments attached to dense bodies
• Dense bodies transmit contractions from cell to
cell
© 2012 Pearson Education, Inc.
10-9 Smooth Muscle Tissue
• Functional Characteristics of Smooth Muscle
Tissue
1. Excitation–contraction coupling
2. Length–tension relationships
3. Control of contractions
4. Smooth muscle tone
© 2012 Pearson Education, Inc.
10-9 Smooth Muscle Tissue
• Excitation–Contraction Coupling
• Free Ca2+ in cytoplasm triggers contraction
• Ca2+ binds with calmodulin
• In the sarcoplasm
• Activates myosin light–chain kinase
• Enzyme breaks down ATP, initiates contraction
© 2012 Pearson Education, Inc.
10-9 Smooth Muscle Tissue
• Length–Tension Relationships
• Thick and thin filaments are scattered
• Resting length not related to tension development
• Functions over a wide range of lengths
(plasticity)
© 2012 Pearson Education, Inc.
10-9 Smooth Muscle Tissue
• Control of Contractions
• Multiunit smooth muscle cells
• Connected to motor neurons
• Visceral smooth muscle cells
• Not connected to motor neurons
• Rhythmic cycles of activity controlled by pacesetter
cells
© 2012 Pearson Education, Inc.
10-9 Smooth Muscle Tissue
• Smooth Muscle Tone
• Maintains normal levels of activity
• Modified by neural, hormonal, or chemical factors
© 2012 Pearson Education, Inc.
Table 10-4 A Comparison of Skeletal, Cardiac, and Smooth Muscle Tissues
© 2012 Pearson Education, Inc.