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CHAPTER
Elaine N. Marieb
Katja Hoehn
PART A
Human
Anatomy
& Physiology
SEVENTH EDITION
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Muscles and
Muscle Tissue
Muscle Overview
The three types of muscle tissue are cardiac,
smooth and skeletal
These types differ in structure, location, function,
and means of activation
PLAY
InterActive Physiology ®:
Anatomy Review: Skeletal Muscle Tissue, page 3
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Muscle Function
Skeletal muscles are
responsible for all locomotion,
plus to maintain posture,
stabilize joints, and generate
heat.
Cardiac muscle is responsible
for pushing the blood through
the body
Smooth muscle helps maintain
blood pressure, and squeezes or
propels substances (i.e., food,
feces) through organs
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Muscle Similarities
Skeletal and smooth muscle cells are elongated
and are called muscle fibers
Muscle contraction depends on two kinds of
myofilaments – actin and
myosin
Muscle terminology is similar
Sarcolemma – muscle plasma membrane
Sarcoplasm – cytoplasm of a muscle cell
Prefixes – myo, mys, and sarco all refer to muscle
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Cardiac Muscle Tissue
Occurs only in the heart
Is striated like skeletal
muscle but is not voluntary
Contracts at a fairly steady rate
set by the heart’s pacemaker
Neural controls allow the
heart to respond to
changes in bodily needs
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Smooth Muscle Tissue
Found in the walls of hollow visceral organs,
such as the stomach, urinary bladder, and
respiratory passages
Forces food and other substances through internal
body channels
It is not striated
and is not voluntary
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Skeletal Muscle
Each muscle is a discrete organ composed of
muscle tissue, blood vessels, nerve fibers, and
connective tissue
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Skeletal Muscle Tissue
Is controlled voluntarily
(by conscious control)
Contracts rapidly but tires
easily
Is responsible for overall
body motility
Is extremely adaptable
and can exert forces
ranging from a fraction of
an ounce to over 70
pounds
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Contracted vs, not contracted
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Functional Characteristics of Muscle Tissue
Excitability, or irritability – the ability to receive
and respond to stimuli
Contractility – the ability to shorten forcibly
Extensibility – the ability to be stretched or
extended
Elasticity – the ability to recoil and resume the
original resting length
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Structure and Organization of Skeletal Muscle
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Table 9.1a
Structure and Organization of Skeletal Muscle
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Table 9.1b
Skeletal Muscle
The three connective tissue sheaths are:
Endomysium –areolar and reticular fibers
surrounding each muscle fiber
Perimysium – fibrous connective tissue that
surrounds groups of muscle fibers (cells) called
fascicles
Epimysium –dense irregular connective tissue that
surrounds the entire muscle
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Skeletal Muscle: Nerve and Blood Supply
Each muscle is served by one nerve, an artery,
and one or more veins
Each skeletal muscle fiber is supplied with a
nerve ending that controls contraction
Contracting fibers require continuous delivery of
oxygen and nutrients via arteries
Wastes must be removed via veins
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Skeletal Muscle: Attachments
Most skeletal muscles span joints and are attached
to bone in at least two places
When muscles contract, the movable bone
(insertion) moves toward the immovable bone
(origin)
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Skeletal Muscle: Attachments
Muscles attach:
Directly – epimysium of the
muscle is fused to the
periosteum of a bone
Indirectly – connective tissue
wrappings extend beyond the
muscle as a tendon or
aponeurosis
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Microscopic Anatomy of a Skeletal Muscle
Fiber
Each fiber is a long, cylindrical cell with multiple
nuclei just beneath the sarcolemma (plasma
membrane)
Fibers are huge cells, 10 to 100 µm in diameter,
and up to 30 centimeters long (about 10x larger
than the average body cell!)
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Microscopic Anatomy of a Skeletal Muscle
Fiber
Sarcoplasm has numerous glycosomes and a
unique oxygen-binding protein called myoglobin
Fibers contain the usual organelles, myofibrils,
sarcoplasmic reticulum, and T tubules
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Myofibrils
Myofibrils are densely packed, rodlike contractile
elements
They make up most of the muscle volume
The arrangement of myofibrils within a fiber is such
that a perfectly aligned repeating series of dark A
bands and light I bands is evident
Myofibril
Cell
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Myofibrils
Figure 9.3b
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Sarcomeres
The smallest contractile unit of a muscle
The region of a myofibril between two successive
Z discs
Composed of myofilaments made up of contractile
proteins
Myofilaments are of two types – thick and thin
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Sarcomeres
Figure 9.3c
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Myofilaments: Banding Pattern
Thick filaments – extend the entire length of an A
band
Thin filaments – extend across the I band and
partway into the A band
Z-disc – coin-shaped sheet of proteins that anchors
the thin filaments and connects myofibrils to one
another
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Myofilaments: Banding Pattern
Figure 9.3c,d
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Ultrastructure of Myofilaments: Thick
Filaments
Thick filaments are composed of the protein
myosin
Each myosin molecule has a rod-like tail and two
globular heads
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Ultrastructure of Myofilaments: Thin
Filaments
Thin filaments are chiefly composed of the
protein actin
Contain sites to which myosin heads attach
during contraction
Tropomyosin and troponin are regulatory
subunits bound to actin
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Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Arrangement of the Filaments in a Sarcomere
Longitudinal section within one sarcomere
Figure 9.4d
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Sarcoplasmic Reticulum (SR)
SR is endoplasmic reticulum that mostly runs
longitudinally and surrounds each myofibril
Terminal cisternae form perpendicular cross channels
Functions in the regulation of intracellular calcium
levels (Ca+2)
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Sarcoplasmic Reticulum (SR)
T tubules penetrate into the cell’s interior at each
A band–I band junction
T tubules associate with the paired terminal
cisternae to form triads
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T Tubules
T tubules are continuous with the sarcolemma
They conduct impulses to the deepest regions of
the muscle
These impulses signal for the release of Ca2+ from
adjacent terminal cisternae
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Sliding Filament Model of Contraction
Thin filaments slide past the thick ones so that the
actin and myosin filaments overlap to a greater
degree
In the relaxed state, thin and thick filaments
overlap only slightly
Upon stimulation, myosin heads bind to actin and
sliding begins
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Sliding Filament Model of Contraction
Each myosin head binds and detaches several
times during contraction, acting like a ratchet to
generate tension and propel the thin filaments to
the center of the sarcomere
As this event occurs throughout the sarcomeres,
the muscle shortens
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Skeletal Muscle Contraction
In order to contract, a skeletal muscle must:
Be stimulated by a nerve ending
Propagate an electrical current, or action potential,
along its sarcolemma
Have a rise in intracellular Ca2+ levels, the final
trigger for contraction
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Nerve Stimulus of Skeletal Muscle
Skeletal muscles are stimulated by motor neurons of
the somatic nervous system
Axons of these neurons travel in nerves to muscle cells
Axons of motor neurons branch profusely as they enter
muscles
Each axonal branch forms a neuromuscular junction
with a single muscle fiber
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Neuromuscular Junction
The neuromuscular junction is formed from:
Axonal endings, have small membranous sacs
(synaptic vesicles) that contain the
neurotransmitter acetylcholine (ACh)
The motor end plate of a muscle is a specific part
of the sarcolemma that contains ACh receptors and
helps form the neuromuscular junction
Though exceedingly close, axonal ends and muscle
fibers are always separated by a space called the
synaptic cleft
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Neuromuscular Junction
Figure 9.7 (a-c)
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Neuromuscular Junction
When a nerve impulse reaches the end of an axon
at the neuromuscular junction:
Voltage-regulated calcium channels open and
allow Ca2+ to enter the axon
Ca2+ inside the axon terminal causes axonal
vesicles to fuse with the axonal membrane
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Neuromuscular Junction
This fusion releases ACh into the synaptic cleft via
exocytosis
ACh diffuses across the synaptic cleft to ACh
receptors on the sarcolemma
Binding of ACh to its receptors initiates an action
potential in the muscle
What happens @ the Neuromuscular Junction
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Destruction of Acetylcholine
ACh bound to ACh receptors is quickly destroyed
by the enzyme acetylcholinesterase
This destruction prevents continued muscle fiber
contraction in the absence of additional stimuli
Events at the neuromuscular junction
Generation of an action potential
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Excitation-Contraction Coupling
Excitation-Contraction Coupling
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Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Axon terminal
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1 Net entry of Na+ Initiates
ACh
ACh
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
ACh
Ca2+
Ca2+
SR tubules (cut)
SR
Ca2+
Ca2+
2 Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
ADP
Pi
6
Ca2+
Ca2+
Tropomyosin blockage restored,
blocking myosin binding sites on
actin; contraction ends and
muscle fiber relaxes.
Ca2+
Ca2+
3 Calcium ions bind to troponin;
Ca2+
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
5 Removal of Ca2+ by active transport
into the SR after the action
potential ends.
Ca2+
4 Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
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Figure 9.10
Axon terminal
Synaptic
cleft
Synaptic
vesicle
ACh
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
ACh
ACh
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Figure 9.10
Axon terminal
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1 Net entry of Na+ Initiates
ACh
ACh
ACh
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 9.10
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Axon terminal
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1 Net entry of Na+ Initiates
ACh
ACh
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
ACh
Ca2+
Ca2+
SR tubules (cut)
SR
Ca2+
Ca2+
2 Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Ca2+
Ca2+
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Ca2+
Ca2+
Figure 9.10
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Axon terminal
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1 Net entry of Na+ Initiates
ACh
ACh
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
ACh
Ca2+
Ca2+
SR tubules (cut)
SR
Ca2+
Ca2+
2 Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Ca2+
Ca2+
Ca2+
Ca2+
3 Calcium ions bind to troponin;
Ca2+
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
Figure 9.10
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Axon terminal
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1 Net entry of Na+ Initiates
ACh
ACh
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
ACh
Ca2+
Ca2+
SR tubules (cut)
SR
Ca2+
Ca2+
2 Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Ca2+
Ca2+
Ca2+
Ca2+
3 Calcium ions bind to troponin;
Ca2+
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
4 Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 9.10
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Axon terminal
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1 Net entry of Na+ Initiates
ACh
ACh
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
ACh
Ca2+
Ca2+
SR tubules (cut)
SR
Ca2+
Ca2+
2 Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
Ca2+
Ca2+
Ca2+
Ca2+
3 Calcium ions bind to troponin;
Ca2+
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
5 Removal of Ca2+ by active transport
into the SR after the action
potential ends.
Ca2+
4 Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 9.10
Neurotransmitter released diffuses
across the synaptic cleft and attaches
to ACh receptors on the sarcolemma.
Axon terminal
Synaptic
cleft
Synaptic
vesicle
Sarcolemma
T tubule
1 Net entry of Na+ Initiates
ACh
ACh
an action potential which
is propagated along the
sarcolemma and down
the T tubules.
ACh
Ca2+
Ca2+
SR tubules (cut)
SR
Ca2+
Ca2+
2 Action potential in
T tubule activates
voltage-sensitive receptors,
which in turn trigger Ca2+
release from terminal
cisternae of SR
into cytosol.
ADP
Pi
6
Ca2+
Ca2+
Tropomyosin blockage restored,
blocking myosin binding sites on
actin; contraction ends and
muscle fiber relaxes.
Ca2+
Ca2+
3 Calcium ions bind to troponin;
Ca2+
troponin changes shape, removing
the blocking action of tropomyosin;
actin active sites exposed.
5 Removal of Ca2+ by active transport
into the SR after the action
potential ends.
Ca2+
4 Contraction; myosin heads alternately attach to
actin and detach, pulling the actin filaments toward
the center of the sarcomere; release of energy by
ATP hydrolysis powers the cycling process.
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Figure 9.10
Sequential Events of Contraction
Cross bridge formation – myosin cross bridge
attaches to actin filament
Working (power) stroke – myosin head pivots and
pulls actin filament toward M line
Cross bridge detachment – ATP attaches to myosin
head and the cross bridge detaches
“Cocking” of the myosin head – energy from
hydrolysis of ATP cocks the myosin head into the
high-energy state
PLAY
InterActive Physiology ®: Sliding Filament Theory, pages 3-29
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Myosin head
(high-energy
configuration)
ADP
Pi
1 Myosin head attaches to the actin
myofilament, forming a cross bridge.
Thin filament
ATP
hydrolysis
ADP
ADP
Thick filament
Pi
2 Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
4 As ATP is split into ADP and Pi, the myosin
head is energized (cocked into the high-energy
conformation).
ATP
ATP
Myosin head
(low-energy
configuration)
3 As new ATP attaches to the myosin head, the link between
myosin and actin weakens, and the cross bridge detaches.
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Figure 9.12
Myosin head
(high-energy
configuration)
ADP
Pi
1 Myosin head attaches to the actin
myofilament, forming a cross bridge.
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Figure 9.12
Myosin head
(high-energy
configuration)
ADP
Pi
1 Myosin head attaches to the actin
myofilament, forming a cross bridge.
ADP
2 Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 9.12
Myosin head
(high-energy
configuration)
ADP
Pi
1 Myosin head attaches to the actin
myofilament, forming a cross bridge.
ADP
2 Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
ATP
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Figure 9.12
Myosin head
(high-energy
configuration)
ADP
Pi
1 Myosin head attaches to the actin
myofilament, forming a cross bridge.
ADP
2 Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
ATP
ATP
Myosin head
(low-energy
configuration)
3 As new ATP attaches to the myosin head, the link between
myosin and actin weakens, and the cross bridge detaches.
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Figure 9.12
Myosin head
(high-energy
configuration)
ADP
Pi
1 Myosin head attaches to the actin
myofilament, forming a cross bridge.
Thin filament
ATP
hydrolysis
ADP
ADP
Thick filament
Pi
2 Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
4 As ATP is split into ADP and Pi, the myosin
head is energized (cocked into the high-energy
conformation).
ATP
ATP
Myosin head
(low-energy
configuration)
3 As new ATP attaches to the myosin head, the link between
myosin and actin weakens, and the cross bridge detaches.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 9.12
Myosin head
(high-energy
configuration)
ADP
Pi
1 Myosin head attaches to the actin
myofilament, forming a cross bridge.
Thin filament
ATP
hydrolysis
ADP
ADP
Thick filament
Pi
2 Inorganic phosphate (Pi) generated in the
previous contraction cycle is released, initiating
the power (working) stroke. The myosin head
pivots and bends as it pulls on the actin filament,
sliding it toward the M line. Then ADP is released.
4 As ATP is split into ADP and Pi, the myosin
head is energized (cocked into the high-energy
conformation).
ATP
ATP
Myosin head
(low-energy
configuration)
3 As new ATP attaches to the myosin head, the link between
myosin and actin weakens, and the cross bridge detaches.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 9.12
Contraction of Skeletal Muscle Fibers
Contraction – refers to the activation of myosin’s
cross bridges (force-generating sites)
Shortening occurs when the tension generated by
the cross bridge exceeds forces opposing
shortening
Contraction ends when cross bridges become
inactive, the tension generated declines, and
relaxation is induced
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Overview – Contraction of a Skeletal Muscle
Principles of muscle mechanics:
Force exerted by a contracting muscle on an object
is muscle tension.
Opposing force on the muscle by the weight of the
object is called the load.
A contracting muscle does not always shorten and
move the load.
Isometric vs. Isotonic
A muscle contracts with varying force and for
different periods of time in response to stimuli
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The Motor Unit
Each muscle is
served by at
least one motor
nerve
Contains axons
of up to
hundreds of
motor neurons.
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The Motor Unit
A motor unit = motor neuron and all the muscle
fibers it supplies.
Motor neuron fires - all of the fibers it innervates
contract
May have as many as several hundred or as few as
four
Fine control vs. gross (large) movements
Muscle fibers are spread out within a unit
Stimulation of a single motor unit causes a weak
contraction of the entire muscle.
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The Muscle Twitch
Muscle twitch = the response of the motor unit to a
single action potential
The muscle fibers contract quickly and then relax
1. Latent: excitationcontraction coupling is
occurring
2. Contraction: cross
bridges are active
3. Relaxation: initiated
by reentry of Ca2+ into
the SR
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Developmental Aspects
All muscle tissues develop from embryonic
mesoderm cells called myoblasts
Skeletal muscle myoblasts fuse (multiple nuclei)
Cardiac and Smooth develop gap junctions
Smooth muscle has good ability to regenerate
throughout life
Muscle mass differs between sexes due to
testosterone
Muscle is highly vascularized – resistant to
infection
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