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Chapter 9
The Muscular
System
Skeletal Muscle
Tissue and Muscle
Organization
Lecture Presentation by
Steven Bassett
Southeast Community College
© 2015 Pearson Education, Inc.
Introduction
• Humans rely on muscles for:
• Many of our physiological processes
• Virtually all our dynamic interactions with the
environment
• Skeletal muscles consist of:
• Elongated cells called fibers (muscle fibers)
• These fibers contract along their longitudinal axis
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Introduction
• There are three types of muscle tissue
• Skeletal muscle
• Pulls on skeletal bones
• Voluntary contraction
• Cardiac muscle
• Pushes blood through arteries and veins
• Rhythmic contractions
• Smooth muscle
• Pushes fluids and solids along the digestive tract,
for example
• Involuntary contraction
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Introduction
• Muscle tissues share four basic properties
• Excitability
• The ability to respond to stimuli
• Contractility
• The ability to shorten and exert a pull or tension
• Extensibility
• The ability to continue to contract over a range of
resting lengths
• Elasticity
• The ability to rebound toward its original length
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Functions of Skeletal Muscles
• Skeletal muscles perform the following functions:
• Produce skeletal movement
• Pull on tendons to move the bones
• Maintain posture and body position
• Stabilize the joints to aid in posture
• Support soft tissue
• Support the weight of the visceral organs
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Functions of Skeletal Muscles
• Skeletal muscles perform the following
functions (continued):
• Regulate entering and exiting of material
• Voluntary control over swallowing, defecation, and
urination
• Maintain body temperature
• Some of the energy used for contraction is
converted to heat
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Anatomy of Skeletal Muscles
• Gross anatomy is the study of:
•
•
•
•
Overall organization of muscles
Connective tissue associated with muscles
Nerves associated with muscles
Blood vessels associated with muscles
• Microscopic anatomy is the study of:
• Myofibrils
• Myofilaments
• Sarcomeres
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Anatomy of Skeletal Muscles
• Gross Anatomy
• Connective tissue of muscle
• Epimysium: dense tissue that surrounds the entire
muscle
• Perimysium: dense tissue that divides the muscle
into parallel compartments of fascicles
• Endomysium: dense tissue that surrounds
individual muscle fibers
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Figure 9.1 Structural Organization of Skeletal Muscle
Nerve
Epimysium
Muscle fascicle
Endomysium
Perimysium
Muscle fibers
Blood vessels
SKELETAL MUSCLE
(organ)
Perimysium
Muscle fiber
Endomysium
Epimysium
Blood vessels
and nerves
Endomysium
MUSCLE FASCICLE
(bundle of cells)
Capillary
Endomysium
Mitochondria
Sarcolemma
Tendon
Myosatellite
cell
Myofibril
Perimysium
Axon
Sarcoplasm
Nucleus
MUSCLE FIBER
(cell)
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Anatomy of Skeletal Muscles
• Connective Tissue of Muscle
• Tendons and aponeuroses
• Epimysium, perimysium, and endomysium
converge to form tendons
• Tendons connect a muscle to a bone
• Aponeuroses connect a muscle to a muscle
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Anatomy of Skeletal Muscles
• Gross Anatomy
• Nerves and blood vessels
• Nerves innervate the muscle by penetrating the
epimysium
• There is a chemical communication between a
nerve and a muscle
• The chemical is released into the neuromuscular
synapse (neuromuscular junction)
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Figure 9.2 Skeletal Muscle Innervation
Neuromuscular
synapse
Skeletal
muscle
fiber
Axon
Nerve
LM x 230
a A neuromuscular synapse as seen
on a muscle fiber of this fascicle
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SEM x 400
b Colorized SEM of a neuromuscular
synapse
Anatomy of Skeletal Muscles
• Gross Anatomy
• Nerves and blood vessels (continued)
• Blood vessels often parallel the nerves that
innervate the muscle
• They then branch to form coiled networks to
accommodate flexion and extension of the muscle
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Anatomy of Skeletal Muscles
• Microanatomy of Skeletal Muscle Fibers
• Sarcolemma
• Membrane that surrounds the muscle cell
• Sarcoplasm
• The cytosol of the muscle cell
• Muscle fiber (same thing as a muscle cell)
• Can be 30–40 cm in length
• Multinucleate (each muscle cell has hundreds of
nuclei)
• Nuclei are located just deep to the sarcolemma
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Figure 9.3ab The Formation and Structure of a Skeletal Muscle Fiber
Muscle fibers develop
through the fusion of
mesodermal cells
called myoblasts.
Myoblasts
a Development of a
skeletal muscle fiber.
Myosatellite cell
Nuclei
Immature
muscle fiber
b External appearance
and histological view.
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Anatomy of Skeletal Muscles
• Myofibrils and Myofilaments
• The sarcoplasm contains myofibrils
• Myofibrils are responsible for the contraction of
muscles
• Myofibrils are attached to the sarcolemma at each
end of the muscle cell
• Surrounding each myofibril is the sarcoplasmic
reticulum
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Anatomy of Skeletal Muscles
• Myofibrils and Myofilaments
• Myofibrils are made of myofilaments
• Actin
• Thin protein filaments
• Myosin
• Thick protein filaments
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Figure 9.3b-d The Formation and Structure of a Skeletal Muscle Fiber
b External appearance
and histological view.
Myofibril
Sarcolemma
c The external organization
of a muscle fiber.
Nuclei
Sarcoplasm
MUSCLE FIBER
Mitochondria
Terminal cisterna
Sarcolemma
Sarcolemma
Sarcoplasm
Myofibril
Myofibrils
Thin filament
Thick filament
d Internal organization of a muscle fiber.
Note the relationships among myofibrils,
sarcoplasmic reticulum, mitochondria,
triads, and thick and thin filaments.
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Triad Sarcoplasmic T tubules
reticulum
Anatomy of Skeletal Muscles
• Sarcomere Organization
• Myosin (thick filament)
• Actin (thin filament)
• Both are arranged in repeating units called
sarcomeres
• All the myofilaments are arranged parallel to the
long axis of the cell
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Anatomy of Skeletal Muscles
• Sarcomere Organization
• Sarcomere
•
•
•
•
Main functioning unit of muscle fibers
Approximately 10,000 per myofibril
Consists of overlapping actin and myosin
This overlapping creates the striations that give the
skeletal muscle its identifiable characteristic
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Anatomy of Skeletal Muscles
• Sarcomere Organization
• Each sarcomere consists of:
•
•
•
•
•
Z line (Z disc)
I band
A band (overlapping A bands create striations)
H band
M line
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Figure 9.4b Sarcomere Structure
I band
A band
H band
Zone of overlap
M line
Z line
Titin
Thin
filament
Thick
filament
Sarcomere
I band
A band
H band
Z line
b A corresponding view of a sarcomere in a myofibril in
the gastrocnemius muscle of the calf and a diagram
showing the various components of this sarcomere
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Zone of overlap
M line
Sarcomere
Z line
TEM x 64,000
Anatomy of Skeletal Muscles
• Sarcomere Organization
•
•
•
•
•
•
Skeletal muscles consist of muscle fascicles
Muscle fascicles consist of muscle fibers
Muscle fibers consist of myofibrils
Myofibrils consist of sarcomeres
Sarcomeres consist of myofilaments
Myofilaments are made of actin and myosin
© 2015 Pearson Education, Inc.
Figure 9.5 Levels of Functional Organization in a Skeletal Muscle Fiber
SKELETAL MUSCLE
Surrounded by:
Epimysium
Contains:
Muscle fascicles
MUSCLE FASCICLE
Surrounded by:
Perimysium
Contains:
Muscle fibers
MUSCLE FIBER
Surrounded by:
Endomysium
Contains:
Myofibrils
MYOFIBRIL
Surrounded by:
Sarcoplasmic
reticulum
Consists of:
Sarcomeres
(Z line to Z line)
SARCOMERE
I band
A band
Contains:
Thick filaments
Thin filaments
Z line
M line
H band
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Titin Z line
Anatomy of Skeletal Muscles
• Thin Filaments (Actin)
• Consists of:
• Twisted filaments of :
• F actin strands
• G actin globular molecules
• G actin molecules consist of an active site (binding
site)
• Tropomyosin: A protein that covers the binding
sites when the muscle is relaxed
• Troponin: Holds tropomyosin in position
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Figure 9.6ab Thin and Thick Filaments
Actinin Z line Titin
Sarcomere
a The attachment
of thin filaments
to the Z line
H band
Troponin
Active site
Nebulin
Tropomyosin G actin molecules
F actin
strand
Myofibril
b The detailed structure of a thin filament showing
the organization of G actin, troponin, and
tropomyosin
Z line
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M line
Anatomy of Skeletal Muscles
• Thick Filaments (Myosin)
• Myosin filaments consist of an elongated tail and a
globular head (cross-bridges)
• Myosin is a stationary molecule. It is held in place
by:
• Protein forming the M line
• A core of titin connecting to the Z lines
• Myosin heads project toward the actin filaments
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Figure 9.6cd Thin and Thick Filaments
Sarcomere
H band
Myofibril
Z line
M line
Titin
c The structure of
thick filaments
Myosin
head
M line
Myosin tail
Hinge
d A single myosin molecule detailing the structure and
movement of the myosin head after cross-bridge
binding occurs
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Muscle Contraction
• A contracting muscle shortens in length
• Contraction is caused by interactions between
thick and thin filaments within the sarcomere
• Contraction is triggered by the presence of
calcium ions
• Muscle contraction requires the presence of ATP
• When a muscle contracts, actin filaments slide
toward each other
• This sliding action is called the sliding filament
theory
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Muscle Contraction
• The Sliding Filament Theory
• Upon contraction:
•
•
•
•
The H band and I band get smaller
The zone of overlap gets larger
The Z lines move closer together
The width of the A band remains constant
throughout the contraction
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Figure 9.7 Sliding Filament Theory (1 of 11)
Contracted Sarcomere
Resting Sarcomere
After repeated cycles of “bind, pivot, detach, and reactivate”
the entire muscle completes its contraction.
A resting sarcomere showing the locations of the
I band, A band, H band, M, and Z lines.
I band
A band
M line
Contracted myofibril
I band
Z line
H band
A band
M line
Z line
Resting myofibril
Z line
H band
Z line
In a contracting sarcomere the A band stays the same width,
but the Z lines move closer together and the H band and the
I bands get smaller
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Muscle Contraction
• The Neural Control of Muscle Fiber Contraction
• An impulse travels down the axon of a nerve
• Acetylcholine is released from the end of the
axon into the neuromuscular synapse
• This ultimately causes the sarcoplasmic reticulum
to release its stored calcium ions
• This begins the actual contraction of the muscle
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Figure 9.8 The Neuromuscular Synapse
Arriving action
potential
Synaptic
vesicles
Synaptic
cleft
ACh
ACh receptor
site
Sarcolemma of
motor end plate
Motor
neuron
AChE molecules
Glial cell
Junctional fold
b Detailed view of a terminal, synaptic cleft,
and motor end plate. See also Figure 9.2.
Axon
Path of action
potential
Synaptic
terminal
Muscle Fiber
Myofibril
Motor end plate
Myofibril
Sarcolemma
Mitochondrion
a A diagrammatic view of a
neuromuscular synapse.
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Muscle Contraction
• Muscle Contraction: A Summary
• The nerve impulse ultimately causes the release
of a neurotransmitter (ACh), which comes in
contact with the sarcoplasmic reticulum
• This neurotransmitter causes the sarcoplasmic
reticulum to release its stored calcium ions
• Calcium ions bind to troponin
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Figure 9.7 Sliding Filament Theory (2 of 11)
1 Contraction Cycle Begins
The contraction cycle involves a series of
interrelated steps. The cycle begins with
electrical events in the sarcolemma that
trigger the release of calcium from the
terminal cisternae of the sarcoplasmic
reticulum (SR). These calcium ions enter
the zone of overlap.
Ca2+
Actin
2 Active-Site Exposure
Calcium ions bind to troponin in the
troponin– tropomyosin complex. The
tropomyosin molecule then rolls away
from the active sites on the actin
molecules of the thin filaments.
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Ca2+
Tropomyosin
Active
site
Muscle Contraction
• Muscle Contraction: A Summary (continued)
• The bound calcium ions cause the tropomyosin
molecule to roll so that it exposes the active sites
on actin
• The myosin heads now extend and bind to the
exposed active sites on actin
• Once the myosin heads bind to the active sites,
they pivot in the direction of the M line
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Figure 9.7 Sliding Filament Theory (3 of 11)
3 Cross-Bridge Formation
Once the active sites are exposed, the
myosin heads of adjacent thick
filaments bind to them, forming
cross-bridges.
4 Myosin Head Pivoting
After cross-bridge formation, energy is
released as the myosin heads pivot
toward the M line.
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Myosin head
Cross-bridge
Muscle Contraction
• Muscle Contraction: A Summary (continued)
• Upon pivoting of the myosin heads, the actin
filament slides toward the M line
• ATP binds to the myosin heads causing them to
release their attachment and return to their original
position to start over again
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Figure 9.7 Sliding Filament Theory (4 of 11)
5 Cross-Bridge Detachment
ATP
ATP then binds to the myosin heads,
breaking the cross-bridges between the
myosin heads and the actin molecules.
6 Myosin Reactivation
ATP provides the energy to reactivate
the myosin heads and return them to
their original positions. Now the entire
cycle can be repeated as long as
calcium ion concentrations remain
elevated and ATP reserves are
sufficient.
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ATP
Muscle Contraction
• Muscle Contraction: A Summary (continued)
• Upon contraction:
• I bands get smaller
• H bands get smaller
• Z lines get closer together
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Figure 9.7 Sliding Filament Theory
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Figure 9.9 The Events in Muscle Contraction
STEPS IN INITIATING MUSCLE CONTRACTION
STEPS IN MUSCLE RELAXATION
Synaptic Motor
terminal end plate T tubule Sarcolemma
1 ACh released, binding
to receptors
3 Sarcoplasmic
reticulum
releases Ca2+
4 Active-site
exposure,
cross-bridge
formation
5 Contraction
begins
2 Action
potential
reaches
T tubule
6 ACh removed by AChE
7 Sarcoplasmic
reticulum
recaptures Ca2+
Ca2+
Actin
Myosin
8 Active sites
covered, no
cross-bridge
interaction
9 Contraction
ends
10 Relaxation occurs,
passive return to
resting length
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Motor Units and Muscle Control
• Motor Units (Motor Neurons Controlling Muscle
Fibers)
• Precise control
• A motor neuron controlling two or three muscle
fibers
• Example: the control over the eye muscles
• Less precise control
• A motor neuron controlling perhaps 2000 muscle
fibers
• Example: the control over the leg muscles
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Figure 9.10 The Arrangement of Motor Units in a Skeletal Muscle
Axons of
motor neurons
Motor
nerve
Muscle fibers
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Motor Units and Muscle Control
• Muscle tension depends on:
• The frequency of stimulation
• A typical example is a muscle twitch
• The number of motor units involved
• Complete contraction or no contraction at all (all or
none principle)
• The amount of force of contraction depends on the
number of motor units activated
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Motor Units and Muscle Control
• Muscle Tone
• The tension of a muscle when it is relaxed
• Stabilizes the position of bones and joints
• Example: the amount of muscle involvement that
results in normal body posture
• Muscle Spindles
• These are specialized muscle cells that are
monitored by sensory nerves to control muscle
tone
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Motor Units and Muscle Control
• Muscle Hypertrophy
• Enlargement of the muscle
• Exercise causes:
• An increase in the number of mitochondria
• An increase in the activity of muscle spindles
• An increase in the concentration of glycolytic
enzymes
• An increase in the glycogen reserves
• An increase in the number of myofibrils
• The net effect is an enlargement of the muscle
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Motor Units and Muscle Control
• Muscle Atrophy
• Discontinued use of a muscle
• Disuse causes:
• A decrease in muscle size
• A decrease in muscle tone
• Physical therapy helps to reduce the effects
of atrophy
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Types of Skeletal Muscle Fibers
• Three Major Types of Muscle Fibers
• Fast fibers (white fibers)
• Associated with eye muscles (fast contractions)
• Intermediate fibers (pink fibers)
• Slow fibers (red fibers)
• Associated with leg muscles (slow contractions)
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Figure 9.11a Types of Skeletal Muscle Fibers
Slow fibers
Smaller diameter,
darker color due to
myoglobin; fatigue
resistant
LM x 170
Fast fibers
Larger diameter,
paler color;
easily fatigued
LM x 170
a Note the difference in the size of
slow muscle fibers (above) and
fast muscle fibers (below).
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Types of Skeletal Muscle Fibers
• Features of Fast Fibers
•
•
•
•
•
•
Large in diameter
Large glycogen reserves
Relatively few mitochondria
Muscles contract using anaerobic metabolism
Fatigue easily
Can contract in 0.01 second or less after
stimulation
• Produce powerful contractions
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Types of Skeletal Muscle Fibers
• Features of Slow Fibers
• Half the diameter of fast fibers
• Take three times longer to contract after
stimulation
• Can contract for extended periods of time
• Contain abundant myoglobin (creates the red
color)
• Muscles contract using aerobic metabolism
• Have a large network of capillaries
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Types of Skeletal Muscle Fibers
• Features of Intermediate Fibers
• Similar to fast fibers
• Have low myoglobin content
• Have high glycolytic enzyme concentration
• Contract using anaerobic metabolism
• Similar to slow fibers
• Have lots of mitochondria
• Have a greater capillary supply
• Resist fatigue
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Table 9.1 Properties of Skeletal Muscle Fiber Types
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Types of Skeletal Muscle Fibers
• Distribution of Fast, Slow, and Intermediate
Fibers
• Fast fibers
• High density associated with eye and hand
muscles
• Sprinters have a high concentration of fast fibers
• Repeated intense workouts increase the fast fibers
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Types of Skeletal Muscle Fibers
• Distribution of Fast, Slow, and Intermediate
Fibers (continued)
• Slow and intermediate fibers
•
•
•
•
None are associated with the eyes or hands
Found in high density in the back and leg muscles
Marathon runners have a high amount
Training for long distance running increases the
proportion of intermediate fibers
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Organization of Skeletal Muscle Fibers
• Muscles can be classified based on shape or
by the arrangement of the fibers
• Parallel muscle fibers
• Convergent muscle fibers
• Pennate muscle fibers
• Unipennate muscle fibers
• Bipennate muscle fibers
• Multipennate muscle fibers
• Circular muscle fibers
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Organization of Skeletal Muscle Fibers
• Parallel Muscle Fibers
• Muscle fascicles are parallel to the longitudinal
axis
• Examples: biceps brachii and rectus abdominis
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Figure 9.12ab Skeletal Muscle Fiber Organization
Parallel Muscles
(h)
(d)
(g)
a Parallel muscle
(Biceps brachii muscle)
b Parallel muscle with
tendinous bands
(Rectus abdominis
muscle)
(a)
(b)
(e)
(c)
(f)
Fascicle
Body
(belly)
Cross section
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Organization of Skeletal Muscle Fibers
• Convergent Muscle Fibers
• Muscle fibers form a broad area but come
together at a common point
• Example: pectoralis major
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Figure 9.12d Skeletal Muscle Fiber Organization
(h)
Convergent Muscles
(d)
d Convergent muscle
(g)
(Pectoralis muscles)
(a)
(b)
Tendon
(e)
(c)
Base of
muscle
(f)
Cross
section
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Organization of Skeletal Muscle Fibers
• Pennate Muscle Fibers (Unipennate)
• Muscle fibers form an oblique angle to the tendon
of the muscle
• An example is unipennate
• All the muscle fibers are on the same side of the
tendon
• Example: extensor digitorum
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Figure 9.12e Skeletal Muscle Fiber Organization
Pennate Muscles
(h)
(d)
(g)
e Unipennate
muscle (Extensor
digitorum muscle)
(a)
(b)
(e)
(c)
(f)
Extended
tendon
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Organization of Skeletal Muscle Fibers
• Pennate Muscle Fibers (Bipennate)
• Muscle fibers form an oblique angle to the tendon
of the muscle
• An example is bipennate
• Muscle fibers are on both sides of the tendon
• Example: rectus femoris
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Figure 9.12f Skeletal Muscle Fiber Organization
(h)
(d)
(g)
(a)
(b)
(e)
(c)
(f)
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Pennate Muscles
f Bipennate
muscle
(Rectus femoris
muscle)
Organization of Skeletal Muscle Fibers
• Pennate Muscle Fibers (Multipennate)
• Muscle fibers form an oblique angle to the tendon
of the muscle
• An example is multipennate
• The tendon branches within the muscle
• Example: deltoid muscle
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Figure 9.12g Skeletal Muscle Fiber Organization
(h)
(d)
(g)
Pennate Muscles
g Multipennate muscle
(Deltoid muscle)
(a)
(b)
(e)
(c)
(f)
Tendons
Cross section
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Organization of Skeletal Muscle Fibers
• Circular Muscle Fibers
• Muscle fibers form concentric rings
• Also known as sphincter muscles
• Examples: orbicularis oris and orbicularis oculi
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Figure 9.12h Skeletal Muscle Fiber Organization
(h)
(d)
(g)
Circular Muscles
h Circular muscle
(Orbicularis oris muscle)
(a)
(b)
(e)
(c)
Contracted
(f)
Relaxed
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Muscle Terminology
• Origins and Insertions
• Origin
• Point of muscle attachment that remains stationary
• Insertion
• Point of muscle attachment that is movable
• Actions
• The function of the muscle upon contraction
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Muscle Terminology
• There are two methods of describing
muscle actions
• The first makes reference to the bone region the
muscle is associated with
• The biceps brachii muscle causes “flexion of the
forearm”
• The second makes reference to a specific joint the
muscle is associated with
• The biceps brachii muscle causes “flexion at the
elbow”
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Muscle Terminology
• Muscles can be grouped according to
their primary actions into four types
• Prime movers (agonists)
• Responsible for producing a particular movement
• Antagonists
• Actions oppose the action of the agonist
• Synergists
• Assist the prime mover in performing an action
• Fixators
• Agonist and antagonist muscles contracting at the
same time to stabilize a joint
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Muscle Terminology
• Prime Movers example:
• Biceps brachii – flexes the lower arm
• Antagonists example:
• Triceps brachii – extends the lower arm
• Synergists example:
• Latissimus dorsi and teres major – contract to
move the arm medially over the posterior body
• Fixators example:
• Flexor and extensor muscles contract at the same
time to stabilize an outstretched hand
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Muscle Terminology
• Most muscle names provide clues to their
identification or location
• Muscles can be named for:
•
•
•
•
•
•
•
Specific body regions or location
Shape of the muscle
Orientation of the muscle fibers
Specific or unusual features
Its origin and insertion points
Primary function
References to occupational or habitual action
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Muscle Terminology
• Examples of muscle names related to:
• Specific body regions or locations
• Brachialis: associated with the brachium of the
arm
• Tibialis anterior: associated with the anterior tibia
• Shape of the muscle
• Trapezius: trapezoid shape
• Deltoid: triangular shape
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Muscle Terminology
• Examples of muscle names related to:
• Orientation of the muscle fibers
• Rectus femoris: straight muscle of the leg
• External oblique: muscle on outside that is
oriented with the fibers at an angle
• Specific or unusual features
• Biceps brachii: two origins
• Teres major: long, big, rounded muscle
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Muscle Terminology
• Examples of muscle names related to:
• Origin and insertion points
• Sternocleidomastoid: points of attachment are
sternum, clavicle, and mastoid process
• Genioglossus: points of attachment are chin and
tongue
• Primary functions
• Flexor carpi radialis: a muscle that is near the
radius and flexes the wrist
• Adductor longus: a long muscle that adducts the
leg
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Muscle Terminology
• Examples of muscle names related to:
• References to occupational or habitual actions
• Buccinator (means “trumpet player”): the
buccinator area moves when playing a trumpet
• Sartorius: derived from the Latin term (sartor),
which is in reference to “tailors.” Tailors used to
cross their legs to form a table when sewing
material
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Levers and Pulleys: A Systems Design for
Movement
• Most of the time, upon contraction, a muscle
causes action
• This action is applied to a lever (a bone)
• This lever moves on a fixed point called the
fulcrum (joint)
• The action of the lever is opposed by a force
acting in the opposite direction
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Levers and Pulleys: A Systems Design for
Movement
• There are three classes of levers
• First class, second class, third class
• First class
• The fulcrum (joint) lies between the applied force
and the resistance force (opposed force)
• Example: tilting the head forward and backward
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Figure 9.13 Levers and Pulleys (2 of 6)
First-Class Lever
In a first-class lever, the applied force and the
resistance are on opposite sides of the
fulcrum. This lever can change the amount of
force transmitted to the resistance and alter the
direction and speed of movement. There are
very few first-class levers in the human body.
R
F
AF
Resistance
Fulcrum
Applied force
R
F
AF
Movement
completed
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Levers and Pulleys: A Systems Design for
Movement
• Classes of Levers
• Second class
• The resistance is located between the applied force
and the fulcrum (joint)
• Example: standing on your tiptoes
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Figure 9.13 Levers and Pulleys (3 of 6)
Second-Class Lever
In a second-class lever, the resistance lies
between the applied force and the fulcrum.
This arrangement
magnifies force at the
expense of distance
and speed; the direction
of movement remains
unchanged. There are
very few second-class
AF
levers in the body.
R
F
R
F
Movement
completed
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AF
Levers and Pulleys: A Systems Design for
Movement
• Classes of Levers
• Third class
• The force is applied between the resistance and
fulcrum (joint)
• Example: flexing the lower arm
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Figure 9.13 Levers and Pulleys (4 of 6)
Third-Class Lever
In a third-class lever, which is the most
common lever in the body, the applied force
is between the resistance and the fulcrum.
This arrangement increases speed and
distance moved but requires a larger
applied force.
R
F
AF
F
Movement
completed
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R
Levers and Pulleys: A Systems Design for
Movement
• Sometimes, a tendon may loop around a bony
projection
• This bony projection could be called a pulley
• Example: lateral malleolus and trochlea of the eye
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Figure 9.13 Levers and Pulleys (5 of 6)
The Lateral Malleolus
as an Anatomical Pulley
Fibularis
longus
The lateral malleolus of the fibula is an
example of an anatomical pulley. The
tendon of insertion for the fibularis longus
muscle does not follow a direct path.
Instead, it curves around the posterior
margin of the lateral malleolus of the
fibula. This redirection of the contractile
force is essential to the normal function
of the fibularis longus in producing
plantar flexion at the ankle.
Lateral
malleolus
Pulley
Plantar flexion of the foot
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Figure 9.13 Levers and Pulleys (6 of 6)
Pulley
Quadriceps muscles
The Patella as an
Anatomical
Pulley
The patella is another
example of an anatomical
pulley. The quadriceps
femoris is a group of four
muscles that form the anterior musculature of the
thigh. These four muscles attach to the patella by the
quadriceps tendon. The patella is, in turn, attached to the
tibial tuberosity by the patellar ligament. The quadriceps
femoris muscles produce extension at the knee by this
two-link system. The quadriceps tendon pulls on the
patella in one direction throughout the movement, but
the direction of force applied to the tibia by the patellar
ligament changes constantly as the movement proceeds.
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Quadriceps tendon
Patella
Patellar
ligament
Extension
of the leg
Aging and the Muscular System
• Changes occur in muscles as we age
• Skeletal muscle fibers become smaller in diameter
• Due to a decrease in the number of myofibrils
• Contain less glycogen reserves
• Contain less myoglobin
• All of the above results in a decrease in strength
and endurance
• Muscles fatigue rapidly
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Aging and the Muscular System
• Changes occur in muscles as we age (continued)
• There is a decrease in myosatellite cells
• There is an increase in fibrous connective tissue
• Due to the process of fibrosis
• The ability to recover from muscular injuries
decreases
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