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Chapter 6
The Muscular
System
Lecture Presentation
Betty McGuire
Cornell University
Copyright © 2012 Pearson Education, Inc.
The Muscular System
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Function and characteristics of muscles
Skeletal muscles working in pairs
Contraction of muscles
Voluntary movement
Energy for muscle contraction
Slow-twitch and fast-twitch muscle cells
Building muscle
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Function and Characteristics of Muscles
 Three types of muscle
 Skeletal
 Cardiac
 Smooth
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Function and Characteristics of Muscles
 All muscles are
 Excitable (they respond to stimuli)
 Contractile (they can shorten)
 Extensible (they can stretch)
 Elastic (they can return to their original
length after being shortened or
stretched)
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Function and Characteristics of Muscles
 Skeletal muscles are voluntary muscles
responsible for
 Moving our body
 Maintaining posture
 Supporting internal organs
 Pushing against veins and lymphatic
vessels to move blood and lymph along
 Generating heat
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Skeletal Muscles Working in Pairs
 The body has more than 600 skeletal
muscles
 Synergistic muscles
 Muscles that must contract at the same
time to cause movement
 Antagonistic muscles
 Movement is produced when one
muscle of the pair contracts and the
other relaxes
 Example: the biceps muscle and triceps
muscle of the upper arm
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Copyright © 2012 Pearson Education, Inc.
Copyright © 2012 Pearson Education, Inc.
Copyright © 2012 Pearson Education, Inc.
Copyright © 2012 Pearson Education, Inc.
Copyright © 2012 Pearson Education, Inc.
Copyright © 2012 Pearson Education, Inc.
Skeletal Muscles Working in Pairs
 Tendon
 Band of connective tissue that attaches
a muscle to a bone
 Origin of a muscle
 The end attached to the bone that
remains relatively stationary during
movement
 Insertion of a muscle
 The end attached to the bone that
moves
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Skeletal Muscles Working in Pairs
 Tendinitis
 Condition of having an inflamed tendon
 Caused by overuse, misuse, or age
 Healing is slow because tendons have a
poor blood supply
 Most effective treatment is rest
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Skeletal Muscles Working in Pairs
 Muscle pull
 Also called a muscle strain or tear
 Caused by overstretching that damages
the muscle or tendon
 Treatment includes ice to reduce
swelling and keeping the muscle
stretched
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Contraction of Muscles
 Fascicle
 A bundle of muscle cells
 A skeletal muscle has many fascicles
 Each fascicle is surrounded by its own
connective tissue sheath
 The connective tissue sheaths of fascicles
merge at the ends of muscles to form
tendons that attach the muscle to bone
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Contraction of Muscles
 A muscle cell = a muscle fiber
 When skeletal muscle cells are viewed
under a microscope, they have distinct
bands called striations
 The striations are formed by the
arrangement of myofibrils within the cell
 Myofibrils are specialized bundles of
proteins
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Contraction of Muscles
 Each myofibril contains two types of
myofilaments
 Myosin (thick) filaments
 Actin (thin) filaments
 Actin filaments are more numerous
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Contraction of Muscles
 Each myofibril has tens of thousands of
contractile units, called sarcomeres
 The ends of each sarcomere are marked by
dark protein bands called Z lines
 Within each sarcomere the actin and
myosin filaments are specifically arranged
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Contraction of Muscles
 One end of each actin filament is attached
to a Z line
 Myosin filaments lie in the middle of the
sarcomere, and their ends partially overlap
with surrounding actin filaments
 The degree of overlap increases when the
muscle contracts
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Copyright © 2012 Pearson Education, Inc.
Copyright © 2012 Pearson Education, Inc.
Copyright © 2012 Pearson Education, Inc.
Copyright © 2012 Pearson Education, Inc.
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Contraction of Muscles
 Muscle contraction occurs at the
molecular level
 According to the sliding filament model, a
muscle contracts when actin filaments
slide past myosin filaments, shortening the
sarcomere
 Myosin molecules are shaped like twoheaded golf clubs
 The club-shaped myosin heads are key
to moving actin filaments
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Contraction of Muscles
 The myosin head, also known as a crossbridge, attaches to a nearby actin filament
 Then the head bends and swivels,
pulling the actin filament toward the
midline of the sarcomere
 The myosin head disengages from the
actin filament
 The movements of myosin require ATP
 The cycle begins again
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Copyright © 2012 Pearson Education, Inc.
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Contraction of Muscles
 Muscle contraction is controlled by the
availability of calcium ions
 Muscle cells contain the proteins troponin
and tropomyosin
 The troponin-tropomyosin complex and
calcium ions regulate muscle contraction
at the actin-myosin binding sites
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Contraction of Muscles
 When a muscle is relaxed, the troponintropomyosin complex covers the actinmyosin binding sites
 Muscle contraction occurs when calcium
ions bind to troponin, causing it to change
shape
 This change in shape moves tropomyosin,
exposing the actin-myosin binding sites
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Copyright © 2012 Pearson Education, Inc.
Contraction of Muscles
 Sarcoplasmic reticulum
 Form of smooth endoplasmic reticulum
found in muscle cells
 Stores calcium ions
 Transverse tubules (T tubules)
 Pockets in the plasma membrane of a
muscle cell
 Carry signals from motor neurons deep
into the muscle cell to every sarcomere
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Contraction of Muscles
 Rigor mortis
 Muscle contraction will occur as long
ATP is present
 Without ATP, cross-bridges cannot
be broken
 Within 3 to 4 hours after death, the
muscles become stiff = rigor mortis
 Actin and myosin gradually break
down and muscles relax again after
2 to 3 days
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Contraction of Muscles
 Role of nerves in muscle contraction
 Neuromuscular junction
 Junction between the tip of a motor
neuron and a skeletal muscle cell
 A nerve impulse travels down a motor
neuron to the neuromuscular junction, where
it causes the release of acetylcholine (a
neurotransmitter) from the motor neuron
 Acetylcholine diffuses across a small gap
and binds to receptors on the plasma
membrane of the muscle cell
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Contraction of Muscles
 Role of nerves in muscle contraction
(cont.)
 The acetylcholine causes changes in
the permeability of the muscle cell,
resulting in an electrochemical message
similar to a nerve impulse
 The message travels along the plasma
membrane into the T tubules and then to
the sarcoplasmic reticulum, releasing
calcium ions for muscle contraction
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Contraction of Muscles
Web Activity: Muscle Structure and Function
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Contraction of Muscles
 Muscular dystrophy (MD)
 If too many calcium ions enter a muscle cell, then
proteins may be destroyed, eventually causing the
cell to die; on a large scale, muscles weaken
 MD = a group of inherited conditions in which
muscles weaken
 Duchenne muscular dystrophy
 One of the most common forms
 The gene for production of the protein
dystrophin is defective
 Lack of dystrophin allows excess calcium ions to
enter muscle cells, eventually killing the cells
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Voluntary Movement
 Motor unit
 A motor neuron and all the muscle cells
it stimulates
 All the muscle cells in a given motor unit
contract together
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Voluntary Movement
 The number of muscle cells in a motor unit
is highly variable
 Muscles responsible for precise movements
have fewer muscle cells in each motor unit
than do muscles responsible for less precise
movements
 On average, there are 150 muscle cells in a
motor unit
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Voluntary Movement
 Motor units and recruitment
 The strength of muscle contraction can
be increased by increasing the number
of motor units that are stimulated
 This process, performed by the
nervous system, is called recruitment
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Voluntary Movement
 Muscle twitch
 Contraction of a muscle in response to a
single stimulus
 Twitches are very brief and typically not part
of normal movements
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Voluntary Movement
 If a second stimulus is received before the
muscle is fully relaxed, the second twitch
will be stronger than the first, due to
summation
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Voluntary Movement
 Tetanus
 A sustained, powerful contraction
caused by very frequent stimuli
 Fatigue sets in when a muscle is unable to
contract even when stimulated
 Changing the frequency of stimulation is
another way to vary the contraction of
muscles
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Copyright © 2012 Pearson Education, Inc.
Energy for Muscle Contraction
 Muscle contraction requires an enormous
amount of energy
 ATP for muscle contraction comes from
many sources, typically used in sequence
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ATP stored in muscle cells
Creatine phosphate stored in muscle cells
Anaerobic metabolic pathways
Aerobic respiration
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Slow-Twitch and Fast-Twitch Muscle Cells
 Slow-twitch muscle cells
 Contract slowly, with great endurance
 Abundant mitochondria
 Packed with myoglobin (oxygen-binding
pigment)
 Dark, reddish appearance
 Myoglobin
 Rich blood supply
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Slow-Twitch and Fast-Twitch Muscle Cells
 Fast-twitch muscle cells
 Contract rapidly and powerfully but with
much less endurance
 Can make and break cross-bridge
attachments more rapidly
 Have more actin and myosin
 Rely on anaerobic metabolic pathways to
generate ATP and therefore tire quickly
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Building Muscle
 Aerobic exercise
 Enough oxygen is delivered to the muscles
to keep them going for long periods
 Increases endurance and coordination
 Promotes development of new blood vessels
 Increases the number of mitochondria
 Typically does not increase size of muscles
 Examples: walking, jogging, swimming
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Building Muscle
 Resistance exercise
 Builds strength
 Muscles increase in size when they are
repeatedly made to exert more than 75% of
their maximum force
 Increases in muscle size reflect increases in
the diameter of existing muscle cells
 Example: weight lifting
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Building Muscle
PLAY
| Mitchell Report
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Building Muscle
PLAY
| Steroids
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