The Muscular
System
Chapter 8
Read
Page 168
8.1 Introduction
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Muscle – an organ composed of specialized
cells that uses chemical energy stored in
nutrients to contract
Functions
 movement
 muscle tone
 propel
body fluids and food
 generate heartbeat
 distribute heat
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Types
 skeletal
 smooth
 cardiac
8.2 Structure of a Skeletal Muscle
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Fascia
 layer
of fibrous connective tissue
 separates muscles & holds in position
 projects beyond muscle to form tendon
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attaches to periosteum of bone
Aponeuroses
 broad
fibrous sheet
 attaches to bone or coverings of other
muscles
8.2 Structure of a Skeletal Muscle
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Epimysium
 layer
of connective tissue most closely
surrounding muscle
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Perimysium
 connective
tissue extending inward from
epimysium to separate the muscle tissue into
smaller compartments
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Fasicles
 compartments
muscle fibers
containing bundles of skeletal
8.2 Structure of a Skeletal Muscle
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Skeletal Muscle Fiber
 single
cell
 contracts in response to stimulation
 Contains:
Sarcolemma = cell membrane
 Sarcoplasm = cytoplasm
 nuclei
 mitochondria
 myofibrils = parallel threads
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8.2 Structure of a Skeletal Muscle
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Myofibrils
 Contain
two types of protein filaments causes
striations in muscle fiber
Myosin – thick
 Actin - thin
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I bands – light, thin
actin filaments,
connect to Z line
A bands – dark,
thick myosin
overlaps thin actin
H zone – thick
filaments only
M line – proteins
holding thick
filaments in place
Sarcomere – area
from Z line to Z line
Sarcoplasmic reticulum – membranous channels surrounding
myofibril, run parallel
T tubules – channels extending inward, passes all the way through
Cisternae – enlarged portion of sarcoplasmic reticulum
These areas activate muscle contration
Life Connection

Muscle strain
 tearing
of muscle fibers and connective
tissues
 Mild
few fibers torn
 fascia intact
 minimal loss of function
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 Severe
everything torn
 function may be completely lost
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Review
Describe how connective tissue is a part of
a skeletal muscle.
 Describe the general structure of a
skeletal muscle fiber.
 Explain why skeletal muscles appear
striated.
 Explain the relationship between the
sarcoplasmic reticulum and transverse
tubules.
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8.2 Structure of a Skeletal Muscle
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Neuromuscular
Junction
 connection
of
motor neuron
and muscle fiber
8.2 Structure of a Skeletal Muscle
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Motor End Plate
 specialized
part of muscle fiber
 abundant mitochondria and nuclei
 sarcolemma folded
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Neurotransmitters
 chemical
excreted by axon
 stimulates muscle contraction
Muscle fiber has single motor
end plate but axons are
densely branched
 Motor Unit

 motor
neuron and fibers it controls
Review
Which two structures approach each other
at a neuromuscular junction?
 Describe a motor end plate.
 What is the function of a neurotransmitter?
 What is a motor unit?

8.3 Skeletal Muscle Contraction
Myosin molecule – two twisted protein strands with cross
bridges
Myosin filament – many molecules put together
Actin molecule – contains binding site for cross bridges
Actin filament – many
molecules twisted into
double helix containing
troponin and tropomyosin
Sliding Filament Theory
1 & 2 – Calcium ion
concentration rises,
binding sites on actin
filaments open,
cross bridges attach
Sliding Filament
Theory
3 & 4 – Upon binding to
actin, cross bridges spring
from the cocked position
and pull on actin filament
Sliding Filament
Theory
5. ATP binds to cross bridge
causing it to release from the
actin filament.
6. ATP breakdown provides
energy to cock the
unattached myosin cross
bridge.
Cycle continues as long as
ATP and calcium are present.
8.3 Skeletal Muscle Contraction
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Neurotransmitter = Acetylcholine
Nerve impulse causes release into synaptic cleft
Binds to receptors in muscle fibers
Stimulates muscle impulse
Impulse travels through T-tubules
Reaches sarcoplasmic reticulum
Calcium ions diffuse into sarcoplasm
Troponin and Tropomyosin expose binding sites on actin
Actin and myosin filaments link
Muscle fiber contracts
When a skeletal muscle
contracts the individual
sarcomeres shorten as
thick and thin filaments
slide past one another.
8.3 Skeletal Muscle Relaxation
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Nerve impulses stop
Acetylcholine broken down by
acetylcholinesterase
Calcium transported back into sarcoplasmic
reticulum
Links between actin and myosin break
Troponin and Tropomyosin block binding sites
on action
Muscle fiber relaxes
Real World

Bacteria Clostridium botulinum
 Prevents
release of acetylcholine
 Muscle fibers aren’t stimulated – paralyzed
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bad if you’re trying to breathe
 “Botox”
injections used to smooth wrinkles by
preventing local muscles from contracting
8.3 Energy Sources
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ATP only lasts short time – have to make more
from ADP and Phosphate
Creatine Phosphate – molecule with high energy
phosphate bonds
 more
abundant than ATP in muscle fibers
 stores excess energy from mitochondria
 when ATP low this excess energy is transferred to
ADP molecules to make more ATP
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Cellular respiration of glucose used when other
sources depleted
8.3 Oxygen Supply
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Needed to breakdown glucose in mitochondria
Blood carries oxygen from lungs to body cells
During rest or moderate activity respiratory and
cardiovascular systems have no trouble
supplying oxygen.
Aerobic respiration-glucose broken into CO2 and
O2
8.3 Oxygen Debt
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Anaerobic Respiration
Glucose broken down into pyruvic acid
Pyruvic acid then produces lactic acid
Lactic acid gets into bloodstream and is taken to
the liver
Liver uses ATP to make glucose out of the lactic
acid
During exercise not enough oxygen for liver to
make glucose
Debt = amount of oxygen needed for liver to
make glucose + amount muscles need to restore
ATP and creatine phosphate to their original
concentrations
8.3 Oxygen Debt
Repayment may take several hours
 Can change metabolism with training
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 increase
amount of glycolytic enzymes
 more capillaries and mitochondria form
8.3 Muscle Fatigue
Fatigue – loss of ability to contract
 Can be caused by interruption of blood
supply of lack of acetylcholine
 Usually caused by too much lactic acid
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 lowers

pH and fibers can’t contract
Cramp – sustained involuntary contraction
Real Life
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Rigor Mortis
 Calcium
ions easily diffuse into membrane
 Decrease in ATP prevents relaxation
 Actin and Myosin stay linked until muscles
start to decompose
8.3 Heat Production
½ of body’s energy used for metabolic
purposes
 ½ becomes heat
 All cells generate heat, but muscle is big
part of body mass
 Blood takes heat generated in muscles to
other parts of body to maintain
temperature
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Review
Which biochemicals provide the energy to
regenerate ATP?
 What are the sources of oxygen for
aerobic respiration?
 How are lactic acid, oxygen debt, and
muscle fatigue related?
 What is the relationship between cellular
respiration and heat production?
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8.4 Muscular Responses
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Threshold Stimulus
 minimal
strength required to cause a
contraction
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All-or-None Response
 fibers
don’t partly contract
 either it contracts all the way or not at all
8.4 Recording Contractions
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Twitch
 single
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contraction
Latent Period
 delay
time
between stimulus
and response
Myogram
Real World
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Normal people
½
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fast twitch and ½ slow twitch
Olympic sprinter
 80%
fast twitch
muscles bigger
 stronger contractions
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Olympic marathoner
 90%
slow twitch
resists fatigue
 abundant mitochondria (aerobic)
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Individual
Twitches
Summation
muscle not
completely relaxed
before next stimulus
arrives
Tetanic
Contraction –
Tetanus – sustained
contraction with no
relaxation
8.4 Muscular Responses
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Recruitment
 Increase in the number of motor units being
 low stimulus = few motor neurons stimulated
 high stimulus = many neurons stimulated
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activated
Summation and Recruitment produce sustained
contractions of increasing strength
Muscle Tone
 response
to nerve impulses that stimulate a few
muscle fibers
 needed to keep us from collapsing – like when we
lose consciousness
Review
Define threshold stimulus.
 What is an all-or-none response?
 Distinguish between a twitch and a
sustained contraction.
 How is muscle tone maintained?
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8.5 Smooth Muscle
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Not striated
 contains
actin and myosin filaments, but they
aren’t well organized
 sarcoplasmic reticulum not developed
8.5 Smooth Muscle
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Two types
 Multiunit
muscle fibers separate
 irises of eyes and walls of blood vessels
 contract only in response to stimulation by motor
nerve impulses or hormones
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 Visceral
sheets of cells in close contact with each other
 walls of hollow organs (stomach, intestines, etc.)
 stimulate each other
 rhythmicity – pattern of repeated contractions
 Peristalsis – forces the content of the organs along
their lengths
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8.5 Smooth Muscle Contraction
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Like skeletal
 include
actin and myosin
 triggered by membrane impulses and increased
calcium concentration
 use ATP
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Different from skeletal
– acetylcholine and norepinephrine
stimulate contractions in some muscles and inhibit in
others
 affected by hormones
 slower to contract and relax
 maintain forceful contractions longer
 fibers can change length without changing tautness
 Neurotransmitters
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stomach can fill up without losing pressure
Review
Describe two major types of smooth
muscle.
 What special characteristics of visceral
smooth muscle make peristalsis possible?
 How does smooth muscle contraction
differ from that of skeletal muscle?
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8.6 Cardiac Muscle
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Only found in heart
Composed of
 branching
striated cells
 sarcoplasmic reticulum
 many mitochondria
 large transverse tubules
 cisternae not well developed
 intercalated discs
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crossbands connecting opposing ends of cardiac cells
helps impulses pass quickly
Cells contract as unit
 Self-exciting
and rhythmic
Review
How is cardiac muscle similar to smooth
muscle?
 How is cardiac muscle similar to skeletal
muscle?
 What is the function of intercalated discs?
 What characteristic of cardiac muscle
contracts the heart as a unit?
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Origin – The
immovable
end of the
muscle
Insertion –
The movable
end of the
muscle
When a
muscle
contracts the
insertion is
pulled toward
its origin
8.7 Interaction of Skeletal Muscles
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Muscles usually function in groups
Prime mover = Agonist
 The
muscle doing the main work
 Deltoid lifts arm horizontally
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Synergist
 Muscles
that contract to assist the prime mover
 Makes prime mover’s actions more effective
 Hold shoulder steady
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Antagonist
 Resist
prime mover’s action and cause movement in
opposite direction
 Lowers the arm
Review
Distinguish between the origin and the
insertion of a muscle.
 Define prime mover.
 What is the function of a synergist? An
antagonist?
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FYI
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Human body contains over 600 muscles
Face has 60
 40
used to frown
 20 to smile
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Smallest
 stapedius
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Largest
 gluteus
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– middle ear
maximus
Longest
 sartorius