Muscles
13.8 Muscles are effectors which
enable movement to be carried out
Muscle
 Is
responsible for almost all the
movements in animals
 3 types
Cardiac muscle
Smooth muscle
Involuntary
controlled by
autonomic
nervous system
voluntary
Skeletal muscle
controlled by
(aka striped or
somatic nervous
striated muscle)
system
Muscles & the Skeleton
 Skeletal
muscles cause the skeleton to
move at joints
 They are attached to skeleton by
tendons.
 Tendons transmit muscle force to the
bone.
 Tendons are made of collagen fibres &
are very strong & stiff
Antagonistic Muscle Action
 Muscles
are either contracted or relaxed
 When contracted the muscle exerts a
pulling force, causing it to shorten
 Since muscles can only pull (not push),
they work in pairs called antagonistic
muscles
 The muscle that bends the joint is called
the flexor muscle
 The muscle that straightens the joint is
called the extensor muscle
Elbow Joint
 The
best known example of antagonistic
muscles are the bicep & triceps muscles
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Muscle Structure

A single muscle e.g.
biceps contains approx
1000 muscle fibres.
 These fibres run the
whole length of the
muscle
 Muscle fibres are joined
together at the tendons
Bicep Muscle
Muscle Structure

Each muscle fibre is actually a
single muscle cell
 This cell is approx 100 m in
diameter & a few cm long
 These giant cells have many
nuclei
 Their cytoplasm is packed full
of myofibrils
 These are bundles of protein
filaments that cause
contraction
 Sarcoplasm (muscle
cytoplasm) also contains
mitochondria to provide energy
for contraction
nuclei
stripes
m
yofibrils
Muscle Structure

The E.M shows that each myofibril is made up of
repeating dark & light bands
 In the middle of the dark band is the M-line
 In the middle of the light band is the Z-line
 The repeating unit from one Z-line to the next is called
the sarcomere
1 myofibril
Z dark light M
line bandsbands line
1sarcom
ere
Muscle Structure

A very high resolution E.M reveals that each myofibril
is made up of parallel filaments.
 There are 2 kinds of filament called thick & thin
filaments.
 These 2 filaments are linked at intervals called cross
bridges, which actually stick out from the thick
filaments
Thick
filament
Thin
filament
Cross
bridges
The Thick Filament (Myosin)

Consists of the protein
called myosin.
 A myosin molecule is
shaped a bit like a golf
club, but with 2 heads.
 The heads stick out to
form the cross bridge
 Many of these myosin
molecules stick
together to form a
thick filament
onem
yosin
m
olecule
m
yosintails
m
yosinheads
(crossbridges)
Thin Filament (Actin)

The thin filament consists of a protein called
actin.
 The thin filament also contains tropomyosin.
 This protein is involved in the control of
muscle contraction
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•Sarcomere = the basic contractile unit
The Sarcomere
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I Band = actin
filaments
Anatomy of a Sarcomere

The thick filaments produce the dark A band.
 The thin filaments extend in each direction
from the Z line.
 Where they do not overlap the thick filaments,
they create the light I band.
 The H zone is that portion of the A band where
the thick and thin filaments do not overlap.
 The entire array of thick and thin filaments
between the Z lines is called a sarcomere
Sarcomere shortens when
muscle contracts
 Shortening
of the
sarcomeres in a
myofibril produces
the shortening of
the myofibril
 And, in turn, of the
muscle fibre of
which it is a part
Mechanism of muscle contraction
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
The above micrographs show that the
sarcomere gets shorter when the muscle
contracts
 The light (I) bands become shorter
 The dark bands (A) bands stay the same length
The Sliding Filament Theory
 So,
when the muscle contracts,
sarcomeres become smaller
 However the filaments do not change in
length.
 Instead they slide past each other (overlap)
 So actin filaments slide between myosin
filaments
 and the zone of overlap is larger
What makes the filaments
slide past each other?
 Energy
for the movement comes from
splitting ATP
 ATPase that does this is located in the
myosin cross bridge head.
 These cross bridges attach to actin.
 The energy from the ATP causes the angle
of the myosin head to change.
 So they are able to cause the actin
filament to slide relative to the myosin.
 This movement reduces the sarcomere
length.
The Cross Bridge Cycle
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 The
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cross bridge cycle has 4 steps
 It is analogues to 4 steps in rowing a boat
Step 1
T
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 The
Cross bridge
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attaches to the actin h
filament.
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 Put Oars in water
n
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Step 2 – The Power Stroke

The cross
bridge
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 Causes the filaments
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to slide.
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 Energy from ATP is
used for
this
power
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stroke
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 ADP + Pi are released
 Pull oars through
water
Step 3
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tA new ATP
molecule
binds
to myosin
s
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
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 The
Cross bridge s
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filament
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Push
oars out of water
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Step
4
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
The Cross bridge
changes back to its
original
 shape

 This
is
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tch
detached (to ensure
theA
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isA
not
D
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pushed back again).
 It is now ready for a
new cycle, but further
p
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l actin filament
o
u
t
along
the
 Push oars into starting
position

r
e
co
ve
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Repetition of the cycle

One ATP molecule is split by each cross
bridge in each cycle.
 This takes only a few milliseconds
 During a contraction 1000’s of cross bridges
in each sarcomere go through this cycle.
 However the cross bridges are all out of
synch, so there are always many cross
bridges attached at any one time to maintain
force. http://199.17.138.73/berg/ANIMTNS/SlidFila.htm
Control of Muscle Contraction

How is the cross bridge cycle switched off in a
relaxed muscle?
 This is where the regulatory protein on the actin
filament, tropomyosin is involved.
 Actin filaments have myosin binding sites.
 These binding sites are blocked by tropomyosin
in relaxed muscle.
 When Ca2+ bind tropomyosin is displaced and the
myosin binding sites are uncovered.
 So myosin & actin can now bind together to start
the cross bridge cycle
Tropomyosin, Ca2+ & ATP
Ca2+ causes
tropomyosin to be
displaced.
 No longer blocks
myosin binding site
 Power stoke can
begin.
 Ca2+ also active
myosin molecules to
breakdown ATP
 So energy is released
to begin contraction

Neuromuscular junction: Note Ach = Acetylcholine
Sarcoplasmic
Reticulum
Sequence of events

1. An action potential arrives at the end of a
motor neurone, at the neuromuscular
junction.
 2. This
causes the release of the
neurotransmitter acetylcholine.
 3 This initiates an action potential in the
muscle cell membrane (Sarcolemma).
 4. This action potential is carried quickly into
the large muscle cell by invaginations in the
cell membrane called T-tubules.
Sequence of events

5. The
action
potential
causes
the
sarcoplasmic reticulum to release its store of
calcium into the myofibrils.
 6. Ca2+ causes tropomoysin to be displaced
uncovering myosin binding sites on actin.
 7. Myosin cross bridges can now attach and
the cross bridge cycle can take place.
 Relaxation is the reverse of these steps