Download Muscles

Muscles
3.5.3 Skeletal muscles are stimulated to contract by nerves and
act as effectors
Muscle
Is responsible for almost all the movements in animals
3 types
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 μm 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
•Sarcomere = the basic contractile unit
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.
onem
yosin
m
olecule
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
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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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
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 heads.
The energy from the ATP causes the angle of the
myosin head to change.
The myosin heads can attach to actin.
Movement of the myosin heads and them attaching
and detaching from actin causes the filaments to slide
relative to one another.
This movement reduces the sarcomere length.
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.
The Cross Bridge Cycle
1.The cycle begins with ATP binding to
the myosin head. This causes the
myosin head to be released from actin.
The Cross Bridge Cycle
2. The ATP molecule is then hydrolysed
while the myosin head is unattached.
The ADP & Pi formed remain bound to
the myosin head.
The Cross Bridge Cycle
3. The energy released by
the hydrolysis of ATP is
absorbed by the myosin
• This causes the myosin
head to change shape
(places it in energised state
or cocked state – also
called the recovery stroke)
• It then binds to the actin
filament.
The Cross Bridge Cycle
4-5. The ADP and Pi are then released from
the myosin head
• Result = Power stroke occurs (the myosin
head changes shape)
•This draws the actin filament over the
myosin filament.
The Cross Bridge Cycle
1.The cycle begins
again when the next
ATP binds to the
myosin head.
Causing the myosin
head to be released
from actin.
Control of Muscle Contraction
How is the cross bridge cycle switched off in a relaxed
muscle?
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,
2+
Ca
& ATP
Ca2+ causes tropomyosin to be displaced.
So it no longer blocks the myosin binding site
So myosin and actin can bind together allowing cross
bridge cycling
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