Sliding filament hypothesis

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  • Created by: Hindleyc
  • Created on: 10-04-19 21:45
When a myofibril contracts what happens
Sarcomere becomes shorter, light bands become shorter (1 band because actin is being pulled inwards by the myosin and so region of overlap increases)
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What else
, dark bands stay the same length (A bands this is bc region contains myosin only which is now smaller and actin and myosin filaments overlap which is now larger
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What is the sliding filament hypothesis
Suggests muscular contraction comes about by the actin filaments sliding between the myosin filaments using ATP energy
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In effect what does this do
Shorten each sarcomere in a muscle fibre
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What do the myosin molecules do
Walk along the actin filaments
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In region of overlap what is present
6 actin filaments arranged neatly around each myosin filament
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What do electron micrographs of the dark bands show
cross-bridges between the myosin and actin filaments
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What are these bridges part of and what they do
the myosin molecules and push on the actin filaments to make the myofibril shorten
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what is this called and why
ratchet mechanism because the actin molecules are moved along 1 step at a time by the myosin head
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What is each myosin filament made up of
Many myosin molecules
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Where do the myosin cross bridges point
in 6 different directions and are arranged in the form of a sprial
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What does each of the cross bridges have
A wider head which acts as an enzyme which can hydrolyse ATP releasing energy (ATP synthase )
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In a resting muscle what does each myosin head have
ADP and Pi bound to it
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When an action potential arrives
myosin head binds to an actin filament at a 'binding site' (a cross bridge forms) and tilts by 45.
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As the myosin head tilts what happens
it pushes the actin filament along by 10nm
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What happens when this happens
the ADP and Pi leave myosin head
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What does the ATP then do
Bind to myosin heads while they are still attached to actin filaments
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What does the ATP synthase in the head then do
hydrolyses ATP into ADP and Pi releasing energy which is used for the detachment of the myosin heads from the actin/breaking of cross bridges and return them to their original angle
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What does the ADP and Pi remain
attached to head which is now ready to do the same thing again
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Because of the spiral arrangement where will the cross bridges be
at different positions in the stroke cycle meaning muscle contraction is smooth rather than jerky
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Although each myosin molecule only produces a tiny force when the head tilts what is the cumulative effect
very large
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What can the cycle of the myosin head attaching, tilting, detaching, straightening can occur
up to 50x a second
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What is it called
ratchet mechanism
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so
therefore a large amount of ATP may be needed
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What do electron micrographs of dark bands show
Cross bridges between the actin and myosin filaments ,
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what are the bridges part of and what do they do
bridges are part of the myosin molecule and push on the actin filaments to make the myofibril shorten
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Why is it called the Ratchet mechanism
Because Actin molecules are moved along 1 step at a time by the myosin heads
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What does each swing of a cross bridge use
the energy from 1 molecule of ATP each time it happens the Actin filaments are pulled 10nm towards the centre of the sarcomere so Z lines are brought closer together
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How much may the sarcomere shorten by
between 30-60 %
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When the action potential stops
the cycle stops, myosin head stays in position but not attached to actin
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What does the muscle do
Stay in this state unless something pulls it out eg muscles own weight or another muscle contracting
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What will the myosin only do
Only walk one way
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there only can
produce a force when it shortens
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What do many 1000s of cross bridges working together create
power of the muscle
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How is the mechanism turned on/off
by the calcium switch
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What is the calcium switch
how the force produced by a muscle is controlled
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As the action potential spreads along the muscle cell what is depolarised
the infolded membranes forming the t tubules
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What does this affect
the membranes of the closely associated sarcoplasmic reticulum
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What is the sarcoplasmic reticulum
specialised ER found in muscle cells
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What is the result
Opening up Ca^2+ channels
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What do the Ca^2+ ions do
Flood out of the cistern of the SR into the cytoplasm of a muscle cell
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What does the Ca2+ do
bind to actin filaments and expose the binding sites for myosin
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What was this previously blocked by
a protein called tropomyosin
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When Ca2+binds to the tropomyosin molecule what happens
it alters the shape of the protein and it can no longer block the binding sites so the fibre is ready to start the process of muscle contraction
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What does this now make it possible for
Myosin to binds with actin and therefore the release of Ca2+ begins muscle contraction
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What can Ca2+ can also activate
The myosin molecules to break down ATP to release the energy needed for muscle contraction
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When the action potential stops what do the ATP driven pumps in the sarcoplasmic reticulum membranes actively transport
Ca2+ out of the cytoplasm and back in the cisternae so there is therefore a high conc of Ca2+ build up in the SR ready for the next AP to arrive
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In living muscles where are calcium ions stored
in membrane sacs called the sarcoplasmic reticulum that surrounds each myofibril
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When muscles resting what happens to the Ca2+ ions
they are pumped into sarcoplasmic reticulum by Active transport
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What does arrive of action potential at neuromuscular junction set off
a chain of events leading to contraction of myofibrils as just described
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After death why do bodies go rigid
rigor mortis due to muscle contraction as ATP production stops and Ca2+ ions leak out of sarcoplasmic reticulum therefore all muscles contract =stiffness
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What does AP in a motor neurone cause
All the muscle fibres in its motor unit to contract together
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If a stronger contraction is needed what happens
Brain recruits more motor units by sending AP along more motor neurones with more motor nerve
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To control movement what can the brain control
How strongly a muscle contracts
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how does it do this
By changing how each muscle fibre contracts or how many fibres contract
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When one AP arrives at at a NMJ what does it do
cross synapse and cause 1 brief contraction or twitch of the fibre
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if a second AP arrive before fibre has fully released what happens
second twitch adds to effect of the first and a greater contraction occurs
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What does a rapid sequence of AP cause
continuous strong contraction of fibre called a tetanus
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The more nerve impulses there are per second in a motor nerve
the stronger the contraction of each muscle fibre will be
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Each motor neurone serves about
150 muscle fibres
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what is a motor unit
1 motor neurone and its associated muscle fibres
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Card 2

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What else

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, dark bands stay the same length (A bands this is bc region contains myosin only which is now smaller and actin and myosin filaments overlap which is now larger

Card 3

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What is the sliding filament hypothesis

Back

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Card 4

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In effect what does this do

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Card 5

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What do the myosin molecules do

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