A-level Biology - Sliding filament Theory help! Watch

science_geeks
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Hi everyone,

I'm struggling to understand the sliding filament theory, in muscle contraction - please could someone help explain it to me?

Thank you!
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unlucky_chances
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this will probably be easier if you look at diagrams as you go through and sorry for bad gramma

a muscle fibre is made up of myofibrils, which are made up of thick (myosin) and thin (actin) myofilaments
a myofibril is made up of short sections called sarcomeres
sliding filament theory is that the sarcomere is shortened (contracts) by the mysosin and actin filaments sliding over one another, the myofilaments themselves don't contract.
contraction of lots of sarcomeres means the myofibril is contracting and therefore so is the muscle fibre.

How they slide over eachother:
myosin filaments have hinged globular heads with binding sites for actin and ATP
actin filaments have binding sites for the myosin heads (actin-myosin binding sites)
tropomyosin ( a protein) is found between actin filaments

when the muscle is resting the tropomyosin is blocking the the actin-myosin binding site

When contraction is triggered, there must be an action potential from a motor nuerone, this depolarises the sarcolemma ( cell membrane of muscle fibre) and sarcoplasmic reticulum ( runs through muscle fibre and stores Ca^2+) releases Ca^2+.
Ca binds to protein thats attached to tropomyosin forcing it to change shape, getting out of the way of the actin-myosin active site
myosin head binds forming a actin-myosin cross bridge.
Ca also activates ATP hydolyse, breaking down ATP releasing energy for muscle contraction, this energy causes myosin head to bend, pulling the actin filament along itself
Another ATP will break cross bridge bond, myosin head detaches, myosin head bends back to normal position and binds to new binding site further along the actin filament
this all happens rapidly, shortening the sarcomere, therefore contacting the muscle
this will happen as long as there is ca ions present

if no ca is present (ca ions actively transport back into sarcoplasmic reticulum) the tropomyosin blocks binding sites again, actin filaments move back, lengthening the sarcomere
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science_geeks
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Hey thank you so much! That's really well explained (not to mention easy to understand too!) Definitely would rep again if I could!
(Original post by unlucky_chances)
this will probably be easier if you look at diagrams as you go through and sorry for bad gramma

a muscle fibre is made up of myofibrils, which are made up of thick (myosin) and thin (actin) myofilaments
a myofibril is made up of short sections called sarcomeres
sliding filament theory is that the sarcomere is shortened (contracts) by the mysosin and actin filaments sliding over one another, the myofilaments themselves don't contract.
contraction of lots of sarcomeres means the myofibril is contracting and therefore so is the muscle fibre.

How they slide over eachother:
myosin filaments have hinged globular heads with binding sites for actin and ATP
actin filaments have binding sites for the myosin heads (actin-myosin binding sites)
tropomyosin ( a protein) is found between actin filaments

when the muscle is resting the tropomyosin is blocking the the actin-myosin binding site

When contraction is triggered, there must be an action potential from a motor nuerone, this depolarises the sarcolemma ( cell membrane of muscle fibre) and sarcoplasmic reticulum ( runs through muscle fibre and stores Ca^2+) releases Ca^2+.
Ca binds to protein thats attached to tropomyosin forcing it to change shape, getting out of the way of the actin-myosin active site
myosin head binds forming a actin-myosin cross bridge.
Ca also activates ATP hydolyse, breaking down ATP releasing energy for muscle contraction, this energy causes myosin head to bend, pulling the actin filament along itself
Another ATP will break cross bridge bond, myosin head detaches, myosin head bends back to normal position and binds to new binding site further along the actin filament
this all happens rapidly, shortening the sarcomere, therefore contacting the muscle
this will happen as long as there is ca ions present

if no ca is present (ca ions actively transport back into sarcoplasmic reticulum) the tropomyosin blocks binding sites again, actin filaments move back, lengthening the sarcomere
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unlucky_chances
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(Original post by science_geeks)
Hey thank you so much! That's really well explained (not to mention easy to understand too!) Definitely would rep again if I could!
no problem, it was good revision recall for me too
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