Contraction of Skeletal Muscle

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Evidence for the Sliding Filament Mechanism

Sliding filament nmechnaism is where actin and myosin filaments slide past one another

There is more of an overlap of actin and myosin in a contracted muscle than in a relaxed one

When a muscle contracts the isotrophic band becomes narrower, the z-lines move closer together (sacromere shortens), the h-zone becomes narrower.

the anisotrophic bands remain the same width. As the width of the band is determind by the length if the myosin filaments, it shows that the myosin filaments haven't become shorter

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Structure of the 3 main protiens

Myosin- made up of 2 types of protien. A fibrous protien arranged into a filament made up of serveral hundred molecules (a tail) and a globular protien formed into 2 bulbous structures at one end

Actin- globular protien whose molecules are arranged into long chainsthat are twisted around one another to form a helical strand

Tropomyosin- forms long thin strands that are wound areound actin filaments

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Sliding filament mechanism- general

Bulbous heads of myosin filament form cross bridges with actin filaments. they do this by attaching themeselves  to binding sights on  theactin filaments and then flexing in unison, pulling the actin filaments along the myosin filaments

They then become detached and (by using ATP) return to their original angle and reattch themselves further along the actin filament.

Process is repaeted up to 100 times per second

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Sliding filmanet mechanism- muscle stimulation

An action potential reaches many neuromuscular junction simultaneously, causing calcium ion channels to open a calcium ions to move into the synaptic knob

Calcium ions cause synaptic vesicles to fuse with presynaptic membrane and release their acetylcholine into the synaptic cleft

Acetylcholine diffuses across synaptic cleft and binds with receptors on post-synaptic membrane, causing it to depolarise

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Sliding filament mechanism- muscle contraction

Action potential travels deep into the fibre through T-tubules in sacroplasm. Tubules are in contact with endoplasmic rectilum of the muscle (sacroplasmic rectilum) which actively transports calicum ions from cytoplasm of that muscle.

Action potential opens the calcuim ion channels on the endoplasmic rectilum and calcium ions flood into the muscle sytoplasm (down a concentration gradient). Calcium ions cause the tropomyosin molecules that were blocking the binding sites on actin filaments to pull away.ADP molecule attached to myosin heads means thet are now in a state to find bind to the actin filament and form a crossbridge

Once attached to actin filament, myosin heads change their angle, pulling the actin filament along with it and releasing a molecule of ADP

An ATP molecule attaches to each myosin head, causing it to become detached from actin filament. Calcium ions activate ATPase, which hydrolyses ATP to ADP. This provides energy got myosin head to return to original position

Myosin head then reattaches itself further along the actin filament ad the cycle is repeated as long as there is nervous stimulation of the muscle

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Sliding filament mechanism- muscle relaxation

When nervous stimulation ceases, calcium ions are actively transported back into the endoplasmic rectilum using energy from the hydrolysis of ATP

This reabsorbtion pf calcium ions allows tropomyosin to block the actin filament again

Myosin heads are now unable to bind to actin filament and contraction ceases

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