Sliding Filament Theory of Muscular Contraction
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What is the Sliding Filament Theory?
This explains the sequence of events leading up to skeletal muscle contraction.
Step 1 - Preparing the Binding Site
- Upon receiving a signal to contract the sarcoplasmic reticulum releases calcium ions.
- The influx of calcium binds to a second protein on the actin filament called troponin which pulls tropomyosin out of the way, thus exposing the myosin binding sites on the actin filament.
Step 2 - The Power Stroke
- The myosin filament extends to a golf-club shaped 'cross-bridge' which attaches to the binding site of the thin actin filament.
- When it binds the myosin head is in an energised state and pulls on the actin filament pulling it inwards towards the centre of the sarcomere.
- This is referred to as the 'Power Stroke'.
Step 3 - The Binding of ATP
In order to re-energise the myosin head ATP is required. Therefore to expose the ATP binding site the myosin filament uncouples from the actin filament allowing ATP to bind on to it causing the myosin head to return to its high-energy state.
Step 4 - The Ratchet Mechanism
Assuming sufficient calcium is available Step 2 can now be repeated. The power stroke pulling actin filaments past the myosin filaments will continue in a ratchet-like manner until each sarcomere shortens and the H zone disappears.
Step 5 - Return of Calcium Ions
Once the action potential has diminished, all the calcium ions responsible for revealing the myosin binding site on the actin filament will be returned into the sarcoplasm. The shape of troponin will once again be changed allowing tropomyosin to return to its protective position of covering the myosin cross bridge binding site. Tropomyosin will remain in this position as long as the muscle is relaxed.
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