Sliding filament theory overview

Sliding filament theory - basic overview

1. Sliding Filament Theory

2. Phase 1 A nervous impulse arrives at the neuromuscular junction (NMJ) – this causes a release of a chemical called acetylcholine . Acetylcholine causes the depolarisation of the motor end plate which travels throughout the muscle. Calcium (Ca+) is then released from the sarcoplasmic reticulum (this encircles each myofibril). This calcium triggers a muscle contraction .

3. If there is a lot of calcium, it binds to troponin. This causes the troponin-tropomyosin to move away from the actin . This means the myosin can now attach to the actin – this forms a cross-bridge . Phase 2

4. ATP is broken down (into ADP and P ) so energy is released. The myosin pulls the actin inwards (the Z-line /Z-discs move closer together) and the sarcomere shortens e.g. the muscle shortens. The length of the thin and thick filaments do not change – they (actin and myosin) just move closer together / overlap and shorten the whole muscle fibre. The release of the P triggers what we call the ‘power-stroke’ of contraction!!! This is because the ADP which is still attached causes the myosin head to rotate, pulling the actin filaments past the myosin filaments and towards the M-line. Phase 3

5. The myosin then detaches from the actin breaking the cross-bridge as the ATP binds again to the myosin head. The whole process is repeated when the myosin head attaches to an actin-binding site further down the actin filament - another ‘power-stroke’ occurs. Phase 4

6. Muscular contraction can continue so long as there is ATP and enough calcium in the sarcoplasm . If the nervous impulse STOPS , the calcium goes back to the sarcoplasmic reticulum and the actin returns to resting position e.g. the muscle lengthens and relaxes . Phase 5

7. TextWall Answer the question given to you in the lesson or the one that appears on the text wall