Q.(a) Explain the sliding filament theory of muscle contraction. OR
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Start your 14-day free trial to unlock the full solution →The sliding filament theory explains muscle contraction as the sliding of actin (thin) filaments over myosin (thick) filaments, shortening the sarcomere without any change in the length of the filaments themselves, powered by ATP-driven cross-bridge cycling triggered by calcium ions.
Answering part (a): Explain the sliding filament theory of muscle contraction.
Structure: A myofibril is made up of repeating units called sarcomeres, each bounded by Z-lines. Each sarcomere contains thin filaments (actin, with associated troponin and tropomyosin) anchored at the Z-lines, interdigitating with thick filaments (myosin) in the centre.
Mechanism of contraction:
- A nerve impulse arrives at the neuromuscular junction, releasing acetylcholine, which depolarises the muscle fibre membrane (sarcolemma) and travels down the T-tubules.
- This triggers the release of calcium ions (Ca2+) from the sarcoplasmic reticulum into the sarcoplasm.
- Ca2+ binds to troponin on the thin filament, causing a conformational change that shifts tropomyosin away from the myosin-binding sites on actin, exposing them.
- The myosin head (which has ATPase activity and is already bound to ADP and Pi from a previous cycle) binds to the exposed site on actin, forming a cross-bridge.
- The myosin head then bends, pulling the actin filament toward the centre of the sarcomere (the 'power stroke'), releasing ADP and Pi in the process.
- A new ATP molecule binds to the myosin head, causing it to detach from actin.
- ATP is hydrolysed to ADP + Pi, which re-cocks the myosin head to its high-energy position, ready to bind actin again further along.
- This cross-bridge cycle repeats rapidly as long as Ca2+ and ATP are available, progressively pulling the actin filaments inward from both ends of the sarcomere. …
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