The Sliding Filament Theory of Muscle Contraction

The Sliding Filament Theory, put forward independently by Hugh Huxley and Jean Hanson (and by Andrew Huxley and colleagues) explains that a muscle shortens not because its individual filaments contract, but because the thin actin filaments are pulled inward past the stationary thick myosin filaments, so that the filaments overlap more and the sarcomere as a whole becomes shorter. The theory is usually described in five linked stages.

1. Excitation: A nerve impulse arriving at the neuromuscular junction releases acetylcholine, which generates an action potential across the sarcolemma. This electrical signal travels inward along the T-tubules of the triad system and reaches the sarcoplasmic reticulum deep inside the fibre.

2. Cross-bridge formation: The signal triggers the sarcoplasmic reticulum to release stored Ca2+ into the sarcoplasm. The calcium ions bind to troponin C, causing the troponin-tropomyosin complex to shift position along the actin filament and expose the myosin-binding sites that were previously masked. The myosin head, which is already “charged” with energy from a previously hydrolysed ATP molecule (holding ADP and inorganic phosphate), now binds to the newly exposed site on actin, forming a cross-bridge.

3. Power stroke: The myosin head releases the bound ADP and phosphate and bends/pivots at its neck, dragging the attached actin filament a short distance toward the centre of the sarcomere (toward the H-zone). This is the actual force-generating and shortening step.

4. Recovery stroke: A fresh molecule of ATP binds to the myosin head, and this binding causes the head to detach from actin. The myosin ATPase then hydrolyses this ATP, which re-cocks the head back to its original high-energy position, ready to bind a new site further along the actin filament and repeat the cycle. …