The sliding filament theory

Prior to 1954, most suggestions as to the mechanism of muscular contraction involved the coiling and contraction of long protein molecules, rather like the shortening of a helical spring. In that year the sliding filament theory was independently formulated by H. E. Huxley and Jean Hanson (using phase-contrast microscopy of myofibrils from glycerol-extracted muscles) and by A. F. Huxley and R. Niedergerke (using interference microscopy of living muscle fibres). In each case the authors showed that the A band does not change in length either when the muscle is stretched or when it shortens actively or passively. This suggests that contraction is brought about by movement of the thin filaments between the thick filaments. The sliding is thought to be caused by a series of cyclic reactions between the projections on the myosin filaments and active sites on the actin filaments; each projection first attaches itself to the actin filament to form a cross-bridge, then pulls on it and finally releases it, moving back to attach to another site further along the actin filament.

Let us now have a look at some of the further evidence for this theory.

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