Sliding filament theory

Sliding filament theory explains muscle contraction as thin actin filaments moving past thick myosin filaments, shortening sarcomeres without changing either filament’s length.

Cross-bridge cycle: The repeating attachment, power stroke, detachment, and recovery cycle of myosin interacting with actin. It describes the repeating molecular action that makes the filaments slide.
Sarcomere: The repeating contractile unit of striated muscle, extending from one Z disc to the next. Sliding shortens this unit while its actin and myosin filaments keep their lengths.
Hugh Huxley: A British biophysicist whose research helped establish the structural basis of muscle contraction. His X-ray diffraction work provided evidence for movement between filaments.
Filament-shortening hypothesis: The proposal that muscle contracts because actin or myosin filaments themselves become shorter. It predicts filament-length changes that sliding-filament observations do not show.
Myosin: A motor protein that uses ATP to interact with actin and generate force. Myosin heads bind actin and pull thin filaments toward sarcomere centers.
Thick filament: A muscle filament composed mainly of myosin molecules arranged with projecting motor heads. Its myosin heads form cross-bridges with thin filaments.
Jean Hanson: A British cell biologist who investigated muscle structure and helped formulate the sliding-filament model. Her collaboration with Hugh Huxley produced a foundational account of filament sliding.
Muscle contraction: The process by which muscle generates tension and may shorten in response to activation. Sliding filament theory explains a central mechanism behind this broader process.
Actin: A protein that forms thin filaments in muscle and also supports cell structure and movement. Its thin filaments move relative to myosin during contraction.
Thin filament: A muscle filament built chiefly from actin, with regulatory proteins tropomyosin and troponin. It is the filament drawn inward during the sliding process.

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