Sliding filament theory

The sliding filament theory explains the mechanism of muscle contraction based on muscle proteins that slide past each other to generate movement. It was independently introduced in 1954 by two research teams, one consisting of Andrew F. Huxley and Rolf Niedergerke from the University of Cambridge, and the other consisting of Hugh Huxley and Jean Hanson from the Massachusetts Institute of Technology. It was originally conceived by Hugh Huxley in 1953. Andrew Huxley and Niedergerke introduced it as a "very attractive" hypothesis.

According to the sliding filament theory, the actin (thin) filaments of muscle fibers slide past the myosin (thick) filaments during muscle contraction, while the two groups of filaments remain at relatively constant length. Before the 1950s there were several competing theories on muscle contraction, including electrical attraction, protein folding, and protein modification. The novel theory directly introduced a new concept called cross-bridge theory (classically swinging cross-bridge, now mostly referred to as cross-bridge cycle) which explains the molecular mechanism of sliding filament. Cross-bridge theory states that actin and myosin form a protein complex (classically called actomyosin) by attachment of myosin head on the actin filament, thereby forming a sort of cross-bridge between the two filaments. These two complementary hypotheses turned out to be the correct description, and became a universally accepted explanation of the mechanism of muscle movement.

The first muscle protein discovered was myosin by a German scientist Willy Kühne, who extracted and named it in 1864. In 1939 a Russian husband and wife team Vladimir Alexandrovich Engelhardt and Militsa Nikolaevna Lyubimova discovered that myosin had an enzymatic (called ATPase) property that can breakdown ATP to release energy.Albert Szent-Györgyi, a Hungarian physiologist, turned his focus on muscle physiology after winning the Nobel Prize in Physiology or Medicine in 1937 for his works on vitamin C and fumaric acid. He demonstrated in 1942 that ATP was the source of energy for muscle contraction. He actually observed that muscle fibres containing myosin B shortened in the presence of ATP, but not with myosin A, the experience which he later described as "perhaps the most thrilling moment of my life." With Brunó Ferenc Straub, he soon found that myosin B was associated with another protein, which they called actin, while myosin A was not. Straub purified actin in 1942, and Szent-Györgyi purified myosin A in 1943. It became apparent that myosin B was a combination of myosin A and actin, so that myosin A retained the original name, whereas they renamed myosin B as actomyosin. By the end of the 1940s Szent-Györgyi's team had postulated with evidence that contraction of actomyosin was equivalent to muscle contraction as a whole. But the notion was generally opposed, even from the likes of Nobel laureates such as Otto Fritz Meyerhof and Archibald Hill, who adhered to the prevailing dogma that myosin was a structural protein and not a functional enzyme. However, in one of his last contributions to muscle research, Szent-Györgyi demonstrated that actomyosin driven by ATP was the basic principle of muscle contraction.

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