What is the sliding filament model of movement of muscles? How does the sliding movement occur at molecular level?

 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. According to the sliding filament theory, the myosin (thick) filaments of muscle fibers slide past the actin (thin)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 sliding filament theory was born from two consecutive papers published on the 22 May 1954 issue of Nature under the common theme "Structural Changes in Muscle During Contraction". Though their conclusions were fundamentally similar, their underlying experimental data and propositions were different.

Huxley-Niedergerke hypothesis- The first paper, written by Andrew Huxley and Rolf Niedergerke, is titled "Interference microscopy of living muscle fibres". It was based on their study of frog muscle using interference microscope, which Andrew Huxley developed for the purpose. According to them: 

• the I bands are composed of actin filaments, and the A bands principally of myosin filaments; and
• during contraction, the actin filaments move into the A bands between the myosin filaments.    

Huxley-Hanson hypothesis - The second paper, by Hugh Huxley and Jean Hanson, is titled "Changes in the cross-striations of muscle during contraction and stretch and their structural interpretation". It is more elaborate and was based on their study of rabbit muscle using phase contrast and electron microscopes. According to them:

● the backbone of a muscle fibre is actin filaments which extend from Z line up to one end of H zone, where they are attached to an elastic component which they named S filament;

● myosin filaments extend from one end of the A band through the H zone up to the other end of the A band;

● myosin filaments remain in relatively constant length during muscle stretch or contraction;

● if myosin filaments contract beyond the length of A band, their ends fold up to form contraction bands;

● myosin and actin filaments lie side-by-side in the A band and in the absence of ATP they do not form cross-linkages;

● during stretching, only the I bands and H zone increase in length, while A bands remain the same;

● during contraction, actin filaments move into the A bands and the H zone is filled up,

● the I bands shorten, the Z line comes in contact with the A bands; and

● the possible driving force of contraction is the actin-myosin linkages which depend on ATP hydrolysis by the myosin 

A muscle contraction can be explained by the cycle of molecular events that take place between actin and myosin filaments. In a single cycle, a myosin head binds to an actin filament, performs a power stroke, and then releases. Note that for the two filaments to disconnect, the myosin head must bind to a fresh molecule of ATP. After myosin releases actin, it hydrolyzes its ATP and initiates another cycle of actin/myosin interactions.

Although we focused on a single myosin head, in fact a myosin filament has many myosin heads. Each myosin filament is also surrounded by six actin filaments to which the different myosin heads can bind. Therefore, when a myosin head breaks its contact with actin, other myosin heads still connect to actin filaments and thus prevent the sarcomere from sliding back to its relaxed position. 

The relaxation of the sarcomere occurs after calcium returns to the sarcoplasmic reticulum. Whereas the release of calcium from the sarcoplasmic reticulum is by a passive event in which calcium moves through ion channels, the return of calcium is an active event that requires energy. The control of muscle contraction happens at the level of free calcium in the cytoplasm all other components involved in muscle contraction are always present and essentially await calcium ions to begin the action.   

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