How does the sliding filament theory explain muscle contraction?

The sliding filament theory explains muscle contraction as the result of actin and myosin filaments sliding past each other.

The sliding filament theory is a fundamental concept in understanding how muscles contract to produce movement. It was proposed by Hugh Huxley and Jean Hanson in 1954. According to this theory, muscle contraction occurs as a result of thin (actin) and thick (myosin) filaments within the muscle fibre sliding past each other. This process is also known as the cross-bridge cycle.

The cycle begins when a nerve impulse reaches the muscle fibre, causing the release of calcium ions. These ions bind to a protein on the actin filament called troponin, causing a shift in another protein called tropomyosin, which normally blocks the binding sites for myosin on the actin filament. Once these sites are exposed, the myosin heads can bind to the actin, forming a cross-bridge.

The myosin heads then change shape and pull the actin filaments towards the centre of the sarcomere, the functional unit of muscle contraction. This is known as the power stroke. ATP (adenosine triphosphate) then binds to the myosin head, causing it to detach from the actin. The ATP is then hydrolysed into ADP (adenosine diphosphate) and inorganic phosphate, providing the energy for the myosin head to return to its original position. This process is repeated many times during a muscle contraction, with the actin and myosin filaments sliding past each other in an overlapping pattern.

The sliding filament theory is a cyclic process, meaning it repeats as long as the muscle receives signals from the nervous system and has sufficient ATP to power the movement. When the nerve signals stop, calcium ions are pumped back into the sarcoplasmic reticulum, a specialised form of endoplasmic reticulum in muscle fibres, and the muscle relaxes.

In summary, the sliding filament theory explains muscle contraction as a process where actin and myosin filaments within the muscle fibre slide past each other, powered by the hydrolysis of ATP. This process is regulated by nerve impulses and the availability of calcium ions.