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MECHANISM OF MUSCLE CONTRACTION Sliding filament theory In 1954, two researchers, Jean Hanson and Hugh Huxley from the Massachusetts Institute of Technology, made a model for muscle tissue contraction which is known as the sliding filament theory. This theory describes the way a muscle cell contracts or shortens as a whole by the sliding of thin filaments over thick filaments and pulling the Z discs behind them closer. Six different proteins and molecules participate in the contraction of a sarcomere, namely: Myosin Actin Tropomyosin Troponin ATP Ca2+ ions Sliding filament theory, a proposed mechanism of muscle contraction in which the actin and myosin filaments of striated muscle slide over each other to shorten the length of the muscle fibers. Myosin-binding sites on the actin filaments are exposed when calcium ions bind to troponin molecules in these filaments. This allows bridges to form between actin and myosin, which requires ATP as an energy source. Hydrolysis of ATP in the heads of the myosin molecules causes the heads to change shape and bind to the actin filaments. The release of ADP from the myosin heads causes a further change in shape and generates mechanical energy that causes the actin and myosin filaments to slide over one another. Thick Filaments Myosin molecules are bundled together to form thick filaments in skeletal muscles. A myosin molecule has two heads which can move forward and backward and binds to ATP molecule and an actin binding site. This flexible movement of the head provides power stroke for muscle contraction. Thin Filaments The thin filaments are composed of three molecules - actin, tropomyosin, and troponin. Actin is composed of actin subunits, joined together and twisted in a double helical chain. Each actin subunit has a specific binding site to which myosin head binds. Tropomyosin entwines around the actin. This cover the binding sites of actin subunits, preventing myosin heads from binding to them in an unstimulated muscle. Troponin molecules are attached to tropomyosin strands and facilitate tropomyosin movement so that myosin heads can bind to the exposed actin binding sites. The sarcomeres can hence shorten. This, however, can only occur with the binding of Ca2+ ions to troponins first. Mechanism of contraction of the sliding filament Excitation Once an action potential arrives at the axon terminal, acetylcholine is released, resulting in the depolarization of motor end plate. This action potential propagates along the sarcolemma and down the T-tubules causing the release of Ca2+ ions from the terminal cisternae into the cytosol. Ca2+ ions then bind to troponin causing a conformational change in the troponintropomyosin complex, which exposes the binding sites on actin. As illustrated in Figure 3, the myosin head is already energized, as an ATP molecule binds to a myosin head where an enzyme called myosin ATPase hydrolyzes the ATP. This releases the energy resulting in an extension of myosin head, carrying high energy, while holding ADP and a phosphate group temporarily. Contraction This energized and cocked myosin head binds to an active site on the exposed actin binding site as shown in Figure 3. With a power stroke, the thin actin filaments slide along the myosin. The myosin hears changes from a high energy extended position to a low energy flexed position. ADP and a phosphate group are released. The myosin head still remains bound to actin filament until it binds to a new ATP molecule. Once a new ATP binds to myosin head, it releases actin and changes back to a high energy extended position, ready for a next cycle of causing power stroke. Such alternative power stroke occurs concurrently in thousands of myosin heads with actin filaments, resulting in an overall contraction of a muscle fiber. These contractions occurring in millions of muscle fibers, in turn, cause an entire skeletal muscle to contract. Relaxation After a brief time, the acetylcholine diffuses away from their receptor sites causing the acetylcholine receptors to close back as shown in Figure 4. The acetylcholine is then broken down by an enzyme acetylcholinesterase present at the synaptic cleft. Soon after contraction, Ca2+ ions are actively transported from cytosol back to sarcoplasmic reticulum via specialized Ca2+ pumps. ATP is expended in this process of active transport. After the Ca2+ ions are removed from the cytosol, the troponin-tropomyosin complex covers the active binding sites of actin subunits once again, so that myosin heads cannot bind to actin. This results in the relaxation of a muscle cell.