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Difference Between Actin and Myosin

Actin vs Myosin

Actin and myosin are both found in the muscles. Both function for contraction of muscles. Actin and myosin are protein filaments that functions in the presence of calcium ions.actin and myosin are the striations in skeletal muscles. Light striations are called actin filaments. They are also referred as I band. Myosin filaments, on the other hand is the thicker one; thicker than actin myofilaments. Myosin filaments are responsible for the dark bands or striations, referred as H zone. The A band is the length of myosin filament. The M line is the central myosin filament thickening.

Two combined actin strands constitute an actin filament. Actin attachment to myosin is blocked by troponin-tropomyosin-actin complex. Myosin filament, on the other hand is composed of bundles of myosin molecules. The head of a myosin which is globular attaches to actin filaments on proper sites. The myosin bundle tails structured the central stalk. Heads of myosin contain ATPase that converts ATP to ADP.

Muscle contraction where actin and myosin functions, are best explained under the sliding filament theory. The sliding filament theory describes how muscles are contracting. This theory was proposed by Ralph Niedergerke, Jean Hanson, and Andrew Huxley during 1954. In the sliding theory, actin and myosin filaments slide past each other. When the fibers of the muscles are stimulated by the nervous system, the heads of the myosin attach to the binding sites on the lean filaments, and the sliding starts. In the presence of adenosine triphosphate (ATP), the energy giver, each cross bridge attaches at the same time detaches continuously for several times upon contraction. This continuous sliding process produces tension and pulls the thin filaments toward the center of the sarcomere. As this happen concurrently in sarcomeres throughout the cell, the muscle cell shortens. The binding of myosin to actin requires calcium ions. Calcium ions are found deep into the muscle, on the sarcolemma. Action potentials pass onto the sarcolemma to stimulate the sarcoplasmic reticulum to release calcium ions into the cytoplasm. The calcium ions are the one setting off the binding of myosin to actin commencing filament sliding. The end of the action potential to stimulate the sarcoplasmic reticulum causes the reabsorbtion of ions containing calcium particles into the sarcoplasmic reticulum storage areas, and the muscle cells relaxes and return to its original length. The whole sliding filament event occurs within a few thousandths of a second.

Actin and myosin are not only responsible for cellular movements but for non-cellular movements as well. Myosins are also called myosin enzymes since it helps convert ATP to ADP. ATP is needed by myosin to crawl along to actin to create mechanical energy or what we call earlier as muscle contraction. In muscles, two myosin molecules are required. This myosin molecule is a very big protein composed of two similar chains that are heavy and two pairs of chains that are light. This is known as Myosin II. The conversion of chemical energy to mechanical energy is intervened by changes in the myosin shape leading to ATP binding to the actin.

Summary:

1.Actin and myosin are found in muscles and function for muscle contraction. Actins are thinner than myosin and have lighter striations. Myosins are thick and with dark striations.

2.Actin and myosin are not only responsible for cellular movements but for non-cellular movements as well.

3.Muscle contraction where actin and myosin functions, are best explained under the sliding filament theory. The sliding filament theory describes how muscles are contracting in conduction with ATP.

4.Calcium ions are needed for muscle contraction. Action potential is the one stimulating the SR to release calcium ions as well as action potential are the ones responsible for calcium reabsorbtion back to SR storage areas.

5.The contraction of muscles leads to muscle shortening and movement. Relaxation of muscles, on the other hand cause the muscle to return to its usual length.


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