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The Difference Between Actin and Myosin

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Muscle contraction results from attachment-detachment cycles between myosin heads extending from myosin filaments and actin filaments. It is generally believed that a myosin head first attaches to actin, undergoes conformational changes to produce force and motion in muscle, and then detaches from actin

Despite extensive studies, the molecular mechanism of myosin head conformational changes still remains to be a matter for debate and speculation.

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Muscles are made up of proteins. Actin and myosin are two protein molecules present in muscles and are mainly involved in the contraction of the muscle in both humans and animals. Both actin and myosin function by controlling the voluntary muscular movements within the body, along with the regulatory proteins known as troponin, tropomyosin and meromyosin. Actin and myosin proteins build filaments, which are arranged in the myofibrils in a longitudinal manner

They are also responsible for both cellular movements and non-cellular movements. The main difference between actin and myosin is that actin is a protein that produces thin contractile filaments within muscle cells, whereas myosin is a protein that produces the dense contractile filaments within muscle cells.

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For the purpose of determining net interactions between actin and myosin filaments in muscle cells, perhaps the single most informative view of the myofilament lattice is its averaged axial projection (Squire, J. M. 1981). We have studied frozen-hydrated transverse thin sections with the goal of obtaining axial projections that are not subject to the limitations of conventional thin sectioning (suspect preservation of native structure) or of equatorial x-ray diffraction analysis (lack of experimental phases). In principle, good preservation of native structure may be achieved with fast freezing, followed by low-dose electron imaging of unstained vitrified cryosections. In practice, however, cryosections undergo large-scale distortions, including irreversible compression; furthermore, phase contrast imaging results in a nonlinear relationship between the projected density of the specimen and the optical density of the micrograph

To overcome these limitations, we have devised methods of image restoration and generalized correlation averaging, and applied them to cryosections of rabbit psoas fibers in both the relaxed and rigor states (Huxley, H. E., and J. Hanson. 1954).

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In a word, these stereocilia are mechanosensing organelles that respond to the fluid motion in the ear, performing the functions of hearing and balancing. It forms a complex with the PDZ domain-containing protein which, together with actin proteins, assists the response of the stereocilia to sound waves. The myosin allows the stereocilia to adjust to the change in sound waves and fluid motion by moving the cilia. Besides, genes coding for the myosin proteins were also found in some other parts of the ear like the cochlea.

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Huxley, H. E., and J. Hanson. 1954. Changes in the cross-striations of muscle during contraction and stretch and their structural
interpretation. Nature (Lond.). 173:973-976.

Squire, J. M. 1981. The Structural basis of Muscular Contraction. Plenum Publishing Corp., New York. 8-16, 524-527.

Pollard, T. D. 1987. The myosin crossbridge problem. Cell. 48:909-910.

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