How Do Muscles Contract? Specialized Proteins in Muscles Do the Work that Leads to Contraction

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Everyone knows that when muscles contract they generate force. You know this because you use your muscles to do work of various kinds. You know this because you can see your muscles rippling when you do something particularly strenuous. Did you know this happens because of the work being done inside the muscle by two very specialized proteins?

Actin and Myosin

Skeletal, or voluntary, muscle is an unusual tissue. First, individual muscle fibers (myofibers) are made from a number of precursor cells that decide to fuse together and make a single very large cell. Second, these myofibers are filled almost completely by a series of thin and thick filaments each of which is composed largely of a single specialized protein. For thin filaments the protein is known as actin and for thick filaments the protein is known as myosin.

How Do Thick and Thin Filaments Work?

The myosin molecule is a member of a family of proteins known as “motor proteins”. That is they can generate actual physical movement as a result of an energy using process. In the thick filament, many molecules of myosin are wrapped around each other to make a rod-like structure. Importantly, these myosin containing rods have the “head” of the myosin molecules sticking out along their side. The thin filaments are made up of many molecules of actin tightly associated together. The myosin head group can stick to the actin thin filaments and when energy is released by myosin consuming an ATP molecule (the universal energy fuel of all cells), the myosin head groups can actually pull against the thin filament to which they have stuck and force it to move by.

The Contraction Cycle

Other signals inside of the muscle cell are very important to the activation and regulation of the contraction process. When the signal is received for muscle to contract, the calcium ion plays a critical role. Calcium levels rise in the cell cytoplasm and this tells specific regulator proteins to let myosin thick filaments to stick to actin-containing thin filaments. The myosin head groups split ATP, they force the actin filaments by and then they relax and let go of the thin filaments. Calcium inside of the cytoplasm is sequestered inside specialized storage depots and the process ends.

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How Can These Sliding Filaments Do Work?

Obviously, if the thick and thin filaments were simply “floating around” inside of the muscle fibers not much work would get done. The sliding of these filaments past one another generates force because they are fixed at their ends and are attached to the other structures inside of the myofibers in a highly organized fashion. They are so organized, in fact, that if you look at skeletal muscle in the microscope, it has a striped or “striated” appearance. When contraction occurs, the complete “contractile apparatus” within an individual myofiber gets shorter and the muscle can generate force.

For a wonderful animation of the contraction process view the movie online provided by San Diego State University College of Sciences.

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