What are Muscle Fibers Made Of?

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We have three different kinds of muscle, including skeletal muscle, smooth muscle and cardiac muscle. However, all muscles are made up of fibers that contract using the same mechanism: the sliding of thin actin filaments past thick myosin filaments.

Features

  • A muscle fiber is a single cell that contains a bundle of myofibrils. Myofibrils consist of thin filaments and thick filaments arranged in sarcomeres. Thin filaments are made of strands of a protein called actin twisted around strands of a protein called tropomyosin. Thick filaments are made of a protein called myosin.

Contraction

  • In a muscle fiber at rest, the thin and thick filaments only partially overlap in each sarcomere, with the center of the sarcomere containing only thick filaments and the outer edges containing only thin filaments. The lengths of the filaments do not change. Muscle contraction is achieved when filaments slide past each other to increase the amount of overlap between them, shortening the sarcomere. All of the sarcomeres in the muscle fiber contract at once, shortening the entire fiber.

Molecular Mechanism

  • The sliding of the filaments is accomplished by myosin heads of the thick filaments actively pulling the thin actin filaments along. Powered by adenosine triphosphate (ATP), the myosin heads repeatedly bind to the actin, bend, detach from the actin, snap back to their unbent state and then bind again. All of the myosin heads on the thick filament pull the actin in the same direction when they bend, causing the filaments to slide past each other.

Regulation

  • When a muscle fiber is relaxed, the tropomyosin of the thin filament covers the actin, blocking the myosin head from binding. Calcium ions cause the tropomyosin to shift and expose the actin binding sites, leading to contraction. Neurons control muscle contraction and relaxation through the regulation of calcium ion concentration.

Types

  • Different types of muscle fibers have been adapted for specific functions. Fast-twitch muscle fibers are used for brief, powerful contractions and slow-twitch fibers can hold contractions for long periods of time. Most muscles contain a combination of fast- and slow-twitch fibers. Some animals have muscle fibers that contract much faster than human fast-twitch fibers can. For example, the rattle of a rattlesnake is controlled by super fast muscle fibers.

    Muscle fibers also can be classified into oxidative and glycolytic fibers. Oxidative fibers obtain ATP through aerobic respiration, while glycolytic fibers rely on glycolysis. When eating chicken, you can easily distinguish these two types. Dark meat is made of oxidative fibers, while white meat is made of glycolytic fibers.

References

  • Biology (8th Edition); Campbell and Reece; 2008
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