Skeletal muscle comprises the largest single organ of the body. It is highly compartmentalized, and we often think of each compartment as a separate entity (such as the bicep muscle). Each of our individual muscles is composed of single cells or fibers embedded in a matrix of collagen. At either end of the muscle belly, this matrix becomes a tendon that attaches the muscle to the appropriate bone.

“Skeletal muscle is the most abundant tissue in the human body and also on of the most adaptable. Vigorous training with weights can double or triple a muscles size, whereas disuse, as in space travel, can shrink it by twenty percent in two weeks” (1).

Muscle cells contain most of the structures common to all cells. Each cell is enclosed by a cell membrane or plasmalemma; they contain mitochondria for the oxidative metabolism of nutrients; and all the machinery necessary for protein synthesis. Skeletal muscle fibers are multinucleated and can be as much as two centimeters long.

The principal force generating components are actin and myosin molecules. These myofilaments are arranged in interdigitating matrices capable of sliding across each other

“The central and structural and functional unit of a muscle cell is the sarcomere, a cylinder of 1.5 micrometers in diameter and 2 micrometers in length which contains about 2 * 2000 thin protein filaments and 1000 thick protein filaments” (2). Each sarcomere is separated by Z-discs. Two Z-discs bound a sarcomere in the direction of stretching. Thin filaments made of actin are attached to each of these discs and extent toward each other inside the sarcomere. There is a dark band visible between the Z-discs. This is made up of the thick myosin filaments, which overlap partially with the actin filaments, which extend into a light half of the I-band left and right of a dark A-band.

The ability of eucaryotic cells to adopt a variety of shapes and to carry out coordinated and direct movements depends on the cytoskeleton, a complex network of protein filaments that extends throughout the cytoplasm. This cytoskeleton is responsible for muscle contraction, and the changes in shape during the development of a vertebrate embryo. These diverse activities depend on three principal types of protein filaments: Actin filaments, microtubules and inter mediate filaments.

“To produce a protein, a muscle cell, like any other cell in the body, must have a “blueprint” to specify the order in which amino acids should be put together to make the protein—in other words, to indicate which protein will be created. This blueprint is a gene is the cell’s nucleus and the process by which the information gets out of the nucleus into the cytoplasm, where the protein will be made starts with transcription. It occurs in the nucleus when a genes information (encoded in DNA) copied into a molecule called messenger RNA. The mRNA then carries this information outside the nucleus to the ribosomes, which assemble amino acids into the proteins—actin or one of the myosin isoforms, for example-as specified by the mRNA. Biologists refer to the entire process of producing a protein from a gene as “expression” of that gene period” (1).

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