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Skeletal muscle fibers

Other Terms: Skeletal muscle fiber, Myocytus striatus

Description

A single skeletal muscle cell, known as a muscle fiber, is relatively large, elongated, and cylinder-shaped, measuring from 10 to 100 micrometers in diameter and up to two feet, in length. These potentially huge skeletal muscle fibers are formed by the fusion of many smaller cells during development creating long cells with multiple nuclei. These cells also have an abundance of mitochondria, the energy-generating organelles, as would be expected with the high energy demands of a tissue as active as skeletal muscle. The most predominant structural feature of a skeletal muscle fiber is the presence of numerous myofibrils. These specialized contractile elements, which constitute 80% of the volume of the muscle fiber, are cylinder-shaped intracellular structures 1 µm in diameter that extend the entire length of the muscle fiber. Each myofibril consists of a regular arrangement of highly organized cytoskeletal elements—the thick and thin filaments. The thick filaments, which are 12 to 18 nm in diameter and 1.6 µm in length, are special assemblies of the protein myosin, whereas the thin filaments, which are 5 to 8 nm in diameter and 1.0 µm long, are made up primarily of the protein actin. These same proteins are found in all other cells of the body but in a less-organized fashion. In the light microscope, a relaxed myofibril displays alternating dark bands (the A bands) and light bands (the I bands). The bands of all the myofibrils lined up parallel to each other collectively lead to the striated appearance of a skeletal muscle fiber. Alternate stacked sets of thick and thin filaments that slightly overlap each other are responsible for the A and I bands. An A band consists of a stacked set of thick filaments along with the portions of the thin filaments that overlap on both ends of the thick filaments. The thick filaments are found only within the A band and extend its entire width. The lighter area within the middle of the A band, where the thin filaments do not reach, is known as the H zone. Only the central portions of the thick filaments are found in this region. The I band consists of the remaining portion of the thin filaments that do not project into the A band. Thus, the I band contains only thin filaments but not their entire length. Visible in the middle of each I band is a dense, vertical Z line. The area between two Z lines is called a sarcomere, which is the functional unit of skeletal muscle. A functional unit of any organ is the smallest component that can perform all the functions of that organ. Accordingly, a sarcomere is the smallest component of a muscle fiber that is capable of contraction. The Z line is actually a flattened disc-like cytoskeletal protein that connects the thin filaments of two adjoining sarcomeres. Each relaxed sarcomere is about 2.5 µm in width and consists of one whole A band and half of each of the two I bands located on either side. During growth, a muscle increases in length by adding new sarcomeres, not by increasing the size of each sarcomere as all sarcomeres are of identical size. Just as the Z lines hold the sarcomeres together in a chain along the myofibril’s length, another system of supporting proteins holds the thick filaments together vertically within each stack. These proteins can be seen as the M line, which extends vertically down the middle of the A band within the center of the H zone. Three-dimensionally, the thin filaments are arranged hexagonally around the thick filaments. Cross bridges project from each thick filament in all six directions toward the surrounding thin filaments. Each thin filament, in turn, is surrounded by three thick filaments. To give you an idea of the magnitude of these filaments, a single muscle fiber may contain an estimated 16 billion thick and 32 billion thin filaments, all arranged in this very precise pattern within the myofibrils.

Latin

Myocytus striatus

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