The structure of skeletal muscles is slightly different from the other two main types of muscles, cardiac and smooth muscles. Also called striated muscle, the skeletal muscle has a striped appearance because of two overlapping proteins that allow the muscle to contract rapidly. Elongated cylindrical cells, which are also called muscle fibers, also make up the structure of skeletal muscles. Each skeletal muscle cell contains several hundred nuclei, as opposed to the single nucleus found in the cells of other muscle types. Muscle fibers have membranes as well as filaments called myofibrils, structures that are particularly important for their contractile function.
Three kinds of muscle tissue, including the skeletal, cardiac, and smooth muscles, make up the human muscular system. Skeletal muscles are the most abundant muscle type, as they cover and provide movement to the entire skeleton of the human body. These have two alternate names: striated muscles, because of their cross-striped appearance, and voluntary muscles because the conscious mind can control their actions. Muscle fibers contract rapidly due to the presence of myofibrils, or tiny filaments that contain two overlapping proteins called actin and myosin. As seen under a microscope, dark bands formed by myosin overlap light bands formed by actin, resulting in the striped appearance of the structure of skeletal muscles.
Muscular contraction is made possible mostly by the chemical reaction between myosin and actin, and controlled by the action of calcium ions and two other proteins that work together called troponin and tropomyosin. When calcium ions are released from the sarcoplasmic reticulum of skeletal muscles then combine with troponin, a chemical reaction, and thus muscular contraction, occurs. The release of calcium ions triggers the troponin to change position, causing the tropomyosin to be pushed away, making way for myosin to be able to “reach” and interact with actin. When calcium ions remain stored in the sarcoplasmic reticulum, no muscular contraction occurs and thus a muscle is said to be “resting” or in a relaxed state.
The structure of skeletal muscles also consists of two membrane systems called the plasma or cell membrane and the sarcoplasmic reticulum, which are both involved in the contraction process. Each muscle fiber is surrounded by the cell membrane, which has tubelike extensions called transverse tubules. Transmission of electrical impulses that activate contraction occurs through transverse tubules that extend deep into muscle fibers. Meanwhile, the sarcoplasmic reticulum, located inside the muscle fibers, releases calcium during muscle contraction and stores calcium during muscle relaxation. A characteristic pattern called triad is formed because the sarcoplasmic reticulum is situated very near the transverse tubules.