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Why do skeletal muscles have a striped appearance under a microscope?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

Skeletal muscles appear striped under a microscope because of the highly organized arrangement of their internal structures, known as myofibrils.

Each myofibril consists of repeating units called sarcomeres, which contain overlapping sets of thick and thin filaments. The thick filaments are composed of myosin, while the thin filaments are primarily made of actin.123

The striped appearance is created by the alternating density of these filaments:
* A bands: These dark regions contain thick filaments, often overlapping with thin filaments.13
* I bands: These light regions contain only thin filaments.13

Within a muscle fiber, all myofibrils are aligned in register, meaning these regions of similar density are positioned side-by-side. This precise alignment causes the entire fiber to exhibit the characteristic striated, or striped, pattern visible under a microscope.3