A myofibril is a tubular arrangement of sarcomere repeat units. Multiple myofibrils form muscle fibres, which in turn form muscles. They are made up of four major proteins: myosin, tropomyosin, actin, and troponin. These proteins are ordered into thick and thin repeating filaments. The thick and thin filaments overlap each other. This arrangement allows the thin filaments to be pulled over the thick filaments to increase the size overlapping section, which reduces the length of the myofibrils; and therefore the muscle, during contraction. The thin filaments can also slide back into their original position to return the myofibrils to their longer length during relaxation of the muscle.
Myofibrils are the basis of contraction in skeletal muscle cells. They are cylindrical in shape and are 1-2 μm in diameter. The thick filaments are made from myosin, and the thin are made from actin, tropomyosin and troponin. Each muscle fibre has many myofibrils running through them. Skeletal muscle cells are often described as striated, which is due to the repeating units of thick and thin filaments. Between each myofibril there are several mitochondria and sarcoplasms.