<Tr> <Td_colspan="2"> Anatomical terminology (edit on Wikidata) </Td> </Tr> <P> A myofibril (also known as a muscle fibril) is a basic rod - like unit of a muscle cell . Muscles are composed of tubular cells called myocytes, known as muscle fibers in striated muscle, and these cells in turn contain many chains of myofibrils . They are created during embryonic development in a process known as myogenesis . </P> <P> Myofibrils are composed of long proteins including actin, myosin, and titin, and other proteins that hold them together . These proteins are organized into thick and thin filaments called myofilaments, which repeat along the length of the myofibril in sections called sarcomeres . Muscles contract by sliding the thick (myosin) and thin (actin) filaments along each other . </P> <P> The filaments of myofibrils, myofilaments, consist of two types, thick and thin: </P>

The thin contractile protein of a muscle cell is known as