noun, plural: myofilaments
Myofilaments are threadlike structures that comprise the myofibril inside the muscle cell (or muscle fiber). There are two main types of myofilaments: (1) thin filaments and (2) thick filaments. In a skeletal muscle, the myofilaments are arranged in a repeating pattern of light and dark bands. Each unit is referred to as a sarcomere, which is the basic functional unit of a muscle. In smooth muscle, the filaments are not arranged in a regular pattern, thus they appear smooth when viewed through the microscope. The thin filament (about 7 nm in diameter) is comprised of two strands of actin and a strand of regulatory protein whereas the thick filament (about 15 nm in diameter) is made up of staggered arrays of myosin molecules.1 Another type of filament is called elastic filament that is only about 1 nm in diameter. It anchors the thick filament to the Z-line of a sarcomere.
Word origin: Greek mûs (mouse; muscle) + Latin filamentum, filare, filum (thread)
1 Campbell, N. A. (1998). Biology. San Francisco: Benjamin Cummings.