The primary components of the skeletal system are cartilage and bone. Cartilage provides a mold for bone development. It also forms pads and surface coverings between bones. Coordinated movements result from specific combinations of contracted and relaxed muscle cells. The major muscle cell types are smooth, cardiac, and skeletal.
Invertebrates use complex combinations of smooth and striated muscle.
Fly wings can beat a thousand times a second. The muscle cell is called a muscle fiber. These fibers are made up of myofibrils, which are composed of myofilaments. Skeletal muscle contraction requires huge amounts of ATP to power the return of calcium and to break the connection between the actin and myosin. A nerve cell and all its connecting muscle fibers constitute a motor unit.