Contraction is the result of a complicated series of events inside muscle cells. Muscles are composed of long, thin cells called muscle fibers. Inside the muscle fibers, there are bands of thick and thin filaments that overlap each other. The thick filaments are a protein called myosin. The thin filaments are a protein called actin. Under a microscope, the areas of overlap appear as bands called Striations. When a muscle is relaxed, the thin actin filaments only overlap the ends of the thick myosin filaments a short distance. When you want to move your arm or leg, your nerves send a signal to the muscles that need to contract. The nerve signal causes numerous small projections called myosin cross bridges or heads to project from the thick filaments. The cross bridges attached to the thin actin filaments and slide them over the myosin filaments, pulling them closer together. The degree of overlap between the active and myosin filaments becomes much greater as the muscle contracts. This is what makes the muscle become shorter and thicker.