Muscles are made up of multiple muscle fibers, which are themselves formed of strings of sarcomeres in series. Each sarcomere is formed of overlapping actin and myosin filaments that allow the sarcomere to change in length. When sarcomeres attempt to reduce length by increasing the amount of actin and myosin filaments that overlap, this produces tensile force within the muscle fiber that ultimately leads to muscle contractions. Muscle fibers produce tensile force in this way in response to pulsed electrical signals sent from the central nervous system (CNS) along efferent nerves. Each pulsed electrical signal sent by the CNS is called an action potential. The efferent nerves are called alpha motor neurons.