APA Dictionary of Psychology

a neuron whose axon connects directly to muscle fibers. Because motor neurons are the final stage of output from the nervous system and are the only means of stimulating muscle fibers, they are known as the final common path. There are two types: lower motor neurons (or alpha motor neurons), found in the cranial nerves and the anterior horn of the spinal cord and that are responsible for muscle contraction; and upper motor neurons (or gamma motor neurons), found in the corticospinal tract and that modulate the sensitivity of muscle spindles, thus influencing activity of the lower motor neurons. Also called motoneuron.