Motor neurons transmit signals to muscles and glands throughout the body. There are two major types: the somatic motor neurons that control voluntary muscle movements and the autonomic motor neurons that control involuntary muscle movements and glands. The autonomic motor neurons are further divided into the sympathetic and parasympathetic systems.
The somatic motor neurons have cell bodies located in the central nervous system in either the brain, brain stem or spinal cord. They extend up all the way out to the skeletal muscles. While these are generally voluntarily controlled, reflex arcs sometimes occur, where certain stimuli cause these neurons to fire involuntarily. In this case, a sensory neuron picks up a signal, generally a pain signal, that causes the stimuli to not only travel to the brain, but also through an interneuron directly to a motor neuron. This is the cause of the involuntary movement in the knee-jerk reflex test.
The autonomic motor neurons in the sympathetic system stimulate involuntary muscles or glands to actions, while those of the parasympathetic system inhibit these actions. The cell bodies of these neurons are not in the central nervous system, but are instead in clusters of nerve cells, called ganglia, in the peripheral nervous system.