A motor neuron is a neuron whose output drives or regulates an effector, commonly muscle. A somatic motor neuron carries action potentials along its axon to a neuromuscular junction, where release of acetylcholine promotes excitation of a muscle fibre. Neural excitation, transmitter release and muscle force are successive steps. [ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. Sections 10.3 and 12.2: neuromuscular junction and functional neuron types
A reflex illustrates the distinction between a sensory input and a motor output: a circuit converts an incoming signal into activity of an efferent neuron. The number and timing of motor-neuron impulses can change force without changing the identity of the contacted muscle. Clinical distinctions between upper and lower motor neurons refer to different positions in a motor pathway, so not every neuron involved in motor control directly innervates muscle.
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