The<unk>  unit: a single<unk>  neuron and the muscle fibers that it innervates. A<unk> consists of all of the<unk>  neurons that innervate a single muscle. Distinct skeletal muscles are controlled by groups of individual<unk>  units. Such<unk>  units are made up of a single<unk>  neuron and the muscle fibers that it innervates.<unk>  neurons are located in the ventral horn of the spinal cord and the brainstem. These neurons innervate skeletal muscle fibers through the propagation of action potentials down their axons (through ventral roots and cranial nerves), and they stimulate skeletal muscle fibers at neuromuscular junctions where they synapse with the<unk>  end plates of muscle fibers. In humans, these axons can be as long as one meter.<unk>  neurons themselves fall into three main classes: alpha-motor neurons control extrafusal muscle fibers, meaning that they innervate skeletal muscles leading to movement; gamma-motor neurons innervate intrafusal muscle fibers, controlling the sensitivity of muscle spindles to stretch; beta-motor neurons are capable of synapsing on either type of muscle fiber. Alpha-motor neurons can further be divided into three separate subclasses, distinguished according to the contractile properties of the<unk>  units that they form: fast-twitch fatigable (FF), fast-twitch fatigue-resistant (FR), and slow-twitch fatigue-resistant (S). The composition of a<unk> may consist of multiple classes and subclasses of<unk>  neurons.<unk>  in the spinal cord are clustered in distinct columns of<unk>  neurons extending over multiple spinal cord segments; although, there is significant overlap.<unk>  that control proximal muscles are generally located medial to the ventral horn, while those that control distal muscles are located laterally.<unk>  that control flexor muscles are located dorsally to the ventral horn while those that control extensor muscles are located ventrally. The number of<unk>  neurons in an individual<unk> is highly variable and can generally be predicted by the level of nuanced control that a specific muscle requires. For example, some muscles have relatively low numbers of<unk>  units in their respective<unk>  while others, with highly nuanced control (such as the muscles in the human hand) have higher densities of<unk>  units.
Motor pool 
Extrapyramidal system
Anterior corticospinal tract
Cerebrospinal fibers
Frontopontine fibers
Pyramidal tracts
Functional spinal unit
Lateral vestibulospinal tract