A<unk> consists of all individual<unk>  neurons that innervate a single muscle. Each individual muscle fiber is innervated by only one<unk>  neuron, but one<unk>  neuron may innervate several muscle fibers. This distinction is physiologically significant because the size of a given<unk> determines the activity of the muscle it innervates: for example, muscles responsible for finer movements are innervated by<unk>  consisting of higher numbers of individual<unk>  neurons.<unk>  are also distinguished by the different classes of<unk>  neurons that they contain. The size, composition, and anatomical location of each<unk> is tightly controlled by complex developmental pathways.
Motor pool 
Extrapyramidal system
Anterior corticospinal tract
Cerebrospinal fibers
Frontopontine fibers
Pyramidal tracts
Functional spinal unit
Lateral vestibulospinal tract