<unk>  function primarily to integrate synaptic input from higher CNS centers into precise and consistent contraction patterns. Individual<unk>  neurons within a given<unk> fire in accordance with what is known as the 'size principle'. The size principle was proposed by Elwood Henneman and his group in the 1960s as an explanation of the characteristic pattern with which individual<unk>  neurons in a<unk> fire. The size principle stipulates that when the<unk>  neurons of a<unk> fire, leading to the contraction of a terminal muscle fiber, the<unk>  units containing the smallest<unk>  neurons fire first. As excitatory signalling increases, larger<unk>  neurons are subsequently recruited and contraction strength increases. Further, this differential recruitment of<unk>  neurons occurs in instances of both increasing and decreasing contraction strength. As contraction strength is increased, the smallest<unk>  units fire first and are also the last to stop firing as the contraction strength decreases. The size principle has important functional benefits. Primarily, this system frees higher centers of the CNS from having to signal specific contraction patterns for distinct levels of muscle contraction. The level of synaptic input that higher centers provide to a given<unk> must determine the contraction strength, and this simplifies the process of contraction strength modulation. This system allows for very precise and consistent modulation of contraction strength from just increased or decreased levels of synaptic input: with additional<unk>  units of increasing size, there will be a consistent and precise effect on the force of contraction. Another key benefit derived from the size principle is that smaller neurons will be fired more regularly and for a longer duration of time compared to larger neurons. These smaller<unk>  units are more resistant to fatigue, and as such, are better suited to this role.
Motor pool 
Extrapyramidal system
Anterior corticospinal tract
Cerebrospinal fibers
Frontopontine fibers
Pyramidal tracts
Functional spinal unit
Lateral vestibulospinal tract