After reading Kandel I am unclear on this point. I understand that the cell bodies of somatic lower motor neurons cluster in motor nuclei (motor pools) in the ventral horn of gray matter in the spinal column. I also understand the size principle whereby lower motor neurons are recruited in an organized fashion. However I cannot find a good source which describes
- the exact ratio of descending efferent fibers to these nuclei and
- Whether descending efferents can selectively connect to individual motor neurons and not the whole nucleus/pool.
Kandel suggests both that motor units within a given muscle all receive the same input and that some muscles are compartmentalized and those compartments need separable control. I can't reconcile those two statements without better understanding the organization of and inputs to motor nuclei.
"In some muscles the fibers of motor units are confined to discrete compartments that correspond to the regions of the muscle supplied by the primary branches of the muscle nerve. Selective activation of different compartments that exert forces in different directions provides a biomechanical advantage." - pg 770
"[T]he brain cannot selectively activate specific motor units." - pg 773