Cross references: Muscle Innervation Motor Neuron Evolution Motor ProgramsThe PDF cited below is 37 pages long and discusses the motoneurons of 8 different animals.
Fast vs. Slow Twitch Muscles Tonic Inhibition Behavioral Disinhibition
Amphioxus Amphioxus Motor Nerves
Amphioxus Fast-Slow Twitch Dorsal Motor Neurons
Ventral Motor Neurons Amphioxus Locomotion
Lamprey Lamprey Motor Nerves Lamprey Muscles
Lamprey Fast-Slow Twitch
Sharks & Rays Shark Muscles Red Nucleus
Teleosts Teleost Muscles
Amphibians Amphibian Muscles
A review of the organization and evolution of
Full length PDF available online for free.
The Local Circuit Neurons:
The most explicit reference I've found so far for the preceding blog entry is in:
Neuroscience, third edition, page 371
ed. Dale Purves, et al.
Sinauer Associates, Publishers
"The neural circuits responsible for the control of movement can be divided into four distinct but highly interactive subsystems, each of which makes a unique contribution to motor control. The first of these subsystems is the local circuitry within the gray matter of the spinal cord and the analogous circuitry in the brain stem. The relevant cells include the lower motor neurons (which send their axons out of the brainstem and spinal cord to innervate the skeletal muscles of the head and body, respectively) and the local circuit neurons (which are the major source of synaptic input to the lower motor neurons)."
I spent some time "reading about the innervation of motor neurons", not in our subcortical ancestors, but in we humans. Paraphrasing Medial Motor Column , it turns out that the rubrospinal tract, the vestibulospinal tract, the tectospinal tract and the reticulospinal tract influence the output from the Alpha motor neurons , not by providing input to the Alpha motor neurons directly, but by providing input to the very numerous Interneurons which then influence the output from the Alpha motor neurons .