Fast vs. Slow Twitch Muscles

Cross references:     Red Nucleus Evolution      Muscle Innervation   
Fast vs. Slow Twitch Muscles
 Motor Nerve Organization   
Amphioxus Motor Nerves     Amphioxus Muscles 
Lamprey Muscles
Shark Muscles    Teleost Muscles  
Amphibian Muscles 
Mammalian Muscles     Human Muscles    
Medial Motor Column      Motor Neuron Disease   Somatic Nervous System 

I'm still trying to determine whether human fast and slow twitch muscles are innervated by nerves with separate, discrete origins as is the case with  Amphioxus Muscles
    I was hoping that 
Motor Neuron Disease would make a clear distinction between fast and slow twitch motor neurons, but it doesn't. 
    An added complication is that
Amphioxus Motor Nerves and Amphioxus Muscles make it clear that the amphioxus actually has only upper motor neurons.  Amphioxus muscle fibers send muscle extensions or tails all the way to the spinal cord, and these muscle extensions act as, or take the place of, lower motor neurons.  However, the dorsal (slow twitch) and ventral (fast twitch) muscle tails contact the spinal cord at different locations.  So, if one accepts these muscle tails as substitute lower motor neurons, then amphioxus fast and slow twitch muscles are innervated by nerves with separate, discrete origins at least as far centrally as the spinal cord. 
    Although I made considerable effort to discern the more central connections within the amphioxus cord, the amphioxus central nervous system is so different from ours that I saw no real analogs.   

Interneuronal synapses with electrical and chemical mechanisms of transmission and the evolution of the central nervous system 
It becomes increasingly evident that not only chemical but also electrotonic synapses are characteristic feature of the vertebrate brain. Experiments carried out on the isolated perfused central nervous system of lampreys and frogs have shown that
    in cyclostomes spinal motoneurons receive at least 4 different electrotonic inputs: from giant reticulospinal axons, from smaller descending fibers intrinsic to the cord, from primary afferents and ventral roots. 
     In amphibian motoneurons it is possible to identify 3 electrotonically mediated monosynaptic actions: from primary afferents, recurrent, and reticulospinal. The exact mode of transmission of monosynaptic excitation 1a and supraspinal actions in mammals remains to be elucidated. The trends of evolutionary development of synaptic transmissional processes and synaptic organization in vertebrate nervous system are discussed."  

    118 Related citations:

Evolution of neuronal connecting mechanisms: electrical, mixed and chemical synapses
    "Investigation of the mechanisms of junctional transmission in the isolated spinal cord in cyclostomes, amphibians, reptilia and mammals reveals the decrease in the number of electrical synapses during evolution from primitive to more advanced vertebrates. Electrical transmission is lacking in reptilian and mammalian cord. On the basis of these data, the analysis of the unitary EPSPs evoked in motoneurons of the lamprey and frog by intracellular stimulation of reticulospinal axons and primary afferent fibers and dendrodendritic interaction between motoneurons, a hypothesis is advanced that neurons of similar type may communicate through pure electrical junctions, whereas successive synaptic articulations between different functional groups of neurons are formed by mixed or chemical synapses. The cellular mechanisms controlling interneuronal communications are discussed."
    104 Related citations: 
    Key Point:  "
Electrical transmission is lacking in reptilian and mammalian cord."     

A review of the organization and evolution of motoneurons innervating the axial musculature of vertebrates. (Goog) - 1987   
from the abstract
In most Anamniotes the axial musculature is Myomeric and is functionally subdivided into superficial red and deep white muscle."   
Among Amniotes, the myomeres divide to form a number of discrete muscles that may be complexly arranged."   
These dramatic differences between the motor columns in anamniotes and amniotes imply that the medial motor column has undergone a major restructuring during the evolution of vertebrates. The available evidence--which is tentative because of the few species that have been studied--suggests that a topographically organized motor column was absent in early vertebrates. A motor column/myotome map appears to have arisen just prior to, or in conjunction with, the origin of amniotic vertebrates."    
My comments
The PDF gives specific information for several different stages of evolution.    
Amphioxus Muscles     Lamprey Muscles   
Shark Muscles   Teleost Muscles    Amphibian Muscles  Mammalian Muscles   
Human Muscles  
Motor Nerve Organization             

Searching Google for "motor neuron evolution" located 1,020,000 references:  

Evolution of nervous systems - Wikipedia   
Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, and therefore no nervous system. They do, however, have homologs of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal-receiving part of a synapse).[2] However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction.[3]"  

Searching PubMed for "motor neuron evolution" located 946 references:   

180701 - 1523