Amphibian Muscles

Cross references:   Motor Neuron Evolution     
Muscle Innervation
Motor Nerve Organization   
Amphioxus Motor Nerves     Amphioxus Muscles 
Lamprey Muscles
Shark Muscles    Teleost Muscles   
Human Muscles        Medial Motor Column    
Fast vs. Slow Twitch Muscles
Brain of the Tiger Salamander    

A review of the organization and evolution of motoneurons innervating the axial musculature of vertebrates. (Goog) - 1987  
See:  Motor Nerve Organization  .  
from the PDF
    "The axial musculature of salamanders is divided into myomeres; however, there is some subdivision of the
Myomeres on the trunk into muscle groups similar to some of those present in amniotes.  
    A relatively large, undifferentiated, muscle mass called the dorsalis trunci forms most of the epaxial (above the spine) muscle of the trunk.  The dorsalis trunci is divided into myomeres by
Myosepta, and is generally similar to the epaxial (above the spine) muscle of fishes. There is some differentiation of the deeper epaxial muscles associated with the vertebrae. 
    The hypaxial (below the spine) trunk muscles are divided into 3 layers: a superficial external oblique, an internal oblique, and a transversus abdominus. The muscle fibers in each layer are oriented differently.  A rectus abdominus muscle runs along the ventral midline.  With a few exceptions (e.g. Proteus, Amphiuma, Salamandra), all of these hypaxial muscles are divided transversely into myomeres by connective tissue septa that penetrate through the different muscle layers and form the origin and insertion sites of the muscle fibers."   
    "The muscle fibers in larval and adult urodeles are divided into at least two fiber types. A red type forms a thin, one to two fibers thick, superficial layer just beneath the skin.  
    The bulk of the muscle fibers are white fibers deep to the red layer. In larvae
of Hynobius tokyoensis, red muscle fibers are also found intermixed with deep white fibers at the dorsoventral level of the notochord (Fig. 10B). The red fibers are smaller, have a higher
Succinate dehydrogenase activity, and a greater density of Mitochondria than the white fibers. 
    Both non-spiking, polyneuronally innervated, tonic muscle fibers, and twitch fibers are present in the extraocular muscles of urodeles. However, I could find no physiological studies of fiber types in the trunk musculature."   

    "Coghill and Youngstrom identified two classes of motoneurons in the motor column of urodeles. The motoneurons in the first class, the primary motoneurons, were the first to develop.  They were relatively large ..."  
    "Cells in the second class, the secondary motoneurons, developed later. They were smaller, unipolar cells ..."  
    "Much later Blight used Golgi preparations to study the development of motoneurons in Triturus helveticus. He recognized two types of motoneurons,
with some similarities to those identified by Coghill and Youngstrom.
    The first motoneurons to develop were a population of large (25-40 pm diameter) primary motoneurons with lateral dendrites in the ventral three-fourths of the ipsilateral gray and a ventral dendrite in the region of the MLF. This ventral dendrite gave rise to the axon which passed medial to the Mauthner axon and ran caudally before it exited in a ventral root."  
    "The second type of motoneuron was a smaller, unipolar cell (15-20 pm in diameter) located slightly ventral to the primary motoneurons."  
    "Similar populations of motoneurons were also identified using HRP in the paedomorphic, permanently larval mudpuppies (Necturus maculosus), although
their development was not examined. A large primary-like group with extensive dendritic arbors occupied the lateral tip of the ventral horn (Fig. 10A). Smaller, unipolar, secondary-like motoneurons were medial to these, and a few morphologically simple, possibly newly born motoneurons were located subadjacent to the ependymal layer."  
    "Blight suggested that the primary motoneurons might generate the early, massive, high frequency bending movements of the embryo, while the secondary
ones produce low frequency movements and provide tonic support of the body.
    The evidence for this interpretation was: the correlation of the sequence of development of the different motor behaviors with the sequence of differentiation of the motoneurons - high frequency movements and primary motoneurons first, low frequency movements and secondary motoneurons later; ...  and the observation that some of the secondary motoneurons project to regions of the myomeres occupied by red muscle fibers in other urodeles. This evidence, in combination with earlier circumstantial evidence for a functional difference between primary and secondary motoneurons is suggestive, but functional studies are needed to provide direct examinations of these hypotheses."  

My comments
1.  Although the author does not draw the conclusion, it looks to me as though the primary neurons are white, fast twitch and the secondary neurons are red, slow twitch. 
2.  There is no discussion of the origin of either the primary or the secondary neurons.     
3.  It is my memory that Brain of the Tiger Salamander did not find any evidence of a red nucleus, which Shark Muscles identified as the source of the red, slow twitch neurons.  However, when I went back and looked, I couldn't find what I remembered as Herrick's statement that he had looked for it and failed to find it.