Lamprey Muscles

Cross references:   Lamprey Fast-Slow Twitch    
Motor Neuron Evolution
Muscle Innervation    
Amphioxus Motor Nerves     Amphioxus Muscles  
Shark Muscles
Teleost Muscles   Amphibian Muscles  
Mammalian Muscles
Medial Motor Column    Human Muscles     

A review of the organization and evolution of motoneurons innervating the axial musculature of vertebrates. (Goog) - 1987   
from the PDF:  
    "The axial musculature of lampreys is myomeric, with each
Myomere formed from a stack of similar subunits (Fig. 3). Each subunit spans the mediolateral extent of the myomere, and contains muscle fibers that run from  Myoseptum to myoseptum. The subunits in Petromyzon marinus consist of 4 central layers of 30-40 large fibers surrounded by one layer of smaller parietal fibers. The individual muscle fibers in a subunit are not each surrounded by a basement membrane as they are in all other vertebrates; instead, the subunit as a whole is surrounded by a basement membrane, which, because of the layered arrangement of the subunit, only contacts the surface of the parietal fibers and the medial side of the most medial central fibers."  

Fig. 3. Organization of the musculature and spinal nerves in lampreys.
    A: cut-away view of the body showing the arrangement of the myomeres (Ml-M7), which are folded so that anterior ones overlap more posterior ones. The spinal cord (S) lies above the notochord (N). Dorsal (D) and ventral (V) roots arise alternately from the spinal cord; the ventral root branches into dorsal and ventral rami.
    B: enlargement of parts of two myomeres showing the structure of the subunits that form them. Each subunit consists of 4 layers of central fibers surrounded, except at the medial and myoseptal surfaces, by a single
layer of parietal fibers. 
    (Ignore the horizontal line in the middle.  The only way I was able to display the image was to paste together two screen shots.) 

    "The motoneurons in lampreys are approximately 25-50 pm in diameter, and are located in the lateral gray matter of the spinal cord."        
    "Unlike most other vertebrates, the dorsal and ventral roots of lampreys do not join after they exit the cord. ... After exiting the cord the ventral root branches into dorsal and ventral rami which provide the innervation of the myomeres."    
    "Motoneurons typically innervate 5-12 myomeric subunits confined to an individual myomere, although occasionally they innervate fibers in 2 or 3 myomeres.  The motoneurons innervating the parietal fibers are more readily activated during spinal reflexes than those innervating the central fibers. The parietal motoneurons are those usually active during fictive swimming in in vitro preparations."  
    "The central fibers are relatively large, they generate spikes following stimulation of peripheral nerves, and they have the ultrastructure and histochemistry typical of fast, white muscle fibers.
    The smaller parietal fibers are non-spiking, and have the ultrastructural and histochemical characteristics of slow, red muscle."  
    "Each of the parietal muscle fibers is multiply innervated by axons that run in the connective tissue between the muscle subunits. Only the most medial of the central fibers are innervated, at their medial edge. The more lateral central fibers are not directly innervated, but are electrically coupled to the medial fibers."      
    "Using simultaneous intracellular recordings from motoneurons and muscle fibers, Teravainen and Rovainen showed that an individual motoneuron innervates only parietal or only central muscle fibers, but not both. The motoneurons innervating parietal muscle fibers are slightly smaller than those innervating central fibers, but are otherwise similar in general morphology, with transversely oriented dendrites ramifying extensively among fiber tracts in the lateral portion of the cord."   
My comment
    This paper makes it clear that the parietal (red, slow twitch) and the central (white, fast twitch) muscle fibers are innervated by separate neuronal systems, but it doesn't describe these systems or trace them back to their origins.  Perhaps part of the problem is that the two kinds of neurons look very much alike.     

Firefox has apparently replaced its Google browser window with Bing, but the transition isn't entirely smooth.  Searching Google-Bing for "lamprey motor neurons" on 09-11-13 yielded 69 claimed hits.   The same search on 09-12-13 yielded 24,900 hits. 

first page:     

Heterogeneity of the Population of Command Neurons in the Lamprey 
Full length with many references, but no mention of fast or slow twitch. 

second page: