Shark Muscles

Cross references:    Sharks & Rays    Motor Neuron Evolution   
Myomere    Muscle Innervation     Motor Nerve Organization         
   
Amphioxus Motor Nerves     Amphioxus Muscles
   
Lamprey Muscles   Teleost Muscles   Amphibian Muscles  
Mammalian Muscles
   
Human Muscles    Medial Motor Column    
Red Nucleus     Fast vs. Slow Twitch Muscles    

A review of the organization and evolution of motoneurons innervating the axial musculature of vertebrates. (Goog) - 1987   
    Abstract
from the PDF:  See:  Motor Nerve Organization .   
    "Most Elasmobranchs that rely on axial muscles for locomotion have a myomeric arrangement of musculature."  
    "The
Myomeres in Elasmobranchs, as in most fishes, are complexly folded. They form a W shape in a lateral view, with the top of the W facing rostrally.  Those muscle fibers in the superficial portion of the myomeres are oriented parallel to the long axis of the body, while the deeper fibers are oriented at an angle between 10“ and 30” relative to the body axis."  
    "The bulk of the myomeres is formed by the two most abundant fiber types that are also clearly functionally different.
    A red muscle fiber type occupies the superficial portion of the myomeres, forming a band along the lateral surface just beneath the skin, as in bony fish. In the tail, the band is thickest near the lateral line, and thins dorsally and ventrally. Red fibers are relatively small and specialized for aerobic metabolism. They contain a large amount of fat and myoglobin, many mitochondria, and a high concentration of oxidative enzymes, and are well vascularized.  Red muscle forms about 18% of the myomeres from the tail of Scyliorhinus
    A white fiber type occupies the deeper portion of the myomeres, and forms roughly 80% of the myomeres in the tail. The white fibers are larger in cross-section than the red ones. They have relatively little fat or myoglobin, fewer mitochondria, and are not well vascularized."  
    "... the red muscle is active during sustained swimming, while the white is active only during ‘vigorous’ movements.  ... This involvement of the red muscle in sustained locomotion is consistent with its anatomical and histochemical specializations for aerobic metabolism."  
   "The red fibers are innervated at nerve terminals spread over the surface of the muscle fibers from myoseptum to myoseptum. These terminals usually arise from more than two axons and are spaced 150-200pm apart in Scyliorhinus. The white fibers are innervated by axons terminating on only one end of the fiber."  
    "Roughly 30% of the axons in the ventral root travel in branches of the spinal nerve that enter red muscle. The largest axons in the ventral root (which presumably arise from the larger motoneurons that innervate white muscle, see below) are grouped together as in most anamniotic vertebrates."  
    "The cell bodies of the motoneurons are located in the ventral portion of the ventral horn. They have dendritic processes that are oriented in the transverse plane and extend throughout much of the ipsilateral white matter. Some dendrites also cross ventrally to the contralateral ventral horn.
    The   motoneurons form a continuous distribution with respect to axonal diameters,  conduction velocities, and the areas of the cell bodies. Horseradish peroxidase labeling of the axial musculature in Scyliorhinus, indicates that there is a rough segregation of motoneurons innervating red and white muscle; the red muscle is innervated by small motoneurons in the lateral portion of the motor column, while the white muscle is innervated by larger motoneurons in its medial part."  
My comment:   
    This last statement is very important.  To repeat, for emphasis: 
"the red muscle is innervated by small motoneurons in the lateral portion of the motor column, while the white muscle is innervated by larger motoneurons in its medial part."  
    So, the red lower motor neurons are lateral and the white lower motor neurons are medial. 

However, there's a possible problem here.  The Wikipedia page on the spinal cord quoted in  

Medial Motor Column

      says: 
"
The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. These are the rubrospinal tract, the vestibulospinal tract, the tectospinal tract and the reticulospinal tract. The rubrospinal tract descends with the lateral corticospinal tract, and the remaining three descend with the anterior corticospinal tract."  

The "Review of the Origin ..." PDF says that "the red muscle is innervated by small motoneurons in the lateral portion of the motor column, while the white muscle is innervated by larger motoneurons in its medial part."  Wikipedia says "The rubrospinal tract descends with the lateral corticospinal tract, and the remaining three descend with the anterior corticospinal tract."  This implies that the red lower motor neurons originate in the Red Nucleus .  Although surprising, it makes real sense. 

The "Review of the Origin ..." PDF says that "the white lower motor neurons are medial".  Wikipedia says "The vestibulospinal tract, the tectospinal tract and the reticulospinal tract ...  descend with the ... anterior corticospinal tract."  These two statements are non-contradictory only if "medial" as used in the Review is the same as "anterior corticospinal" as used in Wikipedia.  

Accepting that "medial" and "anteriorcorticospinal" are equivalent, this ends my search for the separate origins of the slow and fast twitch motor neurons.  The slow twitch originate in the red nucleus and the fast twitch originate in the vestibulospinal, tectospinal and reticulospinal tracts. 

My comments

1.  Most importantly, it's my memory that Herrick's  Brain of the Tiger Salamander  says that he didn't find any grouping of cells which might be equivalent to the red nucleus. 

2.  I should remember that this conclusion depends on a cross reference between two very different species: sharks and humans.  This does not invalidate the conclusion, but it does suggest that I might look at other species. 
    






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