Amphioxus Muscles

Cross references:  Amphioxus Motor Nerves  
Myomere    Myoseptum        Amphioxus Locomotion  
Muscle Innervation    Medial Motor Column   
Motor Neuron Evolution     Motor Nerve Organization  
Fast vs. Slow Twitch Muscles     


A review of the organization and evolution of motoneurons
innervating the axial musculature of vertebrates (Goog) - 1986 
    Full length PDF available online for free. 
For other species, see: 
Motor Neuron Evolution  .  

     "The
Cephalochordates (amphioxus) have some features of the motor organization found in most Anamniotic vertebrates.  These include a Myomeric organization of musculature, functionally different superficial and deep fiber types, and comparable populations of motoneurons."
    "The cephalochordates are the most primitive extant group of chordates having a segmental organization of axial musculaturey [9,12]. In Brunchiostoma (Amphioxus) the axial musculature is formed by a series of V-shaped
Myomeres with the apex of the V pointing rostrally. Each of the myomeres consists of a stack of horizontal, sheet-like, muscle fibers which extend from Myoseptum to myoseptum [50]. The muscle fibers are divided into two major types shown in Fig. 1.
    The deep fibers, which form the first type, span the mediolateral extent of the myomere, and contain regularly arranged, densely packed myofibrils. 
    The superficial fibers form the second type.  They extend from the lateral surface of the myomere only a short distance medially, ending about one-fifteenth of the distance in toward the medial edge of the myomere. They have a more irregular array of myofibrils than the deep fibers, contain a larger number of mitochondria, more glycogen, and stain more strongly for succinic dehydrogenaseso.
    Both fiber types are distributed throughout the dorsoventral extent of the myomeres in a ratio of about one superficial fiber to every 3 deep. There may also be a separate, intermediate fiber type, although Flood [50] was not certain that this type was distinct from the other two."



    "Fig. 1. Organization of the musculature and its relationship to the spinal cord in Branchiostoma. 
    The superficial muscle fibers (S) send processes (t) to a dorsal compartment (Do) of the spinal cord while the deep (D) and intermediate fibers (I) send processes (T) to a ventral compartment (Ve).
     M, myoseptum; N, nuclei. Reproduced (ref. 50) with permission of author and Springer Verlag."  
   
    "The ‘ventral roots’ observed by other workers are muscle processes, not nerves. These processes contact a specialized region of the ventral portion of the cord which lacks a glial covering and contains a large number of boutons with vesicles.
    This region is divided into a dorsal compartment containing large boutons with relatively small vesicles (50-60 nm diameter) and a ventral compartment having smaller boutons and relatively large vesicles (100 nm diameter).
    The processes of the two different types of muscle fibers in the
Myomere contact these two compartments in the cord (Fig. 1). The superficial fibers send long thin processes to contact the dorsal compartment, while the deep fibers send relatively large processes to contact the ventral compartment.
    Thus, there is a spatial segregation of the region of contact of the two different muscle fiber types with the cord."  
My comment
    This review does not distinguish the muscle fibers on the basis of color, i.e. red or white.  Rather, it says "The muscle fibers are divided into two major types ...
    The deep fibers, which form the first type, ... contain regularly arranged, densely packed myofibrils. 
    The superficial fibers ... have a more irregular array of myofibrils than the deep fibers, contain a larger number of mitochondria, more glycogen, and stain more strongly for succinic dehydrogenase.
"        
    The difference between the motor neurons innervating the two fiber types is not based on color, i.e. red or white, either.  Amphioxus Motor Nerves  says "The synaptic zones in each segment consist of two distinct domains, the ventral (VC) and dorsal (DC) synaptic compartments.  Both utilize acetylcholine as a transmitter. The ventral synaptic compartments are where the deep, anaerobic, fast muscle cells receive their innervation.  The dorsal compartment is where the superficial, aerobic, slow muscle cells of the myomeres receive their innervation."     







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