Medial Motor Column

Searching Google for "medial motor column" yielded 941,000 claimed hits. 

A review of the organization and evolution of motoneurons innervating the axial musculature of vertebrates. (Goog) - 1987   
    Abstract
    "The medial motor column is an especially appropriate place to examine the evolution of motor systems, both because of the importance of axial musculature as the ancestral musculature among vertebrates, and because there is as much, if not more, diversity in the arrangements of axial muscles among vertebrates than in any other motor system."  
    See: 
Motor Nerve Organization


Spinal cord (Wiki)   
    "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. 


http://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/Spinal_cord_tracts_-_English.svg/2000px-Spinal_cord_tracts_-_English.svg.png
   
The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge and ... synapse on lower ventromedial (VM) motor neurons in the ventral horn ipsilaterally... . The tectospinal, vestibulospinal and reticulospinal descend ipsilaterally in the anterior column but do not synapse across the anterior white commissure. Rather, they only synapse on VM lower motor neurons ipsilaterally. The VM lower motor neurons control the large, postural muscles of the axial skeleton."  
My comment
    Wikipedia does not have a separate page for "ventromedial lower motor neurons", so I looked at each of the tracts separately.   They only say that the tracts innervate the "lower motor neurons".  So I looked at: 

    Lower motor neuron (Wiki)       

    http://en.wikipedia.org/wiki/Lower_motor_neuron       
It did not make any distinction between fast and slow twitch. 

Searching Google for "lower motor neuron" yielded 2,150,000 claimed hits, but I didn't find anything useful in the first four pages.   

Narrowing the Google search to "fast twitch lower motor neuron" reduced the number of claimed hits to 54,600.    


Principles of Neural Science - Kandel Ch 34.pdf   
PDF available online for free.  Good diagrams. 
     "The brain implements a motor program for limb movement by sending signals to the spinal cord. Some of the signals are transmitted directly to motor neurons, but most are relayed through a variety of interneurons. Most spinal interneurons also receive convergent input from many somatosensory modalities. In turn, they project directly and indirectly to many different motor nuclei.  
    A single motor neuron is thus bombarded with synaptic inputs, and the net result determines whether it reaches threshold and hence whether the muscle fibers it controls will participate in a motor program."  
    Figure 34-11 shows an interneuron simultaneously providing input to both a small, presumably slow, and a large, presumably fast, motor neurons. 



Figure 34-11 Two motor neurons of different sizes have the same resting membrane potential (Em starts at the resting level in both plots) and receive the same excitatory synaptic current (Isyn) from a spinal interneuron.  Because the small motor neuron has a small surface area, it has fewer parallel ion channels and therefore a higher overall resistance (Rhigh). According to Ohm's Law (E = IR) the synaptic current in the small neuron produces a large excitatory postsynaptic potential (EPSP) that reaches threshold, resulting in an action potential. The small motor neuron also has a small-diameter axon that conducts the action potential relatively slowly to the few small muscle fibers that comprise its muscle unit. In contrast, the large motor neuron has a larger surface area, resulting in a lower overall transmembrane resistance (Rlow) and a smaller, subthreshold EPSP in response to Isyn. As a result, its many muscle fibers are not recruited

My comment
    Figure 34-11, above, does not actually refute the possibility that neurons innervating human fast and slow muscle fibers have different origins.  What it does show is that the fast and slow motor units can both receive input from the same interneuron.  This does not necessarily imply that the motor neurons innervating fast (large) and slow (small) muscle fibers have the same origin.           
       


Searching PubMed for "medial motor column" found 278 references:  
http://www.ncbi.nlm.nih.gov/pubmed/?term=medial+motor+column     
 









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