Myelin evolved as a way to speed conduction along axons, and is present in most long-distance projection neurons of the CNS. Corticospinal projections may be more heavily myelinated because they must traverse long distances to reach their spinal cord targets. These distances are much greater for lower versus upper body projections, and larger axons and heavier myelination may help offset the additional time delay.
http://www.jneurosci.org/content/31/32/11597.full?sid=801aa77f-a396-4cf3-a1cc-262ee432e07b
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