As compared to the diameter frequency distribution of myelinated fibers of L3, L4 and L5 ventral spinal roots (VSRs) which have two well developed peaks large-diameter axons (AL) and intermediate-diameter axons (AI) and a poorly developed or non-existent peak of small-diameter axons (AS), D11 VSRs contain small AL and AI peaks and a prominent AS peak. Because the ratio of AL to AI myelinated fibers of D11 VSRs is similar to that in L3, L4 and L5 VSRs and it is known that D11 VSRs contain many pre-ganglionic autonomic fibers it seems likely that AS is composed predominantly of preganglionic autonomic fibers. The mean diameter of the AS peak in D11 VSRs is sim;2.4 mu;m. Average numbers of AS, AI and AL myelinated fibers for D11 VSRs are 3835, 738 and 1545 per root. These values should be useful as controls for morphometric studies of various neuropathies. The average diameter positions of the peak of AL, AI and AS of D11 VSRs as compared to those of L3, L4 and L5 are displaced to smaller diameter categories which may reflect the smaller axon diameters of neurons which are shorter.
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