A study of axonal diameters and areas in lumbosacral roots and nerves in the rat

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Abstract
There has been debate as to whether there is a size difference between central and peripheral processes of dorsal root ganglion cells. In the present study, the mean areas of myelinated and unmyelinated fibers are measured as 27.8 μm2 and 0.55 μm2, respectively, in peripheral nerves and 13.72 μm2 and 0.14 μm2 in dorsal roots. Thus myelinated central processes of dorsal root ganglion cells have mean areas 50% less than the mean areas of the myelinated sensory axons in the same peripheral nerves, and the mean diameters of the central myelinated axons are 30% less than the peripheral myelinated axons. The mean areas of the unmyelinated sensory axons in the dorsal roots are 25% of the mean areas of the unmyelinated sensory axons in the same peripheral nerves. Thus the mean diameters of the central unmyelinated axons are 50% of the mean diameters of the unmyelinated sensory axons in the same peripheral nerve. These data indicate that both myelinated and unmyelinated central processes of dorsal root ganglion cells are smaller than the peripheral processes of these same cells for lumbosacral segments in the rat. It is shown that axonal tapering is not responsible for these striking differences. Finally, documentation of differences in myelinated fiber histograms from dorsal roots of different segments in the rat is provided.