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Folia Morphol (Warsz). 2018 Oct 29. doi: 10.5603/FM.a2018.0100. [Epub ahead of print]

Morphological study of myelinated and unmyelinated fibers in the sacrococcygeal dorsal roots of the rat.

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Department of Life Science, National Taiwan University, No. 1, Section 4, Roosevelt Rd., 10607 Taipei, Taiwan, Province of China.


The number and caliber of myelinated and unmyelinated fibers of the sacrococcygeal dorsal roots innervating the tail of rats were studied by means of light and electron microscopy. There was an estimated total of 12,500 myelinated and 25,500 unmyelinated dorsal root fibers innervating the tail of a rat. The results showed that from the second sacral (S2) to the fourth sacral (S4) segment, the fiber diameter spectrum of myelinated fibers within each dorsal root was bimodal with two peaks at 5 microns and 10 microns, respectively. The first sacral (S1) segment was composed of numerous smaller-size myelinated fibers, thus forming a right-skewed distribution. The coccygeal (Co) segments showed a unimodal distribution peaking at 10 microns for the first (Co1) segment and gradually shifting to 7 microns for the third (Co3) segment. Overall, there was a continuous relative increase of the larger versus the smaller myelinated fibers from the sacral to coccygeal segments. The fiber diameter of unmyelinated fibers of all these roots was unimodal with a single peak at 0.5 microns. The ratio of unmyelinated to myelinated fiber numbers was on average 2.83 for the S1-S2 roots, 1.66 for the S3-S4 roots, and 1.24 for the coccygeal roots. The comparison of the left- and right-side nerve fibers show that there was no significant difference, thus implying a symmetrical sensory innervation of the rat's tail.


axon diameter; dorsal root; frequency distribution; tail

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