The ventral roots L7 and S1 of the cat spinal cord were examined with the light and electron microscopes. Differences in the morphology of Schwann cells associated with large myelinated fibers and with small myelinated or unmyelinated fibers were observed. The blood vessels were largely encircled by pericytes. The most noteworthy finding was that 29% of the axons in these roots were unmyelinated. These unmyelinated axons were greatly reduced in number proximal but not distal to a ventral rhizotomy. Furthermore, they were reduced in number following dorsal root ganglionectomy, but not after dorsal rhizotomy, sympathectomy or peripheral nerve section. It is concluded that the ventral roots of the lumbosacral enlargement contain a large population of unmyelinated fibers originating from dorsal root ganglion cells.
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