Unmyelinated fibers in the trigeminal motor root. Possible relationship to the results of trigeminal rhizotomy.

Spinal ventral roots have been shown by electron microscopy to contain a substantial number of unmyelinated fibers. Physiological studies have confirmed that many of these subserve a sensory function. In this investigation the fiber spectrum of the human trigeminal motor root was studied by electron microscopy and it was determined that up to 20% of fibers were unmyelinated. This accounted for approximately 300 to 1000 fibers per root. Since visceral efferent fibers are thought not to occur in the trigeminal root, it appears likely that these unmyelinated fibers may be afferent in nature. It is believed that the presence of a substantial number of unmyelinated fibers in the trigeminal motor root may account for residual sensation or failure of pain relief after rhizotomy of the trigeminal portio major root.