Unmyelinated primary afferent fibers in dorsal funiculi of cat sacral spinal cord.

The present study tests the hypothesis that there are numerous unmyelinated primary afferent fibers in cat posterior funiculi. The animals have unilateral dorsal rhizotomies from L6 to Ca3. One week later the axons of both S2 dorsal funiculi are counted. The data indicate that there are approximately 22,500 myelinated and 8,500 unmyelinated axons on the unoperated side and 11,000 myelinated and 3,900 unmyelinated axons on the operated side. On this basis, we suggest that 51% of the myelinated and 54% of the unmyelinated axons in cat dorsal funiculi arise from dorsal root ganglion cells and thus are primary afferent axons. If this is correct, then 71% of the primary afferent axons in the cat dorsal funiculus are myelinated and 29% are unmyelinated. The function of this large group of previously unsuspected fine sensory axons remains to be determined.