Many ventral root afferent fibers in the cat are third branches of dorsal root ganglion cells

Abstract

The arrangement of the ventral root afferent fibers was investigated in anesthetized and paralyzed cats. Single unit activity was recorded from a fascicle of the distal stump of the cut S 1 dorsal root. Activity was elicited by stimulating the distal stump of the cut S 1 ventral root. Attempts were then made to collide this activity with that elicited by stimulation of the S 1 spinal nerve. Single unit activity elicited by ventral root stimulation was recorded from a total of 33 axons. In 17 of these, the activity collided with that elicited by peripheral stimulation. These results indicate that more than half the sampled population of ventral root afferent fibers are branches of dorsal root ganglion cells that have at least 3 processes: one in the dorsal root, one in the ventral root and one in a peripheral nerve. In 10 of these units, the conduction velocity of each of 3 processes was determined using the collision technique. The conduction velocities differed in the processes of a given ganglion cell, with conduction in the ventral root process generally being the slowest. The change in conduction velocity along the length of the ventral root was examined by comparing latency differences for the unit activity elicited by ventral root stimulation at different sites in the same root separated by known distances. The conduction velocity was found not to be uniform along the course of the ventral root. In many cases, the conduction velocity slowed down as the fiber approached the spinal cord. We conclude from the present study that many ventral root afferent fibers are the third branches of dorsal root ganglion cells that have processes in the dorsal root and in a peripheral nerve. The sizes of each of these 3 processes of the dorsal root ganglion cell may differ; the ventral root process tends to be the smallest and is usually umyelinated. Furthermore, many of the ventra root afferent fibers may taper as they approach tthe spinal cord.