Proportion and location of spinal neurons receiving ventral root afferent inputs in the cat

Spinal neurons receiving ventral root afferent inputs were investigated in anesthetized and paralyzed cats. We were concerned with the afferent fibers in the ventral root that travel distally and then enter the spinal cord through the dorsal root. The questions to be answered included the proportion and distribution of spinal neurons receiving ventral root afferent inputs and their peripheral input characteristics. The L7 ventral root was cut near the spinal cord and the distal stump was stimulated while making a systematic search for neurons in the entire gray matter of the ipsilateral spinal cord that responded to the stimulation. The following conclusions were made: (i) the afferent fibers in the cat ventral root enter the spinal cord through the dorsal root and evoke a variety of responses (excitation, inhibition, or mixed) in a large proportion of spinal neurons (about 20%); (ii) these responses seem to be mediated largely by spinal mechanisms; (iii) spinal neurons receiving ventral root afferent inputs are situated in a wide region of the ventral spinal cord; (iv) ventral root fibers in a single root enter the spinal cord and exert their responses over a large region of the spinal cord (at least two spinal segments rostrally and caudally); (v) some of the spinal neurons that responded to ventral root stimulation were found to be ascending tract cells, suggesting that ventral root afferent inputs can reach supraspinal structures; (vi) ventral root afferent fibers converge onto spinal neurons that have a variety of peripheral receptive field characteristics; and (vii) with some exceptions, most neurons receiving ventral root inputs were excited best by mechanical and/or thermal noxious stimuli applied to the periphery.