Activation of spinal neurons by afferent fibers in the ventral roots of rats.

The present investigation was aimed to search for spinal neurons which are activated by afferent fibers in the ventral roots, and once found, to study their receptive field and rostral projection. After dorsal roots L1-S4 on the left side were cut, ventral roots were stimulated to record extracellular responses of the neurons using glass microelectrodes filled with Fast green FCF. Thirty-nine neurons were activated by stimulating ventral roots at 2.5-26.7 x threshold (T) of the lowest threshold fibers of the ventral root. Six neurons were classified as wide dynamic range neurons and 16 high threshold neurons according to their response patterns to the periphery. Seventeen neurons were unresponsive to cutaneous stimulation, suggesting they had innervated visceral organs. Most of the neurons (71%) were located in layer V and VI. In 7 neurons (18%), rostral projection was confirmed by collision block method and ability to respond to high frequency stimuli with constant latency.