Comparative study of viscerosomatic input onto postsynaptic dorsal column and spinothalamic tract neurons in the primate.

The purpose of the present investigation was to examine, in the primate, the role of the postsynaptic dorsal column (PSDC) system and that of the spinothalamic tract (STT) in viscerosensory processing by comparing the responses of neurons in these pathways to colorectal distension (CRD). Experiments were done on four anesthetized male monkeys (Macaca fascicularis). Extracellular recordings were made from a total of 100 neurons randomly located in the L(6)-S(1) segments of the spinal cord. Most of these neurons had cutaneous receptive fields in the perineal area, on the hind limbs or on the rump. Forty-eight percent were PSDC neurons activated antidromically from the upper cervical dorsal column or the nucleus gracilis, 17% were STT neurons activated antidromically from the thalamus, and 35% were unidentified. Twenty-one PSDC neurons, located mostly near the central canal, were excited by CRD and three were inhibited. Twenty-four PSDC neurons, mostly located in the nucleus proprius, did not respond to CRD. Of the 17 STT neurons, 7 neurons were excited by CRD, 4 neurons were inhibited, and 6 neurons did not respond to CRD. Of the unidentified neurons, 23 were excited by CRD, 7 were inhibited, and 5 did not respond. The average responses of STT and PSDC neurons excited by CRD were comparable in magnitude and duration. These results suggest that the major role of the PSDC pathway in viscerosensory processing may be due to a quantitative rather than a qualitative neuronal dominance over the STT.