“…In the rat, the VLF includes two main somatosensory pathways originating from cells lying in the marginal zone and in deeper parts of the dorsal horn, the spinothal- amic tract, and the spinoreticular tract [Dennis and Melzack, 1977;Willis et al, 1979;Dubner and Bennett, 19831, These systems are largely concerned with nociceptive afferent information, but also include output axons of wide-dynamic-range cells transmitting inocuous information in addition to pain [Dubner and Bennett, 19831. There exist several reports indicating that dorsal horn neurons can be controlled by pathways descending from the brain stem [Oliveras et al, 1974;Fields et al, 1977;Willis, 19821. In addition, convincing evidence indicates that these descending systems play a major role in opiate-induced analgesia [Fields and Anderson, 1979;Willis, 1982;Gebhart et al, 19841.…”