It is generally accepted that the analgesic effect of opiates results from actions upon an intrinsic system which originates supraspinally

passes through a descending pathway in the spinal cord DLF

and inhibits pain transmission in the dorsal horn. In the DLF-spared group

analgesia produced by 3mg/kg morphine was intermediate (overall 28%MPE

N= 14) between the high level (overall 69%MPE

N=7)found in the sham-operated group and the lack of effect in the spinalized animals (N=12) (p<0.003). The area of bilateral DLF left varied from 3.1% to 12.3% of the spinal cord. The average analgesia level in individual animals varied from 3% to 91% MPE and was directly and highly correlated to the spared DLF surface (Pearson R=0.84

R<0.0003).We conclude the DLF alone or together with immediately surrounding tracts is necessary and probably sufficient for supraspinal pain inhibition