Role for medullary pain facilitating neurons in secondary thermal hyperalgesia.

The rostral ventromedial medulla (RVM) has recently received considerable attention in efforts to understand mechanisms of hyperalgesia and persistent pain states. Three classes of neurons can be identified in the RVM based on responses associated with nocifensive reflexes: on cells, off cells, and neutral cells. There is now direct evidence that on cells exert a net facilitating effect on spinal nociception and that off cells depress nociception.

Nociceptive facilitating neurons in the rostral ventromedial medulla.

The role of the periaqueductal gray-rostral ventromedial medulla (RVM) system in descending inhibition of nociception has been studied for over 30 years. The neural basis for this antinociceptive action is reasonably well understood, with strong evidence that activation of a class of RVM neurons termed 'off-cells' exerts a net inhibitory effect on nociception. However, it has recently become clear that this system can facilitate, as well as inhibit pain.