• The periaqueductal gray matter (PAG), periventral gray matter (PVG), and the nucleus raphe magnus (part of the rostroventral medulla) have high concentrations of opioid receptors, which are known to play an active role in the endogenous descending nociceptive control system.
• These pathways are serotonergic and selectively inhibit the STT, SRT, and SMT ascending pain tracts.
• Electrical stimulation of the PVG increases the response threshold to noxious stimuli.
• Several of the endogenous substances that are part of this control system and exhibit analgesic properties have been identified and include:
  • (beta)-endorphins (from the pituitary and basal hypothalamus)
  • enkephalin, and dynorphin. The latter two pain modulators appear to coexist in some areas of the dorsal horn where there is an abundance of opioid receptors in central terminals of primary afferents.

courtesy of Roxane Pain Institute used with permission http://pain.roxane.com/sitemap.html