Figure 1.

Noxious stimuli activate nociceptors located on primary afferents, which then relay nocispecific information to the dorsal horn of the spinal cord via the dorsal root ganglia (DRG). From here, nociceptive-specific information is relayed supraspinally to the PAG, thalamus and higher cortical regions. Both endogenous and exogenous opioids activate the PAG and its descending projections to the RVM and spinal cord to ultimately inhibit incoming pain signals (adapted from Guo et al., 2006).