The major ascending (a) and descending (b) pain modulatory systems are illustrated in this schematic representation. Nociceptive inputs enter the CNS at the spinal dorsal horn where primary afferent terminals synapse with second-order projection neurons. The ascending tracts in A are represented in red, and the blue 2-headed arrows indicate bilatateral communications. Descending projections in B are shown in blue, and the 2-headed arrows in dark red indicate bilatateral communications. The light red and blue projections from the RVM to the spinal cord are intended to suggest descending inhibition and facilitation. The regions in the illustration are A6 and A7 noradrenergic nuclei, ACC: anterior cingulate cortex, AMY: amygdala, DRG: dorsal root ganglion, INS: insular cortex, PAG: periaqueductal grey, PB: parabrachial nuclei, RVM: rostroventromedial medulla, SI: primary somatosensory cortex, and SII: secondary somatosensory cortex.
