Similarly, afferent fibres from the spinothalamic tract synapse at the periaqueductal grey matter.

Bilaterally symmetric defects are seen in the periaqueductal grey matter, brain stem, basal ganglia, and dentate nucleus.

These neurons are located in the locus coeruleus, dorsal and median raphe nuclei, ventral periaqueductal grey matter, and tuberomammillary nucleus.

The central analgesia system is mediated by 3 major components: the periaqueductal grey matter, the nucleus raphes magnus and the nociception inhibitory neurons within the dorsal horns of the spinal cord, which act to inhibit nociception-transmitting neurons also located in the spinal dorsal horn.