Offset analgesia is mediated by activation in the region of the periaqueductal grey and rostral ventromedial medulla.

Interrupting a continuous noxious heat by a greater noxious heat causes rapid and disproportionate pain reduction when the original noxious heat returns. This reduction in pain experience, known as offset analgesia, is believed to be the consequence of active descending inhibitory control of pain originating in the periaqueductal grey (PAG) and rostral ventromedial medulla (RVM). To test this possibility, brain activation was measured using fMRI in twelve healthy controls during an offset procedure. Each subject experienced six second periods of noxious heat followed by an equal period of more intense heat before returning to the original temperature for a further 6 s. Subjects were also scanned during control trials involving continuous, unchanging, noxious heat for 18 s or involving 6 s of noxious heat followed by an equal period of more intense heat before returning to the non-noxious baseline for a further 6 s. Brain activation during the final 6 s of each trial was compared with activation during the first 6 s and this difference was contrasted across trials. PAG/RVM activation was observed during the final 6 s of offset trials but not during either of the control trials and this difference across trials was significant. Activation throughout the pain neuromatrix was inhibited during the final 6 s of the offset trials and was comparable to the activation observed when the heat returned to a non-noxious baseline. These findings provide strong evidence that offset analgesia engages an endogenous inhibitory mechanism originating in the PAG/RVM region, which inhibits pain experience and activation of the pain neuromatrix.