Figure 9. Variations of triggering thresholds of endogenous descending controls of nociception .

Conceptual diagram of variations of triggering thresholds of endogenous descending controls of nociception governed by thalamic ‘nociceptive discriminators’ during the conditions of physiological pain (A), ineffective pain treatment (B) and pain receiving efficacy control, i.e. α2 receptor agonist induced pre‐emptive analgesia (C). The triggering thresholds of the thalamic ‘nociceptive discriminators’, in control of descending facilitation and inhibition, are not solid, but exhibit plastic characteristics when exposed to pain and its pharmacological interference. Fentanyl decreases the triggering threshold of descending facilitation, and elevates the triggering threshold of descending inhibition (B). By contrast, Dex reverses the triggering thresholds of descending modulation as a result of a lower threshold of descending inhibition associated with higher threshold of descending facilitation (C). For a detailed interpretation see the Discussion.