Nociception is typically the most common explanation for the sensation of pain–small sensory neurons called nociceptors that send electrical signals from peripheral body tissue to the central nervous system and the brain when a noxious stimulus or dangerous event occurs. Although nociception cannot be felt and is not the same thing as the experience of pain, both pain and nociception occur in response to similarly painful or noxious stimuli, so for the sake of simplicity, we will use the model of nociception to help explain the complex, multi-layered understanding of “pain” as a whole.
Nociceptive pain involves the firing of nociceptors in response to disease, stress, injury, or inflammation of tissues in the body. It usually feels like a prickling, sharp, or stabbing sensation, and is felt only around the area of injury.
Another type of chronic pain is related to dysregulated nerve signaling: the areas of the brain that have been previously turned on by nociceptive pain signals remain activated, despite the fact that no more nociceptive signals are being sent. Even when there are no more noxious stimuli or harmful events in the body, the brain areas associated with pain are left running, creating a false danger alarm. This type of chronic pain often feels like a heavy or burning pain, is referred to as neuropathic pain, and involves damage to the nervous system, as opposed to damage to body tissue.
Opioids are XAMPs (Xenobiotic-assisted molecular patterns or unnatural molecules that are not naturally found in our bodies), which means that they take two simultaneous, opposing actions. They act on opioid receptors to produce anti-nociceptive effects (calming the Nervous Nanny) and they also produce inflammatory effects along “toll-like-receptors” (alarming the Nervous Nanny).
Over time, however, the anti-nociceptive effect goes away but the pro-inflammatory effect remains with continued opioid use. This means that overall, the Nervous Nanny is alarmed, meaning that sensitivity to pain increases. This helps explain why pain can gradually worsen, even with increased doses of opioid drugs like synthetic morphine.