Pain systems need to be sensitive enough to detect potentially harmful stimuli. But often they become too sensitive, causing us pain that provides no benefit. This hypersensitivity arises because our pain pathways actually increase in sensitivity when they relay pain messages, and the mechanisms of this sensitization are beginning to be revealed.
Normally, pain is produced only by intense stimuli that are potentially or actually damaging to tissue (technically known as noxious stimuli, although commonly referred to as pain stimuli). This pain is mediated by a specific system of high-threshold peripheral and central neurons designed to respond only to such noxious stimuli (the nociceptive system, also see Sensing damage), which is responsible for the 'ouch' pain we experience in response to a needle prick or on touching a hot surface.
Nociceptive pain is an essential early warning device that helps protect us from the dangerous environment we find ourselves in. To do this the sensation of pain needs to be so unpleasant that we cannot ignore it.
Clinical pain, by contrast, occurs in response to tissue injury and inflammation (inflammatory pain), damage to the nervous system (neuropathic pain) and alterations in the normal function of the nervous system (functional pain). It features both spontaneous pain that arises without any apparent peripheral stimulus and hypersensitivity to peripheral stimuli.
Pain hypersensitivity takes two forms:
thresholds are lowered so that stimuli that would normally not produce pain now begin to (allodynia).
responsiveness is increased, so that noxious stimuli produce an exaggerated and prolonged pain (hyperalgesia).
Pain hypersensitivity after an injury helps healing by ensuring that contact with the injured tissue is minimized until repair is complete – an adaptive response. However, pain hypersensitivity may persist long after an injury has healed or occur in the absence of any injury. In this case, pain provides us with no benefits, and is a manifestation of pathological change in the nervous system.
One way of reversing this is to use electroacupuncture which by sending small currents across the overstimulated area helps to restore the normal pain response.