Pain is a fascinating phenomenon, acting as a physiological warning system that alerts us of potential dangers in our environment. Chronic pain is a leading cause of long-term disability, and certain medications indicated for the treatment of pain have inadvertently led to an opioid crisis of unprecedented proportions. To help us understand these issues, we have to look at what we know about pain and how it works in the human body.
One important point to consider is the fact that any and all pain is technically a central nervous system-generated experience. Pain receptors, part of a vast network of sensory neurons, each respond to a type of pain in a particular area of the body, and they send a simple signal to the brain for processing. From that signal, the brain interprets the type and intensity of the pain. Any disruption along the path and the brain will interpret the signal as pain in the wrong location, or register pain when there is no or little pain.
Categorization of pain
- Nociceptive pain starts with damage or injury to tissues, reported to the brain by the nervous system. Nociceptive pain includes everything from bee stings and toe stubs to nausea, tumors, and inflammatory arthritis.
- Neuropathic pain is a result of damage to the nervous system itself. Most neuropathic pain comes from mechanical injury, like hitting your funny bone. Also included are various central nervous system ailments like multiple sclerosis, alcoholism, chemotherapy, and even phantom limb pain. Neuropathic pain is more likely to lead to chronic pain as nerves don’t heal well.
Nociceptive and neuropathic pain can co-exist. Some injuries or medical conditions affect both tissues and nerves, causing both kinds of pain.
Another way of categorizing pain is somatic and visceral. Somatic pain is experienced in the skin, muscles, bones, and joints. Visceral pain is pain of the organs in the thoracic or abdominal cavities.
Both somatic and visceral pain can be nociceptive or neuropathic. The two classification systems have full overlap.
What about sensitization?
Sensitization is a response that develops after persistent nociceptive stimulation, and results in a heightened response to pain or sensations not normally seen as painful. For example, if you have a long-standing back injury that hurts whenever you stand up, your brain may eventually signal pain on standing even after the injury is healed. Sensitization is a normal part of pain physiology and can be problematic when it’s chronic and seriously disproportionate.
Regardless of type, all pain is under the total control of our brains, and brains have complicated and conflicting priorities that influence how stimuli are interpreted.
If you’re running in fear because your house is on fire, you may not feel a scratch or burn until later. This is normal, even when the system is functioning perfectly; the adrenaline produced overrides the pain. On the other hand, if the nervous system is damaged, as is the case with neuropathic pain, the brain receives flawed information, and tells the person they are experiencing pain. And finally, if the nervous system gets overstimulated (sensitization), it can respond very disproportionally to otherwise normal physical sensations.
Discoveries of gender differences in pain perceptions and responses to treatment are providing new directions for research into pain experience and relief. One example is research into compounds called kappa-opioids, which relieve acute pain in women, but increase pain in men. Interestingly, the same molecules decrease addictive behavior in men, but not in women. One study on this subject can be found here.
Research continues, and new discoveries are sure to be on the horizon.
Altasciences’ experience in pain studies
- Broad range of clinical pharmacology studies on analgesic drugs, narcotic and non-narcotic
- Pain models in healthy participants or patient populations
- A wide range of pain scores using objective measures, questionnaires, or visual analogue scales
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