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Physiology of Pain

Pain, a complex experience consisting of a physiological and psychological response to a noxious stimulus. Pain is a warning mechanism that protects an organism by influencing it to withdraw from harmful stimuli; it is primarily associated with injury or the threat of injury.

Despite its subjective nature, most pain is associated with tissue damage and has a physiological basis.

Not all tissues, however, are sensitive to the same type of injury. For example, although skin is sensitive to burning and cutting, the visceral organs can be cut without generating pain. Over distension or chemical irritation of the visceral surface, however, will induce pain. Some tissues do not give rise to pain, no matter how they are stimulated; the liver and the alveoli of the lungs are insensitive to almost every stimulus. Thus, tissues respond only to the specific stimuli that they are likely to encounter and generally are not receptive to all types of damage.

Pain signals can be selectively inhibited in the spinal cord through a descending pathway, which originates in the midbrain and ends in the dorsal horn. That analgesic (pain-relieving) response is controlled by neurochemicals called endorphins, which are opioid peptides such as encephalin that are produced by the body. These substances block the reception of pain stimuli by binding to neural receptors that activate the pain-inhibiting neural pathway. That system can be activated by stress or shock and is probably responsible for the absence of pain associated with a severe injury. It may also explain the differing abilities amongst individuals to perceive pain.                                                                   

Alleviation of Pain

It is essential that attempts to relieve pain typically address both the physiological and the psychological aspects of pain. The reduction of anxiety, for example, may lessen the amount of medication needed to alleviate pain. Acute pain is generally the easiest to control; medication and the rest are often effective treatments. Some pain, however, may defy treatment and persist for years. Such chronic pain can be compounded by hopelessness and anxiety.

Pain is complex, so there are many treatment options – medications, therapies, and mind-body techniques. Learn the benefits and risks of each, including addiction.

There are many different types of pain-relieving medications and each class works in a slightly different way. Most medications can be grouped under onae of the following:

Non-opioids: a medicine that is not similar to morphine (an opioid) but is not addictive (e.g., acetaminophen, aspirin, NSAIDs)

Weak opioids: a medicine that is similar to morphine (an opioid) but not considered as strong (e.g., codeine, tramadol)

Combination opioids: these contain a no-opioid and either a weak opioid or a strong opioid (e.g., acetaminophen and hydrocodone)

Strong opioids: a medicine such as morphine or similar to morphine that has the potential to cause addiction (e.g., fentanyl, morphine, oxycodone)

Other (e.g., ketamine)

Adjuvant treatments: a medicine that can help relieve pain by relieving inflammation or by improving the functioning of other systems (e.g., cannabidiol, capsaicin cream, gabapentin)

Nonpharmacological treatments (drug-free treatments), such as psychotherapy or counselling.

The choice of pain-relieving medication comes down to how effective it is for that type of pain and the likelihood of side effects in that particular person.

  • Once a pain medication is started, it should be monitored for effectiveness and side effects and the dosage or choice of treatment modified if the pain changes or the choice is deemed unsuitable or ineffective.
  • Some types of pain (such as cancer-related pain) have an unpredictable course that can vary dramatically in severity and duration, depending on the type of treatment and disease progression. Pain management needs to have some flexibility to account for this.
  • Some people will have more than one type of pain.
  • Changing the method of delivery of pain medication may improve its effectiveness; for example, changing from an oral treatment to a patch or a subcutaneous pain pump.

Traditionally, most experts have recommended a stepwise approach to pain management, starting with acetaminophen or NSAIDs, then progressing to a weak opioid (such as codeine, dihydrocodeine, or tramadol), before changing to a strong opioid (such as fentanyl, morphine, oxycodone).