How do our bodies sense pain?

Pain is currently being redefined as by the International Association for the Study of Pain (IASP) (https://www.iasp-pain.org/PublicationsNews/NewsDetail.aspx?ItemNumber=9218)

The Proposed New Definition of Pain:

An aversive sensory and emotional experience typically caused by, or resembling that caused by, actual or potential tissue injury.

Proposed Accompanying Notes Section:

• Pain is always a subjective experience that is influenced to varying degrees by biological, psychological, and social factors.
• Pain and nociception are different phenomena: the experience of pain cannot be reduced to activity in sensory pathways.
• Through their life experiences, individuals learn the concept of pain and its applications.
• A person’s report of an experience as pain should be accepted as such and respected.
• Although pain usually serves an adaptive role, it may have adverse effects on function and social and psychological well-being.
• Verbal description is only one of several behaviours to express pain; inability to communicate does not negate the possibility that a human or a non-human animal experiences pain.

Pain has been classified into three categories —nociceptive pain, neuropathic pain and inflammatory pain. Neuropathic pain is recognised as nerve injury or impairment and pain can occur episodically or continuously. Inflammatory pain can be acute or chronic and occurs when the presence of inflammatory chemicals stimulate the nociceptors. It is widely understood that there are receptors in our skin, muscles, organs and joints and bones attached to nerves called nociceptors. These are activated by extreme temperature (hot or cold), irritating chemicals, and squeezing/deep pressure (what we normally think of as painful stimuli). When activated above their thresholds a signal is sent along their afferent sensory nerve fibres (C-fibres and A-delta fibres) from the receptor to the spinal cord and then the brain, thus involving both the peripheral (neves which connect the parts of the body to the spinal cord) and central nervous system (brain and spinal cord).

When the signal reaches the level of the spinal cord an automatic reflex arc occurs and signals are sent from the spinal cord through efferent nerves to the muscles to produce a reaction. This is without our conscious awareness and it happens very quickly. For example pulling hand away from a hot surface before realising you were touching it.

However nociceptor activity does not per se lead to the perception of pain https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6121522/ This is largely because this basic circuit of the reflex arc is a very small part of the picture. Within the spinal cord are ascending pathways up to the brain. Once the signal reaches the brain there are many centres which are stimulated and the sensory information is integrated, interpreted and logged to memory. The brain sends responses back down to the spinal cord. At the level of the spinal cord these descending pathways can modulate the ongoing sensory information being sent back up to the brain. Sometimes this can be inhibited, and sometimes is can be sensitised or increased.

Therefore the brain plays a massive role in the way that pain is processed in our bodies and how we experience pain. This can be very helpful in some situations, like learning to not touch something hot after the first time you experience being burnt!

However there can be times when this central modulation of pain is not so helpful. Pain can also be experienced from a normally non-painful stimulus such as light touch. This is known as allodynia. Sometimes after an injury and subsequent period of inflammation (which involves inflammatory chemicals stimulating nociceptors), a sensitisation process occurs where there is a reduction in the threshold for a nociceptor to be stimulated. This means an area of the body may start to receive the ‘wrong’ information, such as light touch being interpreted as painful, as in the condition chronic regional pain syndrome. You may have heard of conditions such as phantom limb pain. This is where an amputee experiences pain from a limb which is no longer present. In the case of chronic pain there is a continuation of the experience of pain far beyond the normal time for tissue damage to have healed and so this form of pain is not helpful to our function.

So it is clear that the new, proposed definition of pain is helpful for our understanding of our experience of pain, and therefore the clinical management of it, by showing us that the brain, our emotions, experiences, backgrounds and many other factors affect our experience of pain.