We’ve come a long way in our understanding of the mechanisms of pain and how it affects us.
The International Association for the Study of Pain (IASP) definition of pain is “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” Note the last part – actual or potential tissue damage, or described in terms of such damage. What that suggests is that tissue damage is technically not required for someone to feel pain. Confused? Read on…
A disclaimer: this is a HUGE topic, so I’ll do my best to condense things in a page or three without getting too technical.
Early Pain Models
The earliest & simplest model of how pain works came from the writings of Rene Descartes (he of the “I think, therefore I am” persuasion), the 17th Century French philosopher. Descartes reasoned on the separation of mind and body – an example of Cartesian Dualism, which can be summed up by the suggestion that the degree of pain experienced was directly proportional to the degree of tissue damage that caused it. As a model it was simple & clean, but there are a lot of ‘what abouts?’ that it could not adequately explain.
In the early 1960’s Melzack & Wall published their ‘Gate Control Theory of Pain’, which, among other things, postulated the existence (subsequently proved to be true) of an ‘inhibitory interneuron’ – a nerve ending within the spinal cord that, when triggered, could ‘block’ nerve pathways carrying pain information back from the body to the brain – closing the gate, if you will. This is the principal upon which TENS, those little nerve stimulators, were designed to work. They triggered the closure of the gate, thus providing pain relief. In fact, anything that could close the gate could provide pain relief, and within this concept were the seeds of our current model of understanding pain behavior: the Biopsychosocial model.
Some basic concepts within the Biopsychosocial Model
The Homunculus – a fancy map
The homunculus is a proportionate representation of the amount of ‘brain space’ given to sensation from different parts of the body, a 3-D map, if you will. There are a number of homunculi in the brain, but we’ll focus on just the sensory one, which receives nervous impulses coming back from the body.
And it looks pretty funny too.
Pain is an Output
There’s something else in play, however. Today we know pain as an output, the result of a complicated interplay of a number of systems, of which nerve traffic from your body is just one.
Things like emotions, context & memory, motivation (ever see a little kid trip over, then look around to see who’s watching before deciding whether to cry or not?), hormonal factors (adrenaline is a fantastic inhibitor of pain), fear, mood, anxiety & depression can all influence the amount of pain you feel.
Homuncular man’s proportions are determined by the relative sensitivity of each body part, as determined by the density of nerve endings in each part. Not surprisingly, tongue, lips, hands & face are all very sensitive areas, and accordingly have a greater number of nerve endings and a larger amount of cortex dedicated to monitoring their status.
We also now consider pain as a measure of how much the body feels it is under threat. The brain weighs up all these incoming factors, and then decides if it feels threatened or not, then lets you know about it. More pain = greater threat.
If your brain concludes, rightly or wrongly, that there’s nothing to worry about (regardless of the actual state of the tissues), then you won’t feel pain.
This is how homuncular man is laid out in cross section in the sensory cortex of the brain. You can see that areas containing just the hand, thumb & fingers is almost larger than the whole trunk, legs & feet!
A real example: I moved house last weekend, so there are boxes everywhere, and I banged my shin against the edge of the bed. After the obligatory curse, the pain I felt prompted me, almost unconsciously, to look down at it and give it a rub – a brief examination to rule out anything serious. Once my brain was satisfied that I hadn’t done anything serious, and I remembered having done this sort of thing before and it being OK, the pain subsided: there wasn’t any use for it anymore. Next morning my shin sported a bruise, which signified tissue trauma, but again no pain. Since I’ve already satisfied myself that there’s nothing to worry about, my memory of previous experiences guides my expectations that it should settle in a timely fashion.
Alternatively, if I cut myself and not look after it well and it becomes infected, the ongoing pain and throbbing sensation I feel would prompt me to take another look. Since it hadn’t responded in an expected fashion (ie. healing), and seeing that it was red & swollen, the pain might stimulate me to change my behavior & encourage me to do something more appropriate, like use Neosporin or see a doctor.
This more contemporary model can also neatly account for previously harder to explain phenomena like phantom limb pain – the pain someone may continue to experience in an amputated limb after it’s gone. The limb still exists within the sensory homunculus, so anything that stimulates that part of the brain will cause the sufferer to experience pain in that region, even if that region is no longer physically there.
The good news for people in pain is that research demonstrates that merely understanding the mechanisms by which pain is generated by the body can be therapeutic in managing said pain. It can be very reassuring to learn that fluctuations in pain can occur independent of tissue pathology, and this knowledge can allow your pain to be less of a limitation than first thought. Probably the most important thing to do is talk to your physical therapist or medical specialist about pain so that you can understand its causes and influences.
Watch this space for future posts about further aspects about pain – what it is, how it manifests, and what can be done about it when it doesn’t go away.