Pain 2. A simple explanation of acute pain.

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The International association for the study of pain defines pain like this:

An unpleasant sensory and emotional experience caused by actual or potential tissue damage or described in terms of such damage.

So a wasp sting hurts.  But where we were stung and the implications of the sting play a major part of how much the sting hurts.   For a model with a photo shoot later on, a wasp sting on the nose could be a disaster and hurt more than if he or she was about to spend a few days with the family – or if he/she’d been stung somewhere that would be hidden.

Straightaway we can see the complex nature of pain.  In this blog, I am going to concentrate on some of the nuts and bolts of acute pain- the stuff that happens in the immediate aftermath of a wasp sting or if a hippopotamus stands on our toes, rather than delve into the emotional side or look at long-term pain.

The body has several systems that alert us to danger; the eyes, ears and nose are primary, along with various sensors found all over the body that alert us to changes of pressure, temperature, movement or chemicals.  Otherwise called baroreceptors for pressure – the pressure of the hippopotamus’s foot on ours, thermoreceptors for changes in temperature – a hippopotamus breathing down our neck, mechanoreceptors for sensing tension, stretch, cutting  as when the wasps sting breaks our skin, and chemoreceptors to alert us to chemicals entering the body – as in a wasp sting.  These sensors exist in such places as the skin, muscles, joints, bones, nerves, spinal discs, eye, ears and guts.  If a sensation presents a threat to the body, then the sensors that are stimulated are collectively called nociceptors – which literally means to receive injury.  All sensations connect up to the spinal cord via neurones, collectively called the peripheral nervous system.

The various sensors live in the dendrites, seen in the picture to the left and when we get stung by a wasp we immediately feel a sharp pain followed by a burning sensation. This is then followed by shaking or rubbing the affected part. We feel the sharp pain first followed by the burning sensation because the the various signals from the sensors travel at different speeds; the sensation of the wasp sticking in its stinger travels very fast whereas the burning sensation of the injected venom travels more slowly. The brain responding to the injury by making us rub or shake the affected part and so we excite the nice mechanoreceptors that help override the nociceptors – or pain.

And here we have an example of the 2 way process of pain – not only  a body to brain experience, but also a brain to body event.  So we get stung by a wasp, the signals goes from the dendrite and up the axon to its terminal in the spine.  Because the sting hurt, it generates strong and excited signals which burst out of the axon terminal through a space called a synapse to the second neurone, which lives in the spinal cord, and when the excitement reaches fever pitch, the message now says ‘DANGER’ and the message ascends to the brain up a danger messenger neurone, called second order nociceptors.

Descartes, the 17th century philosopher,  thought that there was one place in the brain that received pain. For many years it was called The Descartes Reflex.  Modern imaging techniques have disproved this, showing us that the brain processes pain in a variety of different sections.

Click on the picture to make out the detail.  Clearly a thesis could be written on all this.  Suffice it to say that the arriving danger signals ascend through the lowest and oldest part of the brain, the cerebellum, then on to the stress response centre (thalamus) and then ping about hitting, amongst others, the memory centre (hippocampus), the fear centre (amygdala), and the boss or problem solving centre, the frontal cortex.  Which means the brain compares the sting to memories, it works out how much to evoke the stress response, it might add a fear response, it weighs up the importance of this event with everything else  going on both now and in the future and so on. And so it comes clear as to why a wasp sting on the nose causes more pain in some situations than in others.  If a model facing a photo shoot in a couple of hours, the various parts of the brain respond with great alarm.  If we are with people we love, then the sting still hurts but causes less alarm.  Add into this how we ourselves deal with pain, and the picture of how painful a wasp sting to the nose becomes individual in both person and circumstance.

So the brain decides how much danger we are in and from what – and acts accordingly.  It sends signals down to the body – rub the nose to help reduce the pain and it can also send very powerful painkillers; if necessary, we can run for our life on a broken foot if the brain considers escaping from the hippopotamus more important than nursing the foot.

OK, a rather simplified explanation of the acute pain response.  Messages from the body alerting the brain to danger and the brain assessing how much danger we are in and from what sources and a pile of other emotional stuff – and all this happens in a split second.  Ongoing pain, called chronic pain, causes the brain to change how it responds to pain and a future blog will look at that.

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