Eli5: What causes pain to come in waves. Why can an injury be fine for 5 seconds then painful for 5 seconds repeat?

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For example, you can hurt your arm and leave it perfectly still, yet pain comes and goes in consistant waves. What is happening here physiologically?

In: Biology

It’s hard to answer this without knowing the exact injury or nociceptive stimulus causing the feeling of pain. For an acute injury, intermittent pain may be caused by a number of factors like desensitization of the afferent (going to the brain) neuron carrying the pain signal that gets reset after a refractory period, habituation at the cortical level (brain shutting off perception of pain due to its sustained yet not life threatening nature) that gets reversed rhythmically to keep you aware that there is injury, desensitization and habituation that get reversed due to a gradual increment in the instigating stimulus (perhaps the injury is causing inflammation that keeps getting worse, and so even as you desensitized a neuron, others may be recruited or even if your brain habituated, the increase in signal reactivated the focus), or even just your conscious brain shifting focus onto other innocuous stimuli and occasionally shifting back to the pain (like when you get transiently distracted from the pain), etc. I’m sure a more specialized scientist (a pain neuroscientist) can give you a better answer, but as a physiologist this is the best I can do without spending a couple of hours searching the literature.

It’s been a minute since my physiology class, but I remember this one.

So you have several different types of nerve endings dedicated to certain signals, one of which is specifically for detecting damage to the body and notifying us of this through pain messages. Call these pain receptors. Depending on the type of injury, pain that comes in waves typically means you’re repeatedly taking the same form of damage after the pain from the first instance should have subsided. In the instance of something minor like a bruise, the afflicted area becomes sensitive to additional stimuli, even something as minor as the pressure caused by the contractions of your blood vessels in some cases, and repeatedly activates your pain receptors in response to that stimuli.

welcome to reason #747384 why the human body is a terribly designed mess