1. Alarm system: Your nerves working like an alarm system to protect you: Neurons, synapses, action potential and nociception

Almost everyone at one time or other has forgot to put on sunscreen and spent a bit too long in the warm sun only to experience the dreaded sunburn the next day. Remember getting into the shower the next day and turning the water to a nice normal temperature and getting into the shower. What happened? Do you think the water is too hot and you have to turn the temperature of the water down? Why do you think that happened? Did someone sneak into your house overnight and turn up the hot water heater and mess up the temperature valve on your shower to make the water so hot? Probably not. This (burning in shower) is not a new injury. This is increased sensitivity from another injury. This acute sensitivity is a good thing when we have had an injury, to tell us to take it easy for a bit, to allow a gradual return to activity. 

2. Extra-sensitive alarm: The nerves (alarm system) in your back becoming extra sensitive (Peripheral sensitisation, neuropathic pain, central sensitisation and hyperalgesia).

Sometimes this oversensitivity doesn’t reset back like it should. The shower temperature that you had previously for eg, continues to cause a burning sensation months after the sunburn. At this point, it is common to equate pain with injury, and assume the injury is not healing and you are continually causing re-injury. Over time, we know that tissues heal. However you can store memories of that injury and the pain you had as a response. Your internal alarm system can sometimes become more sensitive and go overdrive into protect mode; when you adopt certain positions or do certain movements that were once painful or difficult, you can perceive them as being painful again, even when no tissue damage is occurring. 

3. Nerve sensors: Nerve sensors telling you about movement, stress and cold (Ion channel expression, peripheral sensitisation, neuroplasticity and hyperalgesia)

Pain is meant to motivate us to do something when there is the potential for damage or the possible need of safety. However, pain is not proportional to damage and is quite often out of whack with the problem. When in pain you might have muscle guarding, tentative movements or an increased stress response. Think about your response to a car alarm. Now think about renting an unfamiliar car, but the key fob given to you has words and pictures all worn off. Think about how gingerly you think about pressing it because you don’t want to trigger a car alarm. It brings out the same muscle guarding and stress response. That is normal. But imaging if your body doesn't reset back and you have this tense feeling everytime you are about to press on any keyfob. There can be a small physical (shower temperature) or emotional stressor (key fob) and BOOM! we can have pain, tightness, weakness and other symptoms of a flare up. What is unfortunate is that we can learn to have greater responses (i.e. greater flare ups) to smaller insults or stressors. 

4. Fears: Circumstances that keep your alarm system extra sensitive. Biopsychosocial risk factors, fear avoidance and pain catastrophisation

Supposing you see someone break into your home. You immediately call 911. Police come and take care of the situation. The next time, a few days later, you hear a sound on your window and you call 911 again. This time it was a slightly strong breeze has a small branch break and fall on your window. Note that this time, because of your previous experience, you had a lower threshold for calling 911.

Think of your brain as a police station, it is routinely sending out patrols all over your body - this is part of it's job, to make sure you're safe. Once you are injured, do too much of a particular activity, and for apparently a reason you are not aware of (but your brain is), pain occurs. Think of this pain as that area that hurts calling 911. Not only will the patrol go to the painful area, but several extra squads, maybe firefighters, ambulance etc. These are all the body systems, like immune response, endocrine, etc responding to either a real or perceived emergency. These emergencies lower your pain and movement thresholds. The lower your threshold, the less movement and activity you can perform before another "911" call happens again.

5. Pain Generalization: If the neighborhood area is "broken into" or 911 is called enough times, regular patrols will be continuously sent to the area whether or not there is an actual emergency. This is a cycle that needs to be broken by making the brain feel safe. The safer the area is, eventually the patrols stop coming around so often and your pain and movement thresholds return to their normal levels.

adapted from

https://www.sciencedirect.com/science/article/pii/S2212144719301528

https://contextualscience.org/the_six_core_processes_of_act

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