The last post was the first of many on pain and the biopsychosocial model of pain perception. I thought this week would be a good opportunity for us to continue on in this series, and expand on some elements of our previous entry. First let’s review the highlights from last time:
- Pain is a subjective, protective mechanism generated by the brain to motivate a person to take action and survive.
· Pain is in the brain, not the tissues
· Nociception (danger signals) carry information about threats to the brain, from the tissues
· Threat appraisal is the critical step in pain perception-if there is a significant threat, you will experience pain, if there isn’t, you won’t.
· Pain does not indicate tissue damage, and does not mean an injury hasn’t healed properly
· It is possible to have nociception without pain, and pain without nociception.
· The key to eliminating pain is to eliminate the threats that the brain perceives.
For more in depth explanation on the above bullet points check out our last blog post. Today we are going to talk about brain outputs. As was previously mentioned, pain is generated by the brain to motivate action for survival. “So you’re telling me I’m just making this up,” many patients will reply. The answer to that question is a resounding NO. Pain occurs by mechanisms above anyone’s level of control, and accusing someone of confabulating a painful occurrence would be like accusing them of purposely seeing the wrong thing in an optical illusion. Unfortunately, both the illusion and pain are a result of grand biological design that better enables you to survive. The key to remember is that if you are having pain, then your brain wants you to do something to get out of danger.
Pain is one of many brain outputs that we experience on a daily basis. Some others are:
Did you notice how all of the above things are somewhat unpleasant. This is because they are all trying to strongly motivate you to act, because without action your survival is in jeopardy. The things on the above list are frequently mistaken for sensations, when in fact they are experiences that are created by the brain to shape your reality. Let’s break the brain outputs down.
- Thirst- Brain detects low blood volume, high blood viscosity, high body heat, copious sweating, dry environment, low water intake over long period of time, etc. The brain will combine all of these factors to assess the need for fluids. If a need for fluids is perceived, you will be thirsty, because the brain wants you to drink. This may be pre-emptive (future need is perceived) or reactive (dehydration occurs). Perhaps the thirst is social or environmental, and not tied to need for water. An example of this is a person who is well hydrated but feels thirsty when seeing others drinking, or wanting a cool drink on a hot day.
· Fatigue– Brain detects low levels of available energy, high levels of exertion, low food consumption, lack of sleep, etc. The brain combines these factors to assess the need for rest or energy conservation. If energy is depleted or conservation is needed, the brain will make you fatigued. It is also possible that fatigue may set in when energy is plentiful, due to environmental or social factors (i.e. lazy Saturdays).
· Hunger– This is the mirror to thirst. Again low food consumption, energy levels, etc. The brain makes you feel hungry to motivate you to eat.
· Nausea– Brain detects excessive motion, something poisonous eaten, sickness within the body. There is likely a twofold effect here, in that nausea will motivate you not to do what you did again in the future, but also may make you vomit. Your brain is motivating to act, and conditioning you to act differently in the future (i.e. avoiding rotten meat).
· Itch, Tickle, and Pain– These three are grouped together because they are different brain outputs that result from the same sensory inputs. When nociception occurs, the brain begins threat appraisal. If the situation is deemed threatening, the brain will likely give you pain. However, perhaps the situation is only slightly threatening, and perhaps only to the skin (i.e dead skin cells, bug crawling on arm). Motivation for action is needed to remove the dead cells or insect, but the situation is not pain worthy. So your brain creates itch. In another situation, perhaps your ribs and internal organs are threatened by an attacker, but because threat appraisal tells you this is not an enemy, but your Grandpa trying to tickle you, and that this is meant to be playful. So your brain creates tickle. Both itch and tickle are sufficient to motivate you to action (scratch your skin, swat bug, wiggle away from grandpa), but are a result of nociception that is not threatening enough to generate pain.
A big key to remember is that the brain outputs you perceive are situation dependent, and may not always be accurate. Have you ever felt tired after a good night of sleep? Or maybe thirsty after a big glass of water? Have you ever lost your appetite before? All of these are examples of how sensory appraisal can occur in error. Sometimes your brain makes the wrong decision to motivate you to action, and gives you an output you don’t need. The same is true of pain.
Many patients experience pain long after full healing of an injury has occurred. This does not mean that there is something else wrong, or that the injury didn’t heal. It just means that the brain perceives a threat when none exists. This becomes a complicated situation because we begin to develop a vicious cycle. Consider this:
Initially after an injury, muscle tightness causes pain-
However, after an injury has healed, and pain persists, pain now creates muscle tightness-
As you can clearly see, there is still a physical aspect to the pain and dysfunction, but now it is pain, not the dysfunction that is driving the cycle. The key to braking this cycle is to eliminate the pain output from the brain. In our next post we will get into graded motor imagery, and the methods of braking this cycle and treating pain.