Over the past few weeks we have been talking about pain and some misconceptions about it. Today we are going to talk about some of the mechanics of how pain works.
One misconception people frequently have is that the body has pain receptors throughout. This was how pain was originally thought to work. If something damaged the body, you step on a nail, you touch a fire, or you pull a muscle, pain sensors would send signals to your brain and tell the brain pain was occurring. This famous picture about pain was drawn by Descartes something like 400 years ago.
In truth we have a number of ways we experience pain such as nociception, neuropathic pain, and central sensitization. All of these can interact and affect each other in many ways and all can affect acute and chronic pain.
Nociception is the basic way we transmit acute pain, the kind of pain we get when we roll and ankle, touch a hot stove, or stub our toe. Nociceptors are nerve endings throughout the body which can detect potential damage. When a nociceptor detects potential damage it sends a signal along the peripheral nerves to the spinal cord and then the brain. This signal is processed at each level and that is how we experience pain consciously is determined.
Sometimes people call nociceptors pain receptors. Nociceptors actually detect the same things as other receptors, such as pressure, tension, and temperature. They are different in that they have a higher threshold of activation. This means that normally they need a stronger stimulus to activate. They are acting like an alarm system telling you something intense or out of the ordinary is happening.
There are 3 primary types of nociceptors. Mechanical respond to mechanical stresses such as rolling an ankle or tearing a ligament. Temperature nociceptors respond to extreme temperatures to prevent burning. The third are chemical nociceptors. When we have injury or illness we can become inflammed. Inflammation is the body’s way of repairing itself bringing blood and helpful chemicals to a damaged area. These chemical can affect pain in a couple of ways. The chemicals may constantly activate nociceptors giving us constant pain, or the chemicals may lower the threshold of nociceptors making the area much more sensitive to mechanical forces or temperature changes. This is why touch can be very sensitive around an infected cut or any movement may be painful after an acute injury.
Nociceptive pain is generally easily understood. Normally you have done something you can pinpoint that causes the pain, twisted your ankle, have an infected cut, or burned yourself on the stove. But not all nociception causes pain, think of rolling your ankle in an intense game of basketball and not feeling it until later. Not all pain requires nociception. Phantom pain in an amputated limb is an example of this. The limb is no longer present and of course the nociceptors are gone with the limb, but the person can continue to experience limb pain. The important takeaway is that nociception and pain are not the same thing. Nociceptors are not pain receptors and they don’t send “pain signals”. Nociceptors are sensory receptors which sense potential harm. Pain is a conscious experience which nociception can be part of, but depends on many other factors.