This is just meant to be a quick overview of how to think about pain and how the body processes pain. The bottom-up to top-down approach basically looks at pain at where occurs in the body and then how the brain gets the information and what it does with that information.
So say Roger here trips over the hurdle and hurts his elbow. He now has an injury and things happen at the site of the injury.
The first step is a localized inflammatory response at the site of the tissue trauma. Cells like macrophages (which basically act as garbage trucks) and mast cells (which are the cells that also cause your allergies to cats and pollen) are activated and release lots of inflammatory and immune mediators like tumor necrosis factor, interleukin-1, 6, 8, 10, 12, histamine, bradykinin, adenosine, and neurotrophic growth factor are released and cause the blood vessels to dilate and become more permeable like a seize to allow fluid and molecules to cross more easily. This is what leads to the swelling and redness after an acute injury.
Now that the localized response has occurred, the signal is sent to the brain. And for this to happen, the local inflammatory mediators activate C fibers and A-delta fibers, which are neurons that travel through the spinal cord to the brain. So when an injury happens, little receptors called nocireptors at one end of the neuron are activated by the noxious stimulus and tell the brain what’s going on.
Okay so the last step is the brain processing the signal it’s received. And this occurs in multiple ways. There is a sensory-discriminative component, which just identifies the type of pain and where it’s located (like if it’s in your elbow or in your foot). The other major component is the affective-motivational pathway, which involves activation of multiple regions of the brain, which decide how you’re going to respond to the pain- it essentially defines your mood. And finally, there is a part of your brain that sends signals back down to the injury and modulates the painful response so that it’s less painful.
Okay, that’s it for the overview. I’ll go into the details in future episodes.