To understand stress-induced analgesia, it is necessary for us to understand how the body perceives pain and how it responds to the pain signal. Here, we confront an anatomical problem. The sensors for pain are scattered all over the body. The central nervous system (CNS) processes the signals from the sensors. This is housed entirely within the cranial cavity housing the brain, and the vertebral canal, housing the spinal cord.
The response to the pain has to be transmitted to the muscles, which also lie at a distance from the brain and the spinal cord. Therefore, there needs to be some way of relaying sensory input and motor outflow between the periphery and the CNS. This is the function of the peripheral nervous system (PNS). So, how do the pain signals travel from the rest of the body to the brain?
An elegant (now very famous) theory about how pain is perceived is called the Gate Control Theory devised by Patrick Wall and Ronald Melzack in 1965. This theory states that pain is a function of the balance between the information travelling into the spinal cord through slow nerve fibres and information travelling into the spinal cord through fast nerve fibres.
I give below a wildly simplified explanation of this theory. The basic idea is that there are two pathways for communicating pain signals to the brain—a fast path and a slow path. There are two gates (let us call them Gate 1 and Gate 2) through which both these pathways pass. When a signal arrives on the fast path, it passes through both gates and then to the brain where the pain is registered.
However, after the fast signal has passed, Gate 2 plays a trick and closes down Gate 1. This stops additional pain signals from reaching the brain. So, we feel only a short sharp pain. When a signal arrives on the slow path, both gates are opened and the brain registers the pain. However, the slow fibre is well aware of the trick played by Gate 2 and stops it from closing Gate 1. Both gates remain open and we continue to feel a constant pain.
Now for a quick lesson on how to reduce throbbing pain—say from an aching muscle. What you need to do is briefly stimulate the fast fibre by a short painful stimulus. This will result in the Gate 2 pulling off its neat trick of closing Gate 1 and temporarily stopping the slow pain signal from reaching the brain.
Now with this understanding, we know why a good vigorous massage makes our dull pain go away for a while. This is also true in the case of insect bites that throb and itch. A good hard scratch right around the area relieves the pain. In all these cases, the slow pain pathway is shut down temporarily. Naturally, the pain will return after a time if the cause of the pain has not been resolved.
A more advanced use of this same trick is seen in machines called TENS—trans-electrical nerve stimulators. These devices are commonly used by physiotherapists or during labour and they stimulate the sharp pain fibre in peripheral nerves which will in turn shut the slow pain fibre.