The Gate Control Theory of Pain


The Gate Control Theory of pain [Melzack, Wall] revolutionized the understanding of pain. The theory now includes the finding that “the brain can control the degree of pain that is perceived, based on which pain stimuli are to be ignored to pursue potential gains. The brain determines which stimuli are profitable to ignore over time. Thus, the brain controls the perception of pain quite directly, and can be ‘trained’ to turn off forms of pain that are not ‘useful.’ This understanding led Melzack to assert that pain is in the brain.” [en.wikipedia.org/wiki/Gate_control_theory]

Descartes had a direct connection from the pain sensors to the brain. The Gate Control Theory [Melzack and Wall] has gates that can either transmit the signal from the pain sensors to the brain or not. Only one gate for one nerve fiber is shown here. There are many gates along the spine for the many nerve fibers in the body. There are also more general gates in the brain stem that can inhibit or muffle incoming pain signals from the spine by the production of endorphins, which are morphine-like substances that occur naturally in the human body.
The Gate Control Theory explained why rubbing a painful area can actually reduce the pain. The signal from the neurons that transmit the sense of touch can close the gate that would otherwise let the pain signal reach the brain, and thus block the experience of pain. The science of this phenomenon (which is much easier to access experimentally than how retraining the brain works) has benefitted from extensive research. An excellent review article concludes, “Pain is not a passive consequence of the transfer of a defined peripheral input to a pain center in the cortex, but an active process generated partly in the periphery and partly within the [central nervous system] by multiple plastic changes that together determine the gain of the system. The understanding of plasticity is rapidly increasing, and we expect that the future will provide further exciting insights into pain mechanisms.” [Woolf and Salter]
Here are two schematic diagrams based on a modern understanding of the Gate Control Theory of pain. There is no need to understand everything about these very complex diagrams… I certainly don’t! I include them to emphasize that there are already known mechanisms that a retrained brain might use to reduce chronic pain. Clearly much more research will be needed to get a detailed understanding of what actually occurs, which might lead to even more effective ways to retrain brains. But the point is that whether or not we have this specific knowledge, these mechanisms do exist…and we can use them!

 


The pain signals from the “A𝛅 and C fiber nociceptive inputs” may or may not be transmitted to the brain depending on the “Descending inhibitory and facilitatory influences” which come from the brain to the “Spinal dorsal horn” which functions as a gate (as in Gate Control Theory of pain). [Denk] Note that there are very many such gates in the human body.

Signals descend from the brain to modulate the transmission of signals from the pain sensors in the body. Note that these influences can be inhibitory, blocking the transmission of signals from the pain sensors. This is a possible mechanism for the retrained brain to shut off chronic pain. Note also that these influences can be facilitatory, enhancing the transmission of signals from the pain sensors. This is a possible mechanism for the sensitization that will be discussed below in the section “Sensitization and its Reversal.”

This schematic shows the gate formed from the Dorsal Root Ganglion going into the Dorsal Horns that will control whether the signal transmitted from the receptor in the finger through the Pseudounipolar Neuron is allowed to pass up the spine to the brainstem or not. (med.umich.edu/lrc/coursepages/m1/anatomy2010/html/modules/spinal_cord_module/spinalcord_10.html)
In the brain stem other gates will determine if the signal passes to the brain.

A schematic showing sites of action of commonly used pharmacologic and behavioral analgesic therapies. Note especially all the sites for systemic opioids in the medulla (part of the brainstem) and midbrain. [All Care Visiting Nurse Association and Hospice]
This schematic emphasizes that there are many ways to reduce the experience of pain. When the experience of pain is reduced temporarily, you have a choice of passively appreciating the reduction or actively using the reduction to increase activity and fun and begin reversing the downward spiral of pain/inactivity, as suggested by Daniel J. Clauw in his wonderful YouTube video, “Chronic Pain – Is it All in Their Head?” The connections down from the brain suggest possible mechanisms by which a retrained brain can reduce and eventually eliminate chronic pain.
The gate control theory of pain can explain how all the ways shown in the schematic can reduce the experience of pain. But it is not yet clear, at least to me, whether the gates are used by a retrained brain to eliminate chronic pain. Perhaps higher level mental processes that involve meaning and pain are more important.
Meaning and Pain
The brain creates the sensation of pain while trying to help us! It unconsciously evaluates incoming sensory input based on meaning. If it feels that we really need to pay attention to some potentially harmful sensory input, it creates pain to alert us. This point is eloquently made in a story told by Prof. Lorimer Moseley, (a distinguished pain researcher who edited the “Graded Motion Imagery Handbook”) from 2:00 to about 9:00 in his YouTube video, “Body in mind – the role of the brain in chronic pain.” If you like the story, you can get his credentials at the first of the video and find some very interesting research studies after the story. This story is so important and entertaining that I do not want to spoil it for you by summarizing it here. I’ll just attempt to entice you by mentioning that it involves what turns out to be the third deadliest snake in the world.
The goal of watching this video and other videos suggested below, visiting the neuroplastix.com website, reading books from the bibliography below and doing research on your own is to come to a clear understanding that the solution to chronic pain is not just dealing with tissue, but retraining the brain. This clear understanding can provide the motivation to do the relentless work it may take to reduce your chronic pain. (Or you might get lucky and get a breakthrough very quickly like Susan did!)