A Very Short History of Pain Theory

A theoretical understanding of a disease process and a definition of the problem is fundamental and essential if we want to help patients.  Having some idea of the disease process can suggests useful therapies and fruitful lines of investigation.  Without this, we have no basis for discussion, no way to measure either the problem or the efficacy of our intervention.

Reviewing pain theory history might not meet these needs, but it might help to illuminate why there is a problem in defining pain.  We might be able to see if there is any trends or trajectories to pain theory that might suggest the future of pain theory.  We might also find the roots to erroneous assumptions - the origination of cultural biases that color our thinking but have gone unchallenged.

According to my very brief research, in very early times, pain was thought to be the effects of evil entering the body.  These ill effects entered the body, caused pain, and when they left pain resolved.

It does not seem that there was a consensus among the ancient Greeks: some thought pain was an emotion, others an imbalance of vital fluids.  The central pain center was thought to be located in the heart.

The next model suggested that pain is the result of God testing our beliefs or putting us on trial. The therapeutic intervention that was recommended was prayer to confirm our beliefs.

The next big revolution came with René Descartes who conceived of the body as a machine.  He created a division between body and mind.  He moved the pain center from the heart to the brain.  He suggested that pain was damage to the periphery that traveled via some sort of pathway to the brain where something like a bell would ring. The stronger the stimuli the louder the ring.

Throughout the 1800s there was debate about receptors.  It was first thought that there were dedicated pain receptors and transmitters.  The Intensive Theory suggested that any sensation could cause pain as long as it was adequately intense.  In 1943 the summation theory proposed that stimulation of peripheral fibers would have to exceed some threshold for the sensation to spill over and send a signal to the brain.  Another theory proposed that only certain patterns of stimulation are able to create pain.

 In 1953, it was observed that a signal carried from injury traveled on two types of nerves: one with a large diameter, one that was small.  The large diameter fibers carried touch, pressure and vibration and these were found to inhibit the signal carried by the thinner pain sensation carrying fibers.

The final theory is the Gate control presented in 1965 by Ronald Melzack and Patrick Wall, and I'm quoting heavily from wikipedia.  They proposed that both thin (pain) and large diameter (touch, pressure, vibration) nerve fibers carry information from the site of injury to two destinations in the dorsal horn of the spinal cord.  From there, transmission cells carry the pain signal up to the brain, but inhibitory interneurons impede transmission cell activity. Activity in both thin and large diameter fibers excites transmission cells. Thin fiber activity impedes the inhibitory cells (tending to allow the transmission cell to fire) and large diameter fiber activity excites the inhibitory cells (tending to inhibit transmission cell activity). So, if there is more large fiber (touch, pressure, vibration) activity relative to thin fiber activity at the inhibitory cell, the less pain is felt. This is why we rub an injury.
    They pictured not only a signal traveling from the site of injury to the inhibitory and transmission cells and up the spinal cord to the brain, but also a signal traveling from the site of injury directly up the cord to the brain (bypassing the inhibitory and transmission cells) where, depending on the state of the brain, it may trigger a signal back down the spinal cord to modulate inhibitory cell activity (and so pain intensity).   For the first time, the brain itself is an active participant in the transmission of pain.

There are several points that I wish to make about pain theory:
- In reviewing the evolution of pain theory there is a trend to locate the processing of pain from outside the the human, to their periphery, to the spinal column and to the brain.

- There is a trend from ancient times to view the sufferer as a simple receptor for pain to more modern theories that suggest the sufferer to be an active participant in it's creation and modulation.  The sufferer was a passive victim, but is becoming more responsible for their own pain.

- New theories do not necessarily negate established theories.  Reality is actually a vast fog that is beyond our comprehension.  A theory describes a small part of it.  Most of us live quite content in the well described center, but a few will go to the edges.  Here they find unexplained phenomena and then develop a theory to explain the observation.  The function of a theory then is not so much to finally describe reality, but to widen our edges of knowledge and give others the opportunity to explore further.

And here are some lessons that we have learned from the study of pain so far:

- No pain center has yet been found.  Functional MRI imaging has found that the perception of pain is widespread throughout the brain.
- Destruction of pain pathways frequently do not yield long term pain relief.
- We have discovered "phantom pain".  This is pain in a body part that has been surgically removed and no longer exists.  Further, people can have pain and sensation in their bodies even after a verified sectioning of the spinal cord.  People can even have sensation and pain in a limb that was non existent at birth.

And how is pain presently defined?  In 1994 The International Pain Society defined pain as "An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage. "    Perhaps it is simply a reflection of my lack of intelligence, I frankly do not understand this.  The Society also includes a paragraph to help explain their definition.  I won't quote the whole paragraph but two sentence stand out "This definition avoids tying pain to the stimulus. Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause."

All the above is a preface to a discussion of the most current theory, The Body-self Neuromatrix.