Tuesday, June 11, 2013

Melzack and Katz, Pain. Part 6h: Gate Control Theory

The paper, Pain

Part 1 First two sentences Part 2 Pain is personal Also Pain is Personal addendum., Neurotags! Pain is Personal, Always.

Part 3a Pain is more than sensation: Backdrop Part 3b Pain is not receptor stimulation Part 3c: Pain depends on everything ever experienced by an individual

Part 4: Pain is a multidimensional experience across time

Part 5: Pain and purpose

Part 6a: Descartes and his era; Part 6b: History of pain - what’s in “Ref 4”?; Part 6c: History of pain, Ref 4, cont.. : There is no pain matrix, only a neuromatrix; Part 6d: History of Pain: Final takedown Part 6e: Pattern theories in the history of pain Part 6f: Evaluation of pain theories Part 6g: History of Pain, the cautionary tale.

Continued from last week: Still inside the section titled A BRIEF HISTORY OF PAIN, in Melzack and Katz' paper from 2013, Pain.

From the paper:
"Patients who suffered back pain without presenting signs of organic disease were often labeled as psychologically disturbed and sent to psychiatrists. The concept was simple and often failed to help patients who suffered severe chronic pain. To thoughtful clinical observers,5,6 specificity theory was clearly wrong."
The references listed by the authors are 5. Livingston WK. Pain Mechanisms. New York: Macmillan; 1943, and 6. Livingston WK. Pain and Suffering. Seattle: IASP Press; 1998. Neither of these are online - sorry. No links. In any case I think the implication by Melzack and Katz is that they think Livingston was a thoughtful clinical observer. And that specificity theory was wrong, when it came to pain in the absence of clear pathology. 

In the figure inside the paper, Livingston's reverberatory circuits theory and Goldscheider's summation theory are combined into one diagram, with the comment:
".. in none of these theories was there an explicit role for the brain other than as a passive receiver of messages. Nevertheless, the successive theoretical concepts moved the field in the right direction: into the spinal cord and away from the periphery as the exclusive answer to pain. At least the field of pain was making its way up toward the brain." 


At last we arrive at gate control theory. It has been added to the diagram of the evolution of pain theories as the new (d). 
From Figure 1. in  the 2013 paper, Pain, by Melzack and Katz

Go Melzack. Go Wall.
Next is a short little section about GateControlTheory, so short that I'll include it in its entirety, because very soon, we'll be heading up toward the brain, after taking a sharp upward turn at the dorsal columns, or else crossing the spinal cord and taking a broad swing up at the spinothalamic tract. 
"Theories of pain, like all scientific theories, evolve as result of the accumulation of new facts as well as leaps of the imagination.10 In 1965, Melzack and Wall11 proposed the gate control theory of pain. The final model, depicted in Figure 1(d), is the first theory of pain to incorporate the central control processes of the brain."
Melzack mentions Thomas S. Kuhn in just about every single paper or chapter or book he writes - I think that means he embraced Kuhn's idea of sudden leaps forward in scientific progress.  

To continue:
"The gate control theory of pain11 proposed that the transmission of nerve impulses from afferent fibers to spinal cord transmission (T) cells is modulated by a gating mechanism in the spinal dorsal horn. This gating mechanism is influenced by the relative amount of activity in large- and small-diameter fibers, so that large fibers tend to inhibit transmission (close the gate) while small-fibers tend to facilitate transmission (open the gate). In addition, the spinal gating mechanism is influenced by nerve impulses that descend from the brain. When the output of the spinal T cells exceeds a critical level, it activates the Action System—those neural areas that underlie the complex, sequential patterns of behavior and experience characteristic of pain." 
I remember Moseley saying just a few weeks ago, it's easy to recruit neurons (into neurotags), but it's hard to inhibit them once they've started firing together reinforcing each other. See his clarinet analogy, inside this blogpost.
Since spinal cord is CNS, same goes I should think. 

"The theory's emphasis on the modulation of inputs in the spinal dorsal horns and the dynamic role of the brain in pain processes had a clinical as well as a scientific impact. Psychological factors, which were previously dismissed as ‘reactions to pain’, were now seen to be an integral part of pain processing and new avenues for pain control by psychological therapies were opened. Similarly, cutting nerves and pathways were gradually replaced by a host of methods to modulate the input. Physical therapists and other health-care professionals were brought into the picture, and transcutaneous electrical nerve stimulation became an important modality for the treatment of chronic and acute pain. The current status of pain research and therapy indicates that, despite the addition of a massive amount of detail, the conceptual components of the theory have stood the test of time.12"
Yay! He mentions physical therapists in his paper! 

When it comes to electrical modalities, though, I don't like them. None of them. Never have. They feel so ... electrical. TENS is nice for research but I don't think it's great clinically. Sorry Melzack. I'm way over TENS. Way. 
10   Kuhn TS. The Structure of Scientific Revolutions. Chicago: University of Chicago Press; 1970. 
11   Melzack R, Wall PD. Pain mechanisms: a new theory. Science 1965, 150:971–979. 
12   Dickenson AH. Gate control theory of pain stands the test of time. Brit J Anaest 2002, 88:755–757."

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