Monday, October 17, 2005

Neuromatrix 5: The Gate Control Theory

To continue: the gate control theory (p.7, TIP 3):

Melzack discusses how in the 50s he had Donald O. Hebb as a research advisor. Hebb was corresponding with George H. Bishop. Both were fascinated with the nervous system. Bishop was researching sizes of fibers and what that had to do with pain. Hebb had written a book, “The Organization of the Brain” which ascribed pain to abnormal patterns of firing in the brain, and was studying Scottish terriers to research the role of early sensory experience on adult behavior.

Out of this context Melzack suddenly saw a possible PhD thesis, and decided to study the dogs, which were already being raised in sensory-social isolation, to see if this restriction also had an effect on pain perception. When the restricted dogs were let out they got very excited and ran around wildly. The control dogs moved slowly, cautiously exploring. He lit matches. The restricted dogs repetitively sniffed the match. For control dogs one sniff was quite enough and they ran away from the match, requiring Melzack to pursue them.. The behavior was similar for needle pricks. After several years of exposing the restricted dogs and control dogs to flaming matches and sharp needles, he concluded,

..the restricted dogs had difficulty in discriminating among stimuli, largely because of their high level of arousal. I developed the hypothesis that part of the input from an object such as a flaming match must travel rapidly up the largest fibers and fastest pathways to the brain and activate neural processes that act down on the more slowly conducted input.

The information descending from the brain, I proposed, acted at levels that had to be below the brainstem reticular formation. The brains of restricted dogs, without the advantage of prior learning, allowed all information to ascend to the brain. Flaming matches and dissecting needles were no more important than anything else in the environment, and the massive, unfiltered sensory input produced high levels of reticular arousal.

The restricted dogs, I believed, could feel pain, but the irrelevant messages were not being inhibited below the level of the reticular formation, and the relevant injury signals failed to rise above the background noise.

In 1959, while assistant professor of psychology at MIT, Melzack met Patrick Wall who was a biology professor, and they discussed pain:

We both had ideas that we shared: pain is due to patterns of nerve impulses rather than straight-line transmission of modality-specific impulses to a pain center; information that arrives at the spinal cord is filtered and selected on the basis of the total pattern of activity in stimulated fibers as well as by descending information by the brain.

The paper that resulted from their collaboration was published in Science in ‘65. It was the first theory of pain that incorporated the central control processes of the brain. Gate control theory was received with “vigorous (sometimes vicious) debate” and a lot of followup research. By the mid-70s all the textbooks included it. Physiology and pharmacology exploration of the dorsal horns was underway and clinicians were being impacted. Psychological factors in pain began to be seen as integral and not reactive. Surgical efforts were being slowly replaced by ways of moderating input. TENS entered the picture.

Physical therapists and other health care professionals who use a multitude of modulation techniques 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 has been evaluated and indicates that, despite the addition of a massive amount of detail, the conceptual components of the theory remain basically intact after more than 30 years.

Meanwhile Melzack moved on. He and a colleague, Kenneth Casey, coined names such as the sensory-discriminative, motivational-affective, evaluative dimensions of pain. “ These phrases seemed strange when we coined them, but they are now used so frequently and seem so ‘logical’ that they have become part of our language.”

The McGill Pain Questionnaire was developed, analgesic effects of morphine became better understood, spinal and cerebral systems underlying acute and chronic pain continued to be studied. Acupuncture began to be explained by Chinese scientists using the gate control theory and the West started taking a closer look.

Melzack collaborated with W.K. Livingston to discover “the area surrounding the aqueduct in the midbrain exerted a tonic inhibitory effect on pain. This experiment was. in part, the basis for postulating inhibitory control in the gate theory. It also led directly to Reynold’s demonstration that electrical stimulation of the periaqueductal gray produced analgesia. This study was followed by Liebeskind and Paul’s research on the mechanisms of the descending inhibition and by the discovery of pharmacological substances such as endorphins” (Candace Pert) “that contribute to it.” Melzack observed that ‘pain takes away pain’, postulated that descending inhibition tended to be activated by intense inputs, and developed studies with intense TENS. (our PT legacy...)

Then in 1978 Melzack turned his attention to paraplegics in pain. He and John Loeser proposed a central ‘pattern generating mechanism’ above the level of the section, a major advance at the time.

It did not merely extend the gate, it said that pain could be generated by brain mechanisms in paraplegics in the absence of a spinal gate because the brain is completely disconnected from the cord. Psychophysical specificity, in such a concept, makes no sense and we must explore how patterns of nerve impulses generated in the brain can give rise to somaesthetic experience.

This concept does not diminish the role of sensory inputs and spinal processing in pain due to injury, inflammation, and other pathology. It simply provides a new perspective in which the brain synthesizes raw sensory inputs and generates perceptual experience. This approach seems radical and difficult to comprehend, but I am convinced that it is the logical extension of concepts that began with the gate control theory’s incorporation of the brain in the attempt to understand pain.

Still more to come.

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