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What is the Pain Gateway Theory?

Due to the observations that raised questions, a new theory of pain was developed in the early 1960s to account for the clinically recognized importance of the mind and brain in pain perception. It is called the gate control theory of pain, and it was initially developed by Ronald Melzack and Patrick Wall.

Although the theory accounts for phenomena that are primarily mental in nature - that is, pain itself as well as some of the psychological factors influencing it - its scientific beauty is that it provides a physiological basis for the complex phenomenon of pain. It does this by investigating the complex structure of the nervous system, which is comprised of the following two major divisions:

Central nervous system (the spinal cord and the brain)

Peripheral nervous system (nerves outside of the brain and spinal cord, including branching nerves in the torso and extremities, as well as nerves in the lumbar spine region)

In the gate control theory, the experience of pain depends on a complex interplay of these two systems as they each process pain signals in their own way. Upon injury, pain messages originate in nerves associated with the damaged tissue and flow along the peripheral nerves to the spinal cord and on up to the brain. So far, this is roughly equivalent to the specificity theory of pain described above.

However, in the gate control theory, before they can reach the brain these pain messages encounter “nerve gates” in the spinal cord that open or close depending upon a number of factors (possibly including instructions coming down from the brain). When the gates are opening, pain messages “get through” more or less easily and pain can be intense. When the gates close, pain messages are prevented from reaching the brain and may not even be experienced.

Although no one yet understands the details of this process or how to control it, the following concepts are presented to help explain why various treatments are effective and how to find solutions to chronic back pain.

The peripheral nervous system
Sensory nerves bring information about pain, heat, cold and other sensory phenomena to the spinal cord from various parts of the body. At least two types of nerve fibers are thought to carry the majority of pain messages to the spinal cord:

A-delta nerve fibers, which carry electrical messages to the spinal cord at approximately 40 mph (“first” or “fast” pain).

C-fibers, which carry electrical messages at approximately 3 mph to the spinal cord (“slow” or “continuous pain”)

A good example of how these respective nerve fibers work is the activation of the A-delta nerve fibers followed by the activation of the slower C-fibers. The activation of other types of nerve fibers can modify or block the sensation of pain.

After hitting one’s elbow or head, rubbing the area seems to provide some relief. This activates other sensory nerve fibers that are even “faster” than A-delta fibers, and these fibers send information about pressure and touch that reach the spinal cord and brain to override some of the pain messages carried by the A-delta and C-fibers.

The action of these other types of nerve fibers helps to explain why treatments such as massage, heat or cold packs, transcutaneous nerve stimulation, or even acupuncture are often effective in treating back pain. The nerve endings in the back are transmitted by special peripheral nerves first to the spinal cord and then up to the brain. These messages can be overridden by other signals in the manner described above.Treatments such as massage, heat, cold, TNS (transcutaneous nerve stimulation), or acupuncture can change a pain message due to some of these differences in nerve fibers.

The same principles apply in back pain. The nerve endings that detect pain are present in many structures in the back including the muscles and ligaments, the disks, the vertebrae, and the facet joints. When one of these parts is irritated, inflamed, or mechanically malfunctioning, the pain message will be transmitted by special peripheral nerves to the spinal cord and up to the brain. These messages can be over-ridden by other signals produced by the treatments listed previously.

The gate control theory of chronic pain in action
How the theory of chronic pain works
The brain commonly blocks out sensations that it knows are not dangerous, such as when the feel of tight-fitting shoes that are put on in the morning has all but vanished by the second cup of coffee. A similar process is at work in processing some moderately painful experiences.

The following outlines two brief case examples of how the gate control theory of pain may be experienced.

1. This case example shows how the experience of pain may change as information is processed in the brain.

Applying a clothespin to one’s arm initially produces pain that may be quite intense as the skin and surface muscles are compressed. Peripheral nerve fibers detect this pressure and transmit a pain signal to the spinal cord and on to the brain. At first it is the fast pain signals that get through, and the intensity of the pain experience is fairly proportional to the amount of pressure applied. Everyone would agree that this is acute pain.

The slower pain signals are not far behind, however, and a dull ache may soon be noticed. After a short while, the pain coming from the pinched tissue will begin to be decreased by the closing of the spinal nerve gates. This is because the brain begins to view the pain signals as non-harmful. The pressure may be painful initially but it is not injuring the person in any way. As time goes on, the pain message is given less priority by the brain and the person’s awareness of it decreases greatly.

The brain knows that the clothespin is not causing any injury. Therefore, the brain gradually “turns the volume down” on the pain message to the point of it being barely noticeable after about thirty minutes. The compression on skin and muscle is still occurring, but it is now perceived as a mild discomfort if it is noticed at all.

2. This case example shows how other factors can sometimes play a major role in the experience of pain.

The patient reports that she thinks her back pain is due to a spinal tumor. A thorough physical examination and medical history reveals that her spine is normal, except for the onset of the back pain after a recent period of extreme stress.

The stress involved the patient’s elderly father, who had just been diagnosed with a spinal tumor. The patient reported that her father’s symptoms had also initially included back pain.

Upon questioning, it became quite clear that the patient had an extreme fear that she also was suffering from a spinal tumor. This belief was creating intense suffering, which in turn, made the back pain worse.

The patient’s MRI showed no problems with her spine, and the diagnosis of stress related back pain was made. After experiencing tremendous relief that the back pain was not the result of a tumor, the patient’s symptoms began to dissipate rather rapidly and she returned to normal activities

As the above examples illustrate, pain is much more complex than was previously understood (e.g. the specificity theory) and the spine medicine community is now beginning to understand and recognize other factors that contribute to the experience of pain. With this new understanding, it is accepted that treatment of an underlying anatomic lesion may not always relieve the pain (and pain may be present with no anatomic problems)—rather, pain is a complex process that is experienced differently in various situations and is influenced by myriad factors.
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