Pain perception varies across different individuals according to their mood, emotional condition and prior experience, even if the pain is caused by similar physical stimuli and results in a similar degree of damage. In 1965, Ronald Melzack and Patrick Wall outlined a scientific theory about psychological influence on pain perception; the ‘gate control theory’.

If not for this theory, pain perception would be still associated with the intensity of the pain stimulus and the degree of damage caused to the affected tissue. But Melzack and Wall made it evident that pain perception is far more complex.

According to the gate control theory, pain signals are not free to reach the brain as soon as they are generated at the injured tissues or sites. They need to encounter certain ‘neurological gates’ at the spinal cord level and these gates determine whether the pain signals should reach the brain or not. In other words, pain is perceived when the gate gives way to the pain signals and it is less intense or not at all perceived when the gate closes for the signals to pass through. This theory gives the explanation for why someone finds relief by rubbing or massaging an injured or a painful area.

Though the gate control theory cannot present the complete picture of the central system that underlies pain, it has visualized the mechanism of pain perception in a new dimension and it has paved the way for various pain management strategies.

Peripheral nerve fibers involved in transmission of sensory signals

Every organ or part of the human body has its own nerve supply and the nerves carry the electrical impulses generated in response to various sensations like touch, temperature, pressure and pain. These nerves – that constitute the peripheral nervous system – transmit these impulses to the central nervous system (the brain and spinal cord) so that these impulses are interpreted and perceived as sensations. The peripheral nerves send signals to the dorsal horn of the spinal cord and from there the sensory signals are transmitted to the brain through the spinothalamic tract. Pain is a sensation that alerts a person that a tissue or a particular part of the human body has been injured or damaged.

According to the axonal diameter and the conduction velocity, nerve fibers can be classified into three types – A, B and C. The C fibers are the smallest among all the three types. Among the ‘A’ fibers are four subtypes: A-alpha, A-beta, A-gamma and A-delta. Among the A subtypes, the A-alpha fibers are the largest and the A-delta fibers are the smallest.

The A fibers that are larger than the A-delta fibers, carry sensations like touch, pressure, etc. to the spinal cord. The A-delta fibers and the C fibers carry pain signals to the spinal cord. A-delta fibers are faster and carry sharp pain signals while the C fibers are slower and carry diffuse pain signals.

When considering the conduction velocity, the A-alpha fibers (the large nerve fibers) have higher conduction velocity when compared to the A-delta fibers and the C fibers (small nerve fibers). When a tissue is injured, the A-delta fibers are activated first, followed by the activation of the C fibers. These fibers tend to carry the pain signals to the spinal cord and then to the brain. But the pain signals are not transmitted simply like that.

What does the gate control theory say?

The gate control theory suggests that the signals encounter ‘nerve gates’ at the level of the spinal cord and they need to get cleared through these gates to reach the brain. Various factors determine how the pain signals should be treated at the neurological gates. They are:

• The intensity of the pain signals
• The intensity of the other sensory signals (touch, temperature and pressure), if generated at the site of injury
• The message from the brain itself (to send the pain signals or not)

As already mentioned, the nerve fibers, large and small, carrying the sensory signals, end in the dorsal horn of the spinal cord from where the signals are transmitted to the brain. According to the original postulate of Melzack and Wall, the nerve fibers project to the substantia gelatinosa (SG) of the dorsal horn and the first central transmission (T) cells of the spinal cord. The SG consists of inhibitory interneurons that act as the gate and determine which signals should reach the T cells and then go further through the spinothalamic tract to reach the brain.

When the pain signals carried by the small fibers (A-delta and C fibers) are less intense compared to the other non-pain sensory signals like touch, pressure and temperature, the inhibitory neurons prevent the transmission of the pain signals through the T cells. The non-pain signals override the pain signals and thus the pain is not perceived by the brain. When the pain signals are more intense compared to the non-pain signals, the inhibitory neurons are inactivated and the gate is opened. The T cells transmit the pain signals to the spinothalamic tract that carries those signals to the brain. As a result, the neurological gate is influenced by the relative amount of activity in the large and the small nerve fibers.

Emotions and thoughts determine the way how pain is perceived

The theory also proposed that the pain signal transmission can be influenced by emotions and thoughts. It is well known that people do not feel a chronic pain or, to be more appropriate, the pain does not disturb them when they concentrate on other activities that interest them. Whereas, people who are anxious or depressed feel intense pain and find it difficult to cope up with it. This is because the brain sends messages through descending fibers that stop, reduce or amplify the transmission of pain signals through the gate, depending on the thoughts and emotions of a person.

Gate control theory in pain management

The gate control theory has brought about a drastic revolution in the field of pain management. The theory suggested that pain management can be achieved by selectively influencing the larger nerve fibers that carry non-pain stimuli. The theory has also paved way for more research on cognitive and behavioral approaches to achieve pain relief.

One of the tremendous advances in pain management research is the advent of Transcutaneous Electrical Nerve Stimulation (TENS). The gate control theory forms the basis of TENS. In this technique, the selective stimulation of the large diameter nerve fibers carrying non-pain sensory stimuli from a specific region nullifies or reduces the effect of pain signals from the region. TENS is a non-invasive and inexpensive pain management approach that has been widely used for the treatment of chronic and intractable pain that are otherwise non-responsive to analgesics and surgical treatments. TENS is highly advantageous over pain medications in the aspect that it does not have the problem of drug interactions and toxicity.

Many other invasive and non-invasive electrical stimulation techniques have been found to be useful in various chronic pain conditions like arthritic pain, diabetic neuropathy, fibromyalgia, etc. The theory has also been extensively studied in the treatment of chronic back pain and cancer pain. However, favorable results are not attained in some conditions and the long term efficacy of the techniques based on the theory is under question.

Nevertheless, the gate control theory has dramatically revolutionized the field of pain research and it has sown seeds for numerous studies that aim at presenting a pain-free lifestyle to the patients who suffer from chronic pain.

References

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