What occurs at the cellular level when pain is experienced? The gate control theory of pain, by P. D. Wall and Ronald Melzack, offers a useful model of the physiological process of pain. Gate control is recognized as a major pain theory.

According to the gate control theory, pain is a balance between information traveling into the spinal cord through large nerve fibers and information traveling into the spinal cord through small nerve fibers. Without any stimulation, both the large and small nerve fibers are quiet, and the substantia gelatinosa (SG) blocks the signal to the transmission cell (T cell) connected to the brain. The “gate is closed,” and there is no pain. With pain stimulation, small nerve fibers are active. They activate the T-cell neurons but block the SG neuron, making it impossible for the SG to block the T-cell transmission to the brain. The result is that the “gate is open”; therefore, there is pain. In other words, pain is experienced whenever the substances that tend to propagate a pain impulse across each “gate” in a nerve pathway overpower the substances that tend to block such an impulse.


The gate control theory of pain transmission

FIGURE. The gate control theory of pain transmission. The substantia gelatinosa (SG) accepts input both from large-diameter (nonpain) and small-diameter (pain) nerves. Based on the rate of input, the SG allows either the pain or nonpain stimulus to be passed on to the transmission cell (T cell) and up to the brain. Because nonpain impulses travel faster than pain impulses, stimulation of nonpain fibers can override the transmission of pain. In addition, the brain has an inhibiting influence both on the SG and the spinal cord that can work to limit the perception and reaction to pain. (From Starkey, C: Therapeutic Modalities for Athletic Trainers. FA Davis, Philadelphia, 1993, p 28, with permission.)

Studies of coping factors support a wider version of the gate control theory. These factors are to be considered before determining treatment for pain, and they raise a number of questions.

  1. How well is the client experiencing life?
  2. Does the client have pain, and if so, does he or she think that it is under control?
  3. Does the client feel adequately informed about the painful condition?
  4. Is the client occupied? How does the individual fill his or her time?
  5. Is the client coping with other problems?
  6. Does the client feel dissatisfied with his or her past life, or does he or she have any substantial regrets?
  7. Are there any reasons why the client may not be coping?

Answers to these questions may help determine the best treatment protocol for pain.