Gate control theory

Gate control theory is a scientific explanation of how the human mind perceives pain in the body. The theory was developed in the 1960s by scientists Ronald Melzack and Patrick Wall. They proposed that interactions between nerve fibers in the spinal cord act as a "gate," either blocking or allowing pain signals to reach the brain. The theory is used to explain why pain is perceived differently by people in different states of mind. For example, a wounded soldier may continue fighting, unaware of any pain sensation, while stubbing a toe on a table may cause significant discomfort.

Background

Pain is a warning sign that the body sends to the brain to signal that something is wrong. Medical experts define pain as an unpleasant physical or emotional sensation associated with tissue damage. Nerve endings known as pain receptors record the negative stimuli caused by actual or potential damage and send a signal through the central nervous system to the brain, triggering a pain response. The pain response acts as a warning for the body to take action to prevent further harm. The feeling can be a dull ache, mild discomfort, a sharp stab, or an excruciating sensation. Pain receptors are found throughout the body and are most common in the skin, joints, bones, artery walls, and parts of the skull.

Pain receptors can be stimulated by several physiological factors. For example, if a person accidentally cuts his or her hand with a sharp object, the physical breaking of the skin will trigger a pain receptor. The damaged cells release potassium, which alerts the body to the injury. Immune cells that rush to the injured area also release substances that cause a pain response. Pain is categorized in two ways. Nociceptive pain is caused by a direct injury, such as a fresh cut or a broken bone. Neuropathic pain results from damage or disease that affects the nervous system. Pain caused by cancer or diabetes is an example of neuropathic pain.

Pain receptors send their information to the spinal cord and into the brain through nerve fibers called axons. Axons can vary in thickness and may be insulated by a substance known as myelin. Thicker axons and those insulated with myelin are able to transmit information more quickly. Axons are divided into groups based on size: A-alpha axons are the thickest, followed by A-beta, A-delta, and C-nerve fibers. All A fibers contain myelin, while C fibers do not.

Overview

In 1965, psychologist Ronald Melzack and neuroscientist Patrick Wall—both working at the Massachusetts Institute of Technology—published their gate control theory of pain. The theory stated that the human spinal cord contains specialized nerve "gates" that control whether pain signals can pass through to the brain. Small nerve fibers carry pain information; large fibers carry non-pain signals such as sensations of touch and pressure. Melzack and Wall found that signals that travel through smaller nerve fibers are allowed to pass through to the brain, while pain signals from large fibers are blocked or altered. The nerve gates are located in the dorsal horn of the spinal cord. The dorsal horn consists of two columns of cells and nerve fibers located near the back of the spinal cord.

Both small and large nerve fibers pass signals though special nerve cells called the projection neuron and the inhibitory interneuron. When a projection neuron is activated, the brain can receive pain signals. An active inhibitory interneuron decreases the chances that the projection neuron will be activated and may block pain signals from reaching the brain.

Without stimulation, both small and large nerve fibers remain inactive, and the inhibitory interneuron blocks the projection neuron from sending any signals to the brain. In this case, the nerve "gate" is considered closed, and the body does not experience pain. When the body experiences a non-pain stimulus, the large nerve fibers are activated. This fires up the projection neuron, but the inhibitory interneuron steps in and blocks the projection neuron from sending pain signals, again closing the gate.

When tissue damage occurs and the body experiences pain, the signals stimulate the small C-nerve fibers. This activates the projection neuron, but also blocks the inhibitory interneuron. With the inhibitory interneuron unable to prevent the projection neuron from sending pain signals to the brain, the gate is considered open, and the body feels pain.

Melzack and Wall suggested that the gate control theory may be the reason why people's first response is often to rub a body part after an injury. The sensation of touch may activate the large nerve fibers, closing the gate and reducing the feeling of pain. The theory also may explain why massage therapy and physical touch often are used as pain-management techniques.

The nerve gate that controls pain perception may be affected by external factors. Stress, tension, and a lack of physical activity may increase the likelihood of the gate remaining open, and the body may experience more pain. Focusing attention on pain may have the same effect. This may explain why a dull, throbbing headache may seem more painful than it really is.

Conversely, relaxation techniques, physical activity, and shifting the mind's focus away from the pain may close the nerve gate. People who undergo hypnosis often feel less pain after the procedure. Athletes who have been hurt sometimes play though their injuries, only noticing pain at a later time when they have ceased physical activity. The same effect has been observed in wounded soldiers who continue to fight without noticing the pain.

Bibliography

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