Gate Control Theory

Physical pain serves as an indicator of problems in some part of the human body, while also acting as a warning signal from danger. I am sure you don't need to be explained what pain is. That is because pain as they say, is part of being human. In this article, I talk about the gate control theory of pain which tries to explain issues related to the sensation and suppression of pain.

Pain is a sensation of uneasiness and physical discomfort caused due to tissue damage of any kind. This definition has exceptions but it covers most situations when pain is experienced. The sensation of pain is made possible due to the network of nerves spread throughout the human body. Scientists still haven't got an entirely clear idea of how pain is sensed and a response to pain processed in the brain. The processing of a response to pain is a central nervous system function. I present a brief overview here of the gate control theory of pain after discussing how pain sensation is transmitted.

How is Pain Sensed?
Before we can talk about gate control theory of pain, we need to understand some basic concepts about how pain is sensed in our body. The process of pain transmission begins when a stimulus like a cut on the skin, burn, pressure or any kind of harmful impact on the body happens. This impact is received by nerve endings which are present at every point of the body. These nerves transmit this signal to the central nervous system, through a relay mechanism, which uses a continuum of connecting nerves. These nerves that transmit the signal from body towards the central nervous system, are called afferent nerves. This signal is then processed by the brain and appropriate response is generated and transmitted by the efferent nerves to the specific muscles which must come into action to prevent further damage. Like if your hand is burnt, then the nerve response from the brain passes to muscles of your arm to make it draw away.

All this happens within a fraction of a second. These nerve pathways that transmit pain sensation are called nociceptive pathways. They are wired to the brain through the peripheral sensory nerves that grow out of the spine. The signal transmission through these nociceptive neural pathways is slower compared to other sensory nerve fibers and there are different nerves for transmission of different kinds of pain sensations.

The signals are first received in the nerves present in the spinal cord and then travel to the central nervous system through the spinothalamic tract. From there they reach the central nervous system through the brain stem (medulla oblongata) and reach the thalamus, which is where the appropriate response in generated and transmitted back through the peripheral efferent or descending nerves. It has been seen that the sensation of pain can depend on thoughts and mental state of a person. Psychological condition affects how strongly we are affected by pain. The gate control theory of pain was proposed to explain this phenomenon. In the next section, I present a brief overview of gate control theory of pain.

Gate Control Theory of Pain
In 1962, Ronald Melzack and Patrick Wall proposed the gate control theory to explain how emotions and your own thinking can affect your perception of pain. They presented a model which was similar to the concept of a logic gate in electronics, as nerve pathways are actually electrical circuits. A certain set of initial conditions need to be met for the pain sensation signal to be passed or not passed through the gate constituted by specific cells in the spine that control their transmission through spinothalamic tract to the central nervous system.

Here is how gate control theory of Wall and Melzack hypothesizes the perception of pain and how it is suppressed sometimes. The structure of the gate through which pain pathways send a signal to the central nervous system needs to be understood first. The input signals to this gate is of two types, which includes small nerve fibers (pain pathway fibers) and large fibers (sensory neural pathways) which are both connected to the projection cells that carry signals through the spinothalamic tract. The projection cells are the control element of the gate. Both the types of nerve fibers (pain receptors and sensory receptors ) are also connected with inhibitory inter neurons which are situated in dorsal horn of the spinal vertebrae. They can suppress transmission through the spinothalamic tract by controlling the projection cells. There are three cases of pain transmission scenarios that need to be discussed here:

• No Input - Gate Stays Closed: When there is no incoming response from both fibers, inhibitory neurons prevent transmission of signals to the CNS through projection cells.
• Large Sensory Input Closes Gate: In case of a large input signal from the sensory nerves, the inhibitory neurons get activated and again prevent transmission of signals through the projection cells and we feel no pain.
• Pain Receptor Nerve Input Opens Gate: Only when there is an input signal from the small pain receptor neurons and sensory neuron input is not large enough to activate inhibitory cells, then projection cells pass the signal to the CNS (central nervous system). That is the gate opens for only pain reception and otherwise it's closed.

So if you distract yourself through muscular movement and thought to create large sensory signals, pain reception may be stopped due to closing of the gate by high input signal from the large nerve fibers! That explains why rubbing of our hand after it has been hurt, lessens the pain. Also, the descending response signals from CNS, suppress the output of the projector cells and reduce the intensity of pain. How much of pain we feel will be decided by the superimposition of signals from large sensory nerve fibers and pain receptor fibers. In this way, the gate control theory tries to explain how pain perception is affected by emotions and responses from the CNS.

The gate control theory tries to explain how the sensation of pain can be dampened or manipulated by thoughts. That may explain how some people can withstand a large amount of pain through sheer willpower! This theory helped in opening a new way of thinking about pain management and gave an insight into how some people bypass pain.