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Two Painkillers: Music and Imagination
Research has shown that imagining a happy scenario in the mind's eye, serves to lessen the sensation of pain. Also, music helps relieve pain, by causing our mind to be happily distracted.
Physical pain serves as an indicator of underlying problems in some part of the human body, while also acting as a warning signal indicating danger. Nobody needs an explanation of what pain is. Because pain, as they say, is part of being human. It is a sensation of uneasiness and physical discomfort caused due to tissue damage of any kind. Pain may also be a result of a psychologically traumatic or disturbing experience of any kind. The sensation of pain is mediated by the nerve network spread throughout the human body. Pain can be nociceptive (caused by stimulation of peripheral nerve fibers that provide sensory information), neuropathic (caused by damage to the nerves of the somatosensory system), psychogenic (caused by emotional and behavioral issues), or phantom (pain felt in body parts, that are amputated).

A hypothesis that tried to explain how pain sensation is reduced by cognitive activity in the brain or afferent nerve activity, was the Gate Control Theory. It was propounded by Ronald Melzack and Patrick Wall in 1965, through a paper titled 'Pain Mechanisms: A New Theory', in the journal named Science. Though this theory is now superseded by modern ones, it was an important breakthrough that helped motivate better models of pain sensation.

How is Pain Sensed?

The process of nociceptive pain signal transmission begins when a stimulus like a cut on the skin, burn, pressure, or any kind of harmful impact on the body occurs. 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 (CNS), through a relay mechanism, which uses a continuum of connecting nerves.

These nerves, that transmit signals from the body towards the central nervous system, are called afferent nerves. These signals are 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, the nerve response from the brain passes to the muscles of your arm to make it draw away.

All this happens within a fraction of a second. These nerve fibers 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 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, which 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 is generated and transmitted back through the peripheral efferent or descending nerves.

It has been observed that the sensation of pain can depend on the thoughts and mental state of a person. The psychological condition and the cognitive content of the mind affects how strongly we are affected by pain. The gate control theory of pain was proposed to explain this phenomenon.

Gate Control Theory of Pain

Ronald Melzack (a Canadian psychologist) and Patrick Wall (a British Physician) proposed the theory to explain how emotions and your own thinking, as well as activity of the afferent nerves 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 blocked through the gate constituted by specific cells in the spine. These cells control signal transmission through the spinothalamic tract to the central nervous system.

Here's how this theory of Wall and Melzack hypothesizes the working mechanism of pain perception and its suppression. The structure of the gate through which pain pathways send a signal to the central nervous system needs to be understood first.

The input signal pathways to this gate are of two types, which includes small nerve fibers (pain pathways) and large fibers (sensory neural pathways), which are both connected to the projection cells, that carry signals through the spinothalamic tract. The pain pathways are small myelinated (delta) fibers and unmyelinated (C) fibers, while the sensory neural pathways are myelinated nerve fibers.

The projection cell is the control element of the gate. Both the types of nerve fibers (pain receptors and sensory receptors) are connected with inhibitory interneurons, which are situated in the dorsal horn of the spinal vertebrae. They can suppress transmission through the spinothalamic tract by controlling the projection cells. There are three signal 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.

No signal input

Input From Large Sensory Fiber Closes the Gate

In case of a large input signal in the sensory nerve fiber, inhibitory neurons get activated. The projection cells are also activated, but the transmission of pain signals through them is prevented by the inhibitory neurons and the gate remains closed. Ergo, in this event, no pain perception occurs as the brain doesn't receive the signal. Clearly, the sensory signals block out pain.

Large fiber input

Exclusive Input from Pain Receptor Nerve Opens the Gate

The gate opens and pain sensation occurs, only when there is a large pain signal from the small nerve fibers, which deactivates the inhibitory neurons and causes the projection cells to transmit the signal. However, if there is a simultaneous large fiber input, the pain response will be dampened.

Small fiber input

So if you distract yourself through muscular movement and thought, to create large sensory signals, pain reception may be stopped or lessened due to closing of the gate. That explains why rubbing of a 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. The degree of pain felt, will be decided by the superimposition of signals from large sensory nerve fibers and pain receptor fibers. In this way, the theory tries to explain how pain perception is affected by emotions and responses from the CNS.

This hypothesis tries to explain how the sensation of pain can be dampened or lessened by sensory input. It was put forward on the basis of data collected from microelectrode studies of animal spines. The conclusive evidence that could promote this hypothesis to a theory was not found, as the precise mechanism that could explain the inhibitory action caused by afferent nerve input, was never found.

Nevertheless, this theory helped in opening a new way of thinking about pain management and was the primary motivation behind the development of Transcutaneous Electrical Nerve Stimulation (TENS) therapy and generated interest in procedures like acupuncture, designed to relieve pain.