Published 19th May, 2021 by REDO  

In May 2019, chronic pain was acknowledged by WHO (World Health Organization) as an independent disease and not just a symptom of disease. This can, potentially, lead to more precise diagnoses and treatment, which takes the individual patient’s health status and needs into account (dagensmedicin.dk). But what is pain really, which processes lie behind pain, and how do these mechanisms contribute to the development of chronic pain?

Estimated reading time 7 minutes  

Pain is a warning

Thousands of Danes live with muscle- and joint pain every day. Pain is defined as: “An unpleasant sensory- and emotional experience associated with actual or potential tissue damage” (IASP). Put differently: Pain is simply the body, warning us about potential or acute dangers, such as a burning hot stove touching our skin. Pain is something we all experience. How intensively we experience pain impulses is individual, and only you can assess the intensity of your pain.

Pain can be acute or chronic

Pain can be regarded as acute or chronic pain. Acute pains are, typically, short-termed and are the result of sudden tissue damage to bones, muscle- or organs, e.g., when a nail pierces your hand. Chronic pain is defined as pain which has lasted more than three months. Chronic pain may follow a case of acute tissue damage, but chronic pain may also occur in connection with, e.g., cancer, functional- and mental illness. Pain may manifest in many different forms and to gain further insight into the development of chronic pain, we first must define the process behind pain.

How does pain occur?

Our nervous system can be compared to any other electric circuit, where sensors in the periphery constantly rapport back to an electric component, in this case: “the brain”. When we experience pain, the nerve cells, which deal with pain, are activated. The neuron cells pass the signal on to the spinal cord, which transmits the signal to the brain.

Pain detectors

Tissues around your body is scattered by nerves that solely react to pain. These nerve cells are known as nociceptors. Nociceptors send information both ways from the periphery nervous system to the spinal cord. To transmit their signals to another . the nociceptor synthesizes different molecules. Unfortunately, the same molecules may sensitize the nociceptor, reducing its threshold against painful stimuli.


At first, the nervous system will activate motoric functions, to ensure no further tissue damage (in this case, we move our hand away from the burning stove). The body will start an inflammation process, meaning the blood vessels around the wound expanses, which allows beneficial chemicals to aid the immune system in tissuse healing. In the inflammation process, the nerves are sensitized, effectively amplifing the pain signals. This is called neurogenic inflammation, and this is a part of the accute pain process. This reaction takes place in the periphery nervous system, e.g., our skin and limbs, and this process is a key factor in the development of chronic pain.

When chronic pain develops

Why some people develop chronic pain and others do not, we simply do not know for sure. Research suggests, however, that a sensitization process occurs in three different stages of the nervous system: the periphery-, the central nervous system (the spinal cord), and in the brain. A sensitization means the system becomes overly sensitive towards pain impulses.

The “relay station” of the brain

Only when the signal reaches a core area of the brain known as thalamus, does the brain begin to interpret the signal. Thalamus works as a kind of switch which transmits the signal to the relevant parts of the brain, where the information is processed. Within the brain, we find the so-called somatosensory center, which is associated with sensation – including pain impulses.
We know when pain occur in one place on the body (e.g., the right side) is processed in the opposite side of the brain (in this case the left). This matters to the way we view chronic pain-patients, because when we know the bodily location of the pain, we can specify the exact part of the patient’s brain, where the pain is processed. Therefore, it would be benificial to focus solely on this specific area, which ensures the best result.

The system’s hypersensitivity

When painful signals reach the brain, it will, under non-chronic circumstances, try to reduce these signals. The brain does this by emitting morphine-like molecules to the spinal cord, to ease the pain. The signal transmission between the nociceptors and the nerve cells in the spinal cord, is simply blocked. When a patient experiences pain over a longer period of time, this function will, however, cease, despite the periphery sensitization still being active and in spite of the wound being, potentially, healed. This essentilly drives the feeling of being in pain after the trauma has healed. Furthermore, we see a sensitization process in the brain leading to secory and motor impairments. When Thalamus is sensitized, it alters that function amongst others and the system designed to relieve pain, diminishes.


The key to improving the pain reaction of people, suffering from chronic pain, may lie in understanding that chronic pain is individual, and that the treatment of chronic pain must reflect this. The more focus we can create on the fact that chronic pain, in many cases, is driven by an oversensitive nervous system, the closer we are to creating a better and more specialized treatment of people with chronic pain.

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Historical facts about pain

Throughout the ages, pain has been viewed from many interesting angles. An invisible phenomenon which, however, has engaged the minds of many scientists and great thinkers, perhaps because pain has such a great influence on our bodies and lives.

  • 4th century B.C.

    Aristoteles describes pain as an emotion, rooted in the heart.

  • 3rd century B.C.

    The idea of our bodies as nerve networks, which affects our senses and motoric functions stems from the Greek doctor Herophilus.

  • 2nd century A.D.

    The ancient doctor Galen views the brain as a perception processor and describes pain as sensation. He emphasizes the spinal cord’s importance of our motoric and sensory durability.

  • 17th century

    The French philosopher René Descartes expands Galen’s theories about pain being transmitted through sensory nerve paths in a fixed system.

  • 1811

    Charles Bell argues that pain signals are transported to a specific center of the brain. Bell establishes 1st, 2nd, and 3rd order of neurons, as different “pain-stages”.

  • 1856

    Bell and Francois Magendie develop the so-called Bell-Magendie-law, which determines that signals about motoric and sensory fibres are transmitted via different spinal-exits.

  • 1939-45

    During the 2nd world war, field surgeons observed that soldiers reacted individually to tissue damage and they concluded that strong emotional reactions could “block out” pain.

  • 1964

    Melzack and Wall present the Gate control-theory which suggests that non-painful input closes off nerve gates to painful stimuli. This prevents pain perception reaching the central nervous system.