Issues Magazine

The Experience of Pain

By Sue King

What is pain, why does it happen and why do some people continue to experience pain when pain subsides in other people?

Pain is a common symptom that accompanies many accidental injuries, conditions and diseases – indeed it is the most usual reason people seek health care – and it is an expected outcome of surgery.

Acute pain was an evolutionary necessity for human survival. Some people are born without a pain system, and their lives are generally short because of complications associated with trauma or injuries.

For the majority of us who do experience pain, the pain settles when the injury heals or the disease or condition is successfully treated. For some unlucky others it doesn’t, and we struggle to understand why this is.

Explaining Pain

Explanations for pain have varied over the millennia; religious, philosophical and scientific accounts have all been used. The English word “pain” is derived from the Latin word poena meaning “punishment” or “penalty”. In Roman mythology, Poena was the spirit of punishment and the servant of Nemesis, the Greek goddess of divine retribution. These origins may help explain why pain was conceived as a punishment for sins or evil during the Middle Ages, reflecting the “wrath of God” of religious belief during those times.

One of the earliest scientific explanations for pain was proposed by French mathematician and philosopher René Descartes in the 17th century. Somewhat ahead of his time, Descartes conceptualised pain as a sensation generated by tissue injury and conveyed by specific nerves to the brain. His explanation was: “if for example fire comes near the foot … just by pulling at one end of a rope makes to strike at the same instant a bell which hangs at the other end”.

In 1973, the International Association for the Study of Pain (IASP) was established to bring together clinicians and scientists in the field of pain to improve knowledge and clinical management of pain. A taskforce of IASP members worked on developing definitions for a number of key terms, including a consensus on a definition of pain itself. It took four years (see box: Defining and Categorising Pain).

Organ of Pain

In Descartes’ Cartesian model, the pain system was hard-wired – the intensity of pain was thought to reliably reflect the amount of tissue damage, with the brain as the alarm bell. Unfortunately, Descartes’ conceptualisation incorrectly described the role of the brain in pain. More than an alarm bell, the brain is a complex filter and is the key to pain as a sensory and emotional experience.

Pain certainly involves sensory nerves. Sensory nerves have a range of functions and convey messages about how something feels, such as if it is smooth or rough, warm or cold. Sensory nerve fibres called nociceptors sense and transmit signals about “noxious” stimuli that are unpleasant and potentially harmful. Noxious stimuli can be anything from extremes of heat or cold, chemicals such as acid, to mechanical stimuli, such as during surgery. Once activated by stimuli, nerve fibres transmit signals to the central nervous system via the spinal cord, ending in the brain. This is called nociception.

Although pain is usually associated with tissue damage, there can be pain even if there is no tissue damage. An example of this is phantom limb pain, which is common after amputation. Here the brain generates experiences in the absence of real-time sensory inputs from the missing limb.

The brain is the organ responsible for controlling many systems in the body, including pain, which could be considered an output of the brain. It is the brain’s role to make sense of and respond to the perceived threat of tissue damage, which is usually (but not always) brought with nociception. The brain has to gather all the information in order to decide the nature of the threat and how best to respond, so it draws on previous exposure to pain, a person’s culture, knowledge, as well as sensory indicators.

Different regions of the brain are involved simultaneously, and together they:

  • enable the body to respond physically by increasing the blood pressure, heart rate, breathing, sweating, mobilising energy stores, reducing gut activity and increasing vigilance (the body’s “fight or flight” mechanism) via the sympathetic nervous system;
  • effect an emotional response, such as anxiety or fear; and
  • assess the incoming messages. What does this pain mean? Is this dangerous?

This combination leads to a response. It may be to withdraw from the painful stimulus or even ignore it.

One amazing feature of the brain is that it has an inbuilt system to attenuate or “turn down” incoming tissue damage messages. It is by activating this system that most of the drugs we use, including morphine-like drugs, work.

The emotional centres and immune system are closely linked to this pain relief mechanism. Together, the physical, emotional and cognitive factors influence the tissue damage message to either amplify or diminish a person’s pain intensity and their overall experience and response to pain.

Why Does Pain Persist in Some People but Not in Others?

Acute pain is an adaptive response that motivates actions to protect and avoid similar encounters in the future. Chronic or persistent pain, defined as lasting three or more months, is much more complex. Chronic pain occurs when repeated noxious stimuli change the nervous system itself, and psychological and social changes can facilitate the maintenance of pain. “Plasticity” and “central sensitisation” are words used to describe the nervous system and its ability to change, including in response to pain.

With chronic or persistent pain, the brain’s control mechanism has often failed and the pain system does not return to normal but remains in injury mode or on high alert. In the brain, altered processes change the way a person with persistent pain thinks about and responds to their pain. Behavioural changes in the person’s attempts to relieve the pain include fear, avoidance, catastrophising or hypervigilance.

The only way we can know about someone else’s pain is to hear about it or observe behaviours that suggest pain, such as groaning, rocking, limping and rubbing. For some people with persistent pain, the emotional responses and pain behaviours become more extreme as the distress of their situation is communicated to family, friends or healthcare professionals.

Why some people end up with persistent pain while others don’t remains elusive. It is certainly not because of a “pain personality”. Such are the pathological changes that chronic or persistent pain is now regarded by many in the field of pain as a disease in its own right rather than just a symptom.

The Role of Genetics in Pain

Pain, whether it is acute or chronic, is a significant health problem. Despite many advances and new strategies, unacceptably large numbers of people continue to be denied responsive, timely and humane management for their pain.

There is growing interest in a role for genetics in influencing how noxious stimuli are processed and how the brain constructs pain differently for individuals. Research into differences in pain sensitivity, response to analgesics after surgery and genetic predisposition to pain, along with other clinical factors, is showing promising results. It may assist with future understandings of pain and its management.