Pain

Pain has been defined in many ways, including:

"Complex phenomenon, a signal of tissue damage, threat, an integrated defence reaction and a private experience of hurt. "
"An unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage."
"Pain is what the patient says it is and exists when he says it does."

However, it is extremely difficult to come to any exact definition of pain, because it is a very personal experience. There are wide differences in the ways in which patients describe their pain or react to the same painful stimulus. Individual patients may also react differently to the same stimulus at different times or under different emotional conditions.

The biological purpose of pain is to provide a warning to the patient of harm from outside or within the body. Pain is unpleasant, but has considerable biological survival value: if a hand is burned it is quickly removed from the source of heat, so limiting tissue damage. A few individuals are born with no ability to perceive pain and others may lose the ability to perceive it in a limb or part of the body as a result of injury to nerve or brain. Such individuals are at great risk because they may, for example, be quite unaware that they have fractured a bone after a fall and continue to use the limb causing great tissue damage. This section describes pain in terms of physiology, time scale and intensity.

Transmission of pain

Physiology of pain

The sensation of pain is a function of the nervous system, but it is a dual phenomenon. The nervous system is responsible both for the perception of the sensation and for the patient's emotional reaction to it. This is the reason for some of the difficulties encountered when trying to assess or define pain.

Perception

Scattered or clustered on the surface of the body and in the abdominal organs are sensory receptors. Stimulation of these so-called nociceptors results in a nerve signal which is conducted along the sensory nerve fibre. Impulses from the nociceptors are carried in two different types of nerve fibres, fast conducting (type A - delta) and slower conducting (type C) fibres. The former are thought to be responsible for the immediate, sharp pain of the harmful stimulus, while the latter are associated with the duller, diffuse pain that persists after the stimulus has gone. The nerve pathways are illustrated in the diagram.

Pain sensation

A region of the midbrain called the thalamus is thought to be responsible for the actual sensation of pain. Stimulation of the forebrain cortex does not produce a sensation of pain and conversely damage to the forebrain does not reduce the sensation of pain. Nevertheless, the cerebral cortex of the forebrain is thought to assist in identifying and locating the part of the body from which the pain is arising and in comparing the sensory input with past experience (ie. how dangerous is this stimulus?).

Time scale

It is possible to classify pain into two types in terms of duration. Acute pain tends to be of short duration and is largely self limiting. Chronic pain, is a major medical problem because of its duration. Chronic pain may be described as throbbing, dull, aching etc. More details are included in the section on 'Types of Pain'.

Intensity of pain

In general, the intensity of the pain is determined by the number of nociceptors stimulated to 'fire' nerve impulses, and how rapidly they fire. This in turn is related to the size of the stimulus (ie. a large burn is more painful than a small burn). However, there is a limit to the intensity of the pain because there is a maximum rate at which the nociceptor can fire. There are also systems within the brain and spinal cord which can help to block very severe, prolonged pain.