SUMMARY

Pain is an almost universal experience whose perception has been a speculative matter since the most ancient societies. Once magical and religious explanations were left aside, the study of physiology of pain in Greece gave two theories which have been accepted during many centuries. The philosopher Aristotle believed that the heart was the centre of all sensations, including pain. By contrast, Plato, and later Galen, were convinced that the brain was responsible for pain perception and there were followers of each theory in the next centuries. The status quo started to change with the new ideas provided by the French philosopher René Descartes in the XVII century, who gave a new view of the problem in his posthumous book Traité de l’homme (1664). In this work, Descartes also added an example of his dualism to separately consider the body (as a machine) and the mind. This separation had a profound influence in the medical practice during the next centuries. Later, the scientific ideas of the XIX century gave birth to two new concepts on pain physiology: the specificity theory, which followed Descartes’ principles regarding the existence of specific tracks for pain sensation, and the intensity theory, which defended that any sensation other than pain may be painful if given enough intensity.

In 1965 Ronald Melzack and Patrick Wall published a new theory to explain pain, which was named the gate control theory. These scientists wrote that there was an inhibitory mechanisms at spinal cord level that would inhibit the entry of painful stimuli. This mechanism could be triggered, for instance, by touch. Later, the possibility of an inhibitory mechanism coming from brain structures was also accepted. Even though the theory contained several mistakes, it allowed pain to be considered under a new perspective.

Some years later, the limitations of the gate control theory were counteracted with the proposal that there were several types of pain depending on the pathophysiological mechanism which was implied in its initiation and maintenance. Thus, physiological, inflammatory, neuropathic and functional pain were then considered in order to explain the different clinical conditions seen in medical practice. However, these models did not allow to explain some bizarre situations, such the phantom limb pain. In order to understand what happened in this situation, Ronald Melzack suggested the neuromatrix theory which implies the existence of specific neural circuits in the brain related to pain inputs from the periphery. These circuits may also be activated by brain areas to convey the pain perception to the consciousness.

These theories, considered together or separately, allow us to understand how we perceive painful events and how we can modulate them using exogenous substances (analgesic drugs) or endogenously by activating specific neural circuits. In recent years, it has also been established how personal specific characteristics, such as psychological, biological or sociological ones, may strongly influence pain perception. For instance, placebo effect should be outlined, as this mysterious phenomenon is being increasingly known as some neuroimaging and neurochemical studies are giving many clues on how it behaves in people with pain.

From all the evidences that have been collected in the last decade, it should be recognized that pain can not be only considered as a pure sensation, but as a sensorial and emotional experience that it is also influenced by social factors, such as education, culture or expectations. Given its complexity, it has been suggested that the biopsychosocial model of disease should be applied to its better understanding. As a consequence, unimodal treatments are expected to fail in complex chronic pain and a multimodal approach should be tried in patients suffering from it.