Medial and lateral pain systems

Current theory states that the areas of the brain involved in pain processing can be divided into 2 networks: the lateral pain system, which projects through lateral thalamic nuclei to brain regions including the primary and secondary somatosensory cortices (SI and SII); and the medial pain system, which projects through medial thalamic nuclei to brain regions including the prefrontal and anterior cingulate cortices. We have shown that the lateral pain system is responsible for the sensory aspects of pain such as its locating the pain, while the medial pain system is responsible for the emotional aspects of pain, such as how unpleasant it feels (Attention to pain localization and unpleasantness discriminates the functions of the medial and lateral pain systems. Eur J Neurosci. 2005 Jun;21(11):3133-42).

So to give you an example of how we studied this aspect of the medial and lateral pain systems: volunteers were given painful stimuli using a laser and are asked to concentrate on either the location of the pain on their arm (sensory aspect) or the unpleasantness of the pain (emotional aspect). When concentrating on a particular aspect of the pain, brain activity increased in the areas of the brain which process that aspect of the pain. Therefore, by studying brain activity during these two tasks, it was possible to identify brain regions involved in sensory and emotional aspects of pain processing.

Another example of how we study different aspects of pain is imaging the brain working just before the pain stimulus during the expectation of pain. As previously mentioned there seems to be problems with the fine-tuning of this aspect of pain in patients with chronic pain (see above).

So far, we and others have found that most of the problems with pain processing seem to be within the medial pain system. But, encouragingly, these can be, at least partially, reversed. This gives us a target for developing new non-drug and molecular therapies in the future.