Visceral and cutaneous pain : neural correlates and pharmacological intervention

Our brain is involved in processing pain, whether it is superficial cutaneous pain, caused by a scratch or a burn, or deep internal pain, caused by heartburn or gas in the intestines. Moreover, activation of a common cortical network is suggested during different types of pain in humans, implying that as long as the stimulus is painful it will be processed similarly in the cerebral cortex. However, no one has yet made direct comparison between superficial and deep pain of similar intensity and location; direct comparison is necessary in order to see how superficial pain relates to a more clinically relevant deep pain and to further our understanding of the latter.In three separate studies, the perception of visceral and cutaneous pain in humans was examined using psychophysical, brain imaging and pharmacological approaches, respectively. The first study revealed that for a similar given intensity, duration and location, visceral pain is more unpleasant, more varied qualitatively, more diffuse and more persistent after stimulation has ended, suggesting that there are some significant distinctions in the neural processes of external and internal pain in humans. The second study examined such processes with functional magnetic resonance imaging (fMRI), disclosing substantial differences in cortical processing of sensory information from skin and viscera, including limbic areas associated with the emotional component of pain (anterior cingulate and insular cortices), and sensory areas (primary somatosensory cortex). In addition, several similar cortical areas were activated by both superficial and deep pain, consistent with the existence of a common pain network independent of the nature of pain. The final study examined a possible divergence in pharmacological processes underlying deep and superficial pain, which could arise from differences in neuronal processing. The findings revealed that NMDA-receptors mediate both visceral and cutaneous pain in humans, yet the affect of visceral pain might be more susceptible to their blockers, which may be a potential explanation for different treatments of visceral and cutaneous pains.Together these studies provide direct evidence of the differences and similarities between visceral and cutaneous pain in humans within the perceptual, physiological and pharmacological domains