Visceral versus somatic pain: similarities and differences.

Inflammatory bowel disease and the irritable bowel syndrome are conditions characterized by chronic pain that generates persistent, hyperalgesic states in many regions of the body. It is difficult to explain the pain of conditions such as inflammatory bowel disease and irritable bowel syndrome by extrapolating directly from what is known about the mechanisms of somatic pain. Visceral and somatic pain show many differences not only in the psychophysics of the sensation, but also in the neurobiological mechanisms that mediate the sensory process. The activation and sensitization of visceral nociceptors are heavily influenced by the secretory and motor properties of the microenvironment where the sensory receptors are located. In some cases, epithelial cells can play a direct role in the activation of primary sensory neurons. Subclinical alterations of the epithelium can contribute to enhanced visceral sensitivity. Central hypersensitivity induced by visceral activation also shows differences with its somatic counterpart. Mobilization of AMPA receptors from the cytosol to the membrane of nociceptive neurons has been identified as a mechanism of sensitization of visceral pain pathways. Finally, functional pain syndromes, such as irritable bowel syndrome could be triggered or maintained by hormonal alterations, particularly those involving sex hormones such as estrogen.