ARE THERE FUNDAMENTAL DIFFERENCES IN THE PERIPHERAL MECHANISMS OF VISCERAL AND SOMATIC PAIN

There are some conspicuous differences between the sensibilities of cu taneous and visceral tissues: (1) Direct trauma, which readily produce s pain when applied to the skin, is mostly without effect in healthy v isceral tissue. (2) Pain that arises from visceral tissues is initiall y often poorly localised and diffuse. (3) With time, visceral pains ar e often referred to more superficial structures. (4) The site of refer red pain may also show hyperalgesia. (5) In disease states, the afflic ted viscera may also become hyperalgesic. In this target article, I co nsider to what extent differences in the physiology, anatomy, and chem istry of peripheral processing systems explain these different sensibi lities. In almost every aspect, there are subtle differences in the pr operties of the processing mechanisms for cutaneous and visceral infor mation. These may arise because of distinct developmental cues operati ng in the two domains. Many of the differences between visceral and cu taneous afferents are quantitative rather than qualitative. The quanti tative differences, for example in the density of afferent innervation , can be large. The quantitative differences in the numbers of afferen ts alone may be a sufficient explanation for some aspects of the diffe rential sensibility, for example, the poor localisation of sensation a nd the apparent insensitivity to focal yet tissue-damaging stimuli. In addition, the few clear qualitative differences apparent in the inner vations of the two tissue types may be of special importance. That the encoding of visceral nociceptive events may occur by an intensity mec hanism rather than a specificity mechanism could be the key difference in viscerosensory and somatosensory processing.