There are some conspicuous differences between the
sensibilities of cutaneous and visceral tissues: (1) Direct trauma,
which readily produces pain when applied to the skin, is mostly
without effect in healthy visceral tissue. (2) Pain that arises from
visceral tissues is initially often poorly localised and diffuse.
(3) With time, visceral pains are often referred to more superficial
structures. (4) The site of referred pain may also show hyperalgesia.
(5) In disease states, the afflicted viscera may also become
hyperalgesic. In this target article, I consider to what extent
differences in the physiology, anatomy, and chemistry of peripheral
processing systems explain these different sensibilities. In almost
every aspect, there are subtle differences in the properties of the
processing mechanisms for cutaneous and visceral information. These
may arise because of distinct developmental cues operating in the two
domains. Many of the differences between visceral and cutaneous
afferents are quantitative rather than qualitative. The quantitative
differences, for example in the density of afferent innervation, can
be large. The quantitative differences in the numbers of afferents
alone may be a sufficient explanation for some aspects of the
differential sensibility, for example, the poor localisation of
sensation and the apparent insensitivity to focal yet tissue-
damaging stimuli. In addition, the few clear qualitative differences
apparent in the innervations of the two tissue types may be of special
importance. That the encoding of visceral nociceptive events may
occur by an intensity mechanism rather than a specificity mechanism
could be the key difference in viscerosensory and somatosensory
processing.
112. Study on Deep Somatic Pain with Special Reference to Referred Pain experienced during Osteovenography of the Spine