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

McMahon, Stephen B. ARE THERE FUNDAMENTAL DIFFERENCES IN THE PERIPHERAL MECHANISMS OF VISCERAL AND SOMATIC PAIN?.

Short Abstract:

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 which 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. I consider to what extent differences in the physiology, anatomy and chemistry of peripheral processing systems might 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 especially 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 viscero-sensory and somato-sensory processing.

Long Abstract:

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 which 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. I consider to what extent differences in the physiology, anatomy and chemistry of peripheral processing systems might 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 especially 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 viscero-sensory and somato-sensory processing.

Keywords:	visceral pain, somatic pain
Subjects:	BBS Special Issues: Controversies in Neuroscience: V - Persistent Pain Psychology: Cognitive Psychology Neuroscience: Neuroanatomy Neuroscience: Neurochemistry Neuroscience: Neuroendocrinology Neuroscience: Neural Modelling Neuroscience: Neurology Neuroscience: Neuropharmacology Neuroscience: Neurophysiology Neuroscience: Neuropsychology
ID code:	bbs00000521
Deposited by:	S McMahon on 02 May 2001