Pain and the primary somatosensory cortex.

Abstract

In this issue of Pain, Uhelski and colleagues [6] report novel, provocative and potentially important observations supporting the view that sensory input experienced as aversive can access parallel afferent pathways in the central nervous system that are surprisingly independent from each other. The authors confirm that inflammation markedly reduces limb withdrawal threshold to mechanical stimulation and demonstrate that there is an accompanying aversive motivational state revealed by an increase in escape/avoidance behaviors to the applied mechanical stimulation. The novel finding in this report is that bilateral lesions of the hind limb area of primary somatosensory cortex raise the hind limb withdrawal threshold to mechanical stimulation, but do not reduce the escape avoidance behavior elicited by the same stimulus. The authors’ interpretation of these findings is that the SI lesion has reduced the sensory discriminative dimension of pain (asomaesthesia) while leaving its affective dimension intact. This finding complements the earlier work of Johansen et al. (2001) [3] who showed, using the conditioned place aversion paradigm, that lesions of the anterior cingulate cortex block the aversiveness of hind paw intradermal formalin, while leaving the localized behavioral response (i.e. withdrawal, licking) unaffected. Together, these two findings lend direct support to the idea, promoted by Melzack and Casey [5], that different dimensions of the pain experience are processed along largely separate central pathways. Further, the results argue against a hierarchical model where affective responses depend upon upstream processing of an early stage primary sensory discriminative function. As the authors point out, the parallel model is consistent with an extensive body of human functional imaging and animal lesion studies. The idea that there are at least two distinct ascending pathways contributing to pain sensation raises some very interesting questions about widely accepted concepts in the field of pain research. Perhaps most fundamental is the widely accepted distinction between the sensory discriminative and affective motivational dimension of pain. There are some conceptual problems with this formulation. Pain intensity is generally considered to be a sensory discriminative property while unpleasantness is broadly considered a measure of pain affect. I would argue that when the subjective effects of noxious stimulation are assessed using purely subjective psychophysical approaches both intensity and unpleasantness could be considered sensory discriminations [1]. While a warm temperature could be considered either pleasant or unpleasant depending upon core body temperature, it is difficult to imagine experiencing pain that is not unpleasant. This semantic quibble aside, several independent lines of evidence, including the current Uhelski paper support the notion that the nociceptive message is relayed by divergent parallel ascending pathways that access the anterior cingulate, the insula and somatosensory cortex. The evidence also supports the concept that

DOI: 10.1016/j.pain.2012.01.034

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@article{Fields2012PainAT, title={Pain and the primary somatosensory cortex.}, author={Howard L Fields}, journal={Pain}, year={2012}, volume={153 4}, pages={742-3} }