Autonomic neuroscience : basic & clinical
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In clinical practise, the notion that some complex regional pain syndromes (CRPS) are associated with sympathetic facilitation of nociceptive transmission is widespread. However, physiological increases in cutaneous sympathetic nerve activity have not been found to influence the firing properties of cutaneous polymodal nociceptive (high-threshold mechano-heat sensitive) fibers in human subjects. Whether the same applies to low-threshold cutaneous mechanoreceptors is not known. ⋯ Two afferents showed no change, but nor was there significant vasoconstriction in these recordings. Thus, arousal stimuli reduced rather than augmented tactile afferent firing. The close relation to blood flow for all types of afferents, and the different responses among SAII afferents, suggest that sympathetically mediated changes in afferent firing properties are indirect, i.e. secondary to changes in the mechanoreceptors' tissue environment rather than to a direct sympathetic effect on the endings.