Under physiological conditions there is no interaction between the sympathetic and the afferent nociceptive system; stimulation of the sympathetic trunk does not induce any activity in afferent neurons.65,79 However, under pathophysiological conditions the situation dramatically changes.80 Neurophysiological and neuroanatomical experiments in animals show that a pathologic coupling of sympathetic and afferent activity may follow a mechanically induced peripheral nerve lesion. This may take place between sympathetic fibers and regenerating or intact nociceptive C-fibers in the periphery, or between sympathetic vasoconstrictor fibers and afferent somata within the dorsal root ganglion.81 The interaction is chemically via noradrenaline from sympathetic endings and adrenoreceptors that are expressed on afferent neurons under pathophysiological conditions (Figure 4A). Accordingly, mRNA for alpha2A-adrenoceptors is up-regulated in DRG neurons after nerve lesion.82
Complex regional pain syndrome – diagnostic, mechanisms,
CNS involvement and therapy
G Wasner1 (#aff1) , J Schattschneider1 (#aff1) , A Binder1 (#aff1) and R Baron1 (#aff1)
Spinal Cord (2003) 41, 61–75. doi:10.1038/sj.sc.3101404