Abstract
The sensation of pain is carried by A-delta and C afferents to the superficial spinal dorsal horn, while the non-nociceptive information is carried by A-beta fiber to the deep dorsal horn. Inflammation induced by CFA injection to the rat hind paw triggered sprouting of subpopulation of A-beta afferents into the superficial laminae, in particular lamina II, so called substantia gelatinosa (SG). The sprouted A-beta afferents retracted back to deep laminae in 3 to 4 weeks after inflammation that was consistent with a disappearance of pain behavior. On the other hand, the majority of SG neurons received A-beta afferent inputs in an immature condition with lacking of C afferent inputs, because of significant delay of innervation of dorsal horn neurons with small afferents in the spinal cord during development. In analogy with the fact that a change of multiple innervaion in immature to single innervation of skeletal muscle by motor fibers in mature state switches to multiple in pathological conditions, the sprouting of A-beta afferents following inflammation and retraction of the afferents to deeper laminae would be a part of regenerative processes of the sensory pathway. [J Physiol Sci. 2007;57 Suppl:S37]
