Pathobiological reactions of C‐fibre primary sensory neurones to peripheral nerve injury

Mammalian primary sensory neurones display profound anatomical and chemical changes in response to injury of their peripheral processes. Peripheral nerve damage results in transganglionic degeneration of central terminals of A‐fibre primary afferent neurones terminating in the deeper layers of the medullary or spinal dorsal horn. The depletion of neuropeptides and other neurone‐specific macromolecules from neurones in sensory ganglia and from the superficial dorsal horn, the known central projection area of C‐fibre primary afferents, is a salient phenomenon commencing after peripheral nerve lesions. A recently devised new experimental procedure, termed the capsaicin‐gap method, permitted evaluation of the transganglionic degenerative phenomena which develop in C‐fibre primary sensory neurones after a lesion is inflicted upon their peripheral branches. The experimental findings indicated that C‐fibre primary afferent terminals underwent transganglionic degeneration following a perineural treatment with capsaicin, and that this was associated with, and probably resulted from, ganglionic cell degeneration. Studies on the effects of peripheral nerve section yielded similar results. It has therefore been suggested that the observed depletion of specific macromolecules, including sensory peptides, specific glycoconjugates and sensory neurone specific acid phosphatase, may be accounted for, at least in part, by an irreversible loss of sensory ganglion neurones. In contrast, many injured neurones express peptides, including vasoactive intestinal polypeptide, peptide histidine‐isoleucine and galanin, which can be demonstrated in only a few neurones under normal conditions. These seem to be involved in dorsal horn regenerative and/or compensatory processes following peripheral nerve damage. There is suggestive evidence that the partial deafferentation caused by the transganglionic degeneration of C‐fibre primary afferents creates favourable circumstances for an anatomical rearrangement of neuronal connections within the spinal cord dorsal horn. These changes may provide a morphological substrate of some of the functional alterations demonstrated after peripheral nerve lesions.