Properties of regenerated primary afferents and their functional connections

1. The tibial and sural nerves in cats were cut, anastomosed to their distal stumps, and allowed to regenerate for 3-17.5 mo. In the terminal acute experiment, individual afferents were impaled in the dorsal root ganglion to study their receptive field properties, somatic spike parameters, and spinal projections using cord dorsum potential (CDP) measurements. Properties of the CDPs provided evidence on whether the afferent fibers were originally proprioceptive or cutaneous (rapidly or slowly adapting). 2. Fibers with the largest conduction velocity were selectively slowly adapting, suggesting that large muscle afferents maintained their adaptation properties regardless of the peripheral structure innervated. Similarly, the relationship of somatic spike configuration to mechanical threshold was largely normal. Cells with narrow spikes innervated low-threshold mechanoreceptors, whereas cells with broad spikes and an inflection on the descending limb innervated high-threshold mechanoreceptors. 3. Spikes with intermediate properties were observed in some cells that innervated low-threshold mechanoreceptors in the periphery. These were classified as "hybrid" spikes. 4. The largest CDPs were evoked by afferents classified as having originally been cutaneous fibers, regardless of whether they had reinnervated cutaneous or subcutaneous receptors. Fibers classified as has having originally been proprioceptive afferents produced much smaller CDPs; however, these afferents never produce CDPs in intact preparations. Afferents nonresponsive to peripheral stimulation, classified putatively as having been cutaneous, also evoked small CDPs. 5. Fibers classified as putatively cutaneous or proprioceptive could reinnervate foreign target tissue (subcutaneous tissue or skin, respectively), but a propensity to reinnervate the original target tissue was observed. 6. Among putative cutaneous afferents, those with rostrocaudal CDP distributions somatotopically correct for the location of their receptive fields evoked the largest CDPs regardless of the peripheral tissue innervated. 7. We conclude that receptive field properties (adaptation, mechanical threshold) of regenerated afferents are well matched with the electrophysiological properties of the soma and axon. The properties of the central projections of these afferents are not as well matched with their peripheral receptor properties. This is discussed in terms of the plasticity of the central projections of axotomized afferents.