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 termina l acute experiment, individual afferents were impaled in the dorsal ro ot ganglion to study their receptive field properties, somatic spike p arameters, and spinal projections using cord dorsum potential (CDP) me asurements. Properties of the CDPs provided evidence on whether the af ferent 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 ma intained their adaptation properties regardless of the peripheral stru cture innervated. Similarly, the relationship of somatic spike configu ration to mechanical threshold was largely normal. Cells with narrow s pikes innervated low-threshold mechanoreceptors, whereas cells with br oad spikes and an inflection on the descending limb innervated high-th reshold 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 l argest CDPs were evoked by afferents classified as having originally b een cutaneous fibers. regardless of whether they had reinnervated cuta neous of subcutaneous receptors. Fibers classified as having originall y been proprioceptive afferents produced much smaller CDPs; however, t hese afferents never produce CDPs in intact preparations. Afferents no nresponsive to peripheral stimulation, classified putatively as having been cutaneous, also evoked small CDPs.5. Fibers classified as putati vely cutaneous or proprioceptive could reinnervate foreign target tiss ue (subcutaneous tissue or skin, respectively), but a propensity to re innervate the original target issue was observed, 6. Among putative cu taneous afferents, those with rostrocaudal CDP distributions somatotop ically correct for the location of their receptive fields evoked the l argest CDPs regardless of the peripheral tissue innervated, 7. We conc lude that receptive field properties (adaptation, mechanical threshold ) of regenerated afferents are well matched with the electrophysiologi cal properties of the soma and axon. The properties of the central pro jections of these afferents are not as well matched with their periphe ral receptor properties. This is discussed in terms of the plasticity of the central projections of axotomized afferents.