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Twenty-five physiologically identified spinocervical tract (SCT) neurons in the sixth lumbar segment of the cat were filled with HRP by intracellular injection. All were reconstructed from sagittal sections using the camera lucida, and a subset (n = 18) was also reconstructed using a computer reconstruction system. Thirteen cells were in intact preparations, nine were in spared root preparations (L5, L6, S1, S2 cut; L7 spared), and three were in preparations with L5 through S2 cut. Analysis of the dendritic tree of these neurons revealed little change in gross morphology after partial deafferentation despite increased proportions sensitive to nociceptive input (Sedivec et al., 1983). The dendrites still largely respected the lamina II-III border, and relatively few dendrites were directed ventrally from the cell body, although the ratio of ventral to dorsal dendrites was greater than normal. The major change was an increase in surface area and volume caused by changes in diameter (but not length) of the dendrites. Larger- than-usual maximum branch order of individual dendritic trees of some cells was also observed after chronic deafferentation. Thus, SCT cells in deafferented segments do not undergo atrophy, but show, rather, limited signs of growth and the possibility of dendritic reorganization. We have also computed correlations between different parameters of these cells (cell body size, number and size of primary dendrites, total area and length of individual dendrites) and have found that, as in motoneurons, diameter of the primary dendrite measured 30 micron from the soma is significantly correlated with total dendritic surface area and length. SCT neurons tend to have more dendrites than spinal alpha-motoneurons, but total surface area is smaller for a given diameter of a proximal dendrite.

Recordings from antidromically identified spinocervical tract (SCT) cells in the partially deafferented spinal cord of chloralose- anesthetized cats have revealed reorganization of their input from peripheral receptors in the skin. Immediately after dorsal rhizotomy of segments L5, L6, S1, and S2, (sparing L7), units in L6 had a lower than normal probability of responding to moderate pressure, mechanical nociceptive, noxious heat, and cooling inputs. This persisted for about 6 weeks, after which normal or even larger than normal proportions of SCT units could be driven by these inputs. The proportion of units driven by hair deflection remained large throughout this period. Responses in the L7 segment with intact L7 dorsal root were unchanged. We suggest that afferent fibers from different receptor types differ in the extent of their normal functional projection, accounting for the selective changes after acute deafferentation. The chronic changes may reflect a differential ability of surviving afferent fibers to undergo compensatory changes in their projection (e.g., sprouting) after chronic rhizotomy.