Changes in dorsal horn synaptic disc numbers following unilateral dorsal rhizotomy

The present study estimates the numbers of synaptic discs and numbers of degenerating synaptic terminals in laminae I–IV of the rat S2 dorsal horn ipsi‐ and contralateral to unilateral dorsal rhizotomy. These data allow us to estimate the loss of synapses of primary afferents and to correlate this loss with the rate of axon disappearance in the proximal stump of a transected S2 dorsal root. Our first findings are that 47% of the ipsilateral synapses and 27% of the contralateral synapses disappear within a day following unilateral rhizotomy. Conclusions are that the predominant synaptic population in this part of the rat spinal cord is of primary afferent origin and that there is an extensive bilateral projection of the dorsal root fibers. The contralateral projection is confirmed by the appearance of numerous degenerating terminals on the contralateral side. We also find that synaptic loss and appearance of degenerating terminals occur relatively synchronously in laminae I–IV. Finally we find that the time course of the synaptic loss correlates primarily with the disappearance of unmyelinated fibers in the proximal stump of the transected dorsal root.