Dorsal root projections to the motor nuclei in the cat spinal cord

The dorsal root projections to the motor nuclei of the L 7 spinal cord segment were examined with light and electron microscopy in cats surviving dorsal rhizotomies of L 6 –S 2 for periods of 2–15 days. Light microscopy demonstrated maximal Nauta‐Gygax and Fink‐Heimer degeneration around large neurons and their dendrites of the motor nuclei within the four to eight day period. Electron microscopy revealed axonal degeneration without any accompanying synaptic knob degeneration. Counts of the synaptic populations in the 12 and 15 day animals revealed that one particular synaptic type, the M knob, occurred less frequently and was smaller in size. In the normal animals the M knob, which was present up to 8 days postoperatively, was a large terminal containing rounded vesicles and making multiple synaptic contacts which had specialized subsynaptic densities. It was found primarily on proximal dendrites of large neurons and was often accompanied by a smaller flattened vesicle knob. All other synaptic knobs described in the normal animal were intact throughout all postoperative periods. The possibility that dorsal root afferents synapse farther out on the motoneuronal dendritic tree, as well as the possibility that few dorsal root terminals synapse on the motoneuronal surface, is discussed. The lack of any morphological evidence for presynaptic inhibition of dorsal root afferents in lamina IX is also discussed and possible alternative mechanisms are considered.