Ultrastructural identification of degenerating boutons of monosynaptic pathways to the lumbosacral segments in the cat after spinal hemisection

Summary Electrophysiological results suggest that the supra-spinal monosynaptic pathways to the lumbosacral motoneurones in cats are located in the vestibulospinal and reticulospinal tracts. In the present study these pathways were interrupted in the upper thoracic region by hemisection of the spinal cord. The boutons undergoing degeneration were then studied both qualitatively and quantitatively by means of serial and single sections of selected areas of the ventral horn in spinal cord segments L 6, L 7 and S 1. The degenerating boutons were distributed over the cell soma and larger dendrites (but not the most proximal portions) of the motoneurones. Two days postoperatively boutons showing signs of degeneration were readily identified in the latero-ventral motoneurone nucleus and Rexed's lamina VIII. These boutons which were swollen had a greatly increased number of neurofilaments and a decreased number of synaptic vesicles. Five days postoperatively these boutons which were now shrunken, contained swollen mitochondria and a dense, granular material. Between 5 and 8 days post-operatively the boutons were invaded by glial cell processes and digested. A small percentage of the F- and S-type boutons (approximately 0.5 and 3.0% respectively) were identified as forming the terminals of the experimentally interrupted pathways. This study did not exclude the possibility that a proportion of these boutons were the terminals of the propriospinal, monosynaptic fibers derived from the cervical region of the cord.On sabbatical leave from the Department of Zoology, University of Melbourne