Fine structure of the rubrospinal terminals in the cervical cord of the cat

Rubrospinal fibers in cat do not terminate on anterior motor horn cells in the spinal cord but on the interneurons, mostly in the lateral portion of Rexed's lamina of V-VII, the so-called Lateral Basal Region. The ultrastructure of rubrospinal terminals at the cervical cord level (C8) was investigated in 8 adult cats with partial or complete stereotaxic lesions of the red nuclei. Animals were sacrificed from 1 to 7 days after the lesions were made and the earliest forms of degeneration found were infrequent dense preterminal axon changes and clumping and coalescing of synaptic vesicles in the terminals. Multiple dense or polymembranous inclusions were seen in some terminals and these contained vesicular profiles; dense terminal shrinking was seen in others after 24 hours. Neurofilamentous proliferation appeared in some degenerating terminals after 48--72 hours. Degenerating terminals were related to different sized dendrites to determine regional axodendritic specificity for rubrospinal endings on LBR neurons.