An ultrastructural examination of dorsal root input to the sacral secondary visceral gray.

Dorsal root input to the sacral secondary visceral gray was examined in the adult cat using an orthograde degeneration technique. Five days after unilateral, intradural section of sacral dorsal roots electron-microscopic evidence of synaptic degeneration was seen throughout the nucleus ipsilateral to the side of the lesion. Degenerating sacral primary afferents in this nuclear region were associated mainly with S and GS type boutons. S type synapses showed two distinct degenerative responses. Some of the degenerating boutons were swollen with synaptic vesicles clustered tightly near the presynaptic membrane. Others appear shrunken with consequent packing of vesicles and mitochondria within the presynaptic terminal. Degenerating S terminals were found mainly on dendrites. S type synapses on nerve cell bodies appeared to be unaffected by dorsal root section. The degenerative response of sacral primary afferents associated with GS boutons was characterized by marked swelling of the presynaptic terminal and the presence of significant amounts of glycogen. Degenerating GS boutons were observed on both cell bodies and dendrites. These observations provide morphologic evidence of a direct synaptic link between dorsal root afferents and nerve cells in the sacral secondary visceral gray.