Primary afferent projections to the sacral secondary visceral gray: a quantitative electron microscopic examination in the cat.

Afferent input to the sacral secondary visceral gray matter was studied by quantitatively assessing synaptic degeneration that follows ipsilateral sacral dorsal rhizotomy. Five days after the lesions, slightly more than 25% of the synaptic contacts showed degenerative changes. The majority of the reactive synapses contained clear spherical vesicles (S type). Boutons containing dense core vesicles (GS type) also showed degenerative changes, and together with S boutons constituted more than 96% of the reactive terminals. Degenerating S type synapses were found only on dendrites whereas GS boutons were observed on both cell bodies and dendrites. Further examination revealed that although there were more S than GS synapses in the nucleus, the percentage of GS terminals affected was greater than that for S type boutons. These results demonstrate a direct primary afferent input to the sacral secondary visceral gray matter utilizing chiefly two different synaptic types.