Synaptic organization in the spinal and rubral fiber-recipient regions in the inferior olive (IO) of the cat was examined electron microscopically; the spinal fiber-recipient regions were the caudolateral portions of the medial accessory olive (MAO) and the lateral portions of the dorsal accessory olive (DAO), and the rubral fiber-recipient regions were the middle portions of the dorsal lamella of the principal olive (PO).
The density of axosomatic (AS) synaptic terminals (the number of AS terminals per 100μm length of somatic membrane) was calculated on each somatic profiles cut through the nuclear plane. It ranged from 0 to 12.0 with an average of 2.4±0.32. No correlations were recognized between the cell size and the density of AS terminals. About 25% of AS terminals were filled with round synaptic vesicles (Rtype), and 75% contained pleomorphic synaptic vesicles (P-type). About 60%axodendritic (AD) axon terminals were of R-type, and 40% were of P-type.
In the cats allowed to survive for 2 to 7 days after hemicordotomy at the 2nd cervical cord segment, or after placing lesions in the red nuclear regions, electronaxon terminals were seen ipsilaterally in the MAO and DAO regions, or in the PO regions, respectively. The vast majority of these degenerated axon terminals were in contact with dendritic profiles and appeared to be of R-type. Although electron-dense axon terminals never exceeded more than 7% of the total population of axon terminals in the IO regions examined, the ratio of AD terminals of R-type to those of P-type was decreased in the IO regions deafferented from spinal or mesencephalic fibers for 35 days.