Reticulo- and trigemino-hypoglossal connections: a quantitative comparison of ultrastructural substrates

Axon terminals were identified and characterized by electron microscopy after injections of horseradish peroxidase (HRP) into the spinal V nucleus (SPVN) or the medullary reticular formation adjacent to the XIIth nucleus. The synaptic organization and topology of these two different populations of hypoglossal afferents (T-XII and R-XII respectively) were determined by quantitative comparisons. Significant differences were obtained in the ratios of morphological types of terminals, sizes of axonal endings and their location on postsynaptic structures. Axon terminals containing spherical vesicles (S-terminals) and those with flattened/pleomorphic vesicles (F-terminals) were anterogradely labeled with HRP from both injection sites. However, the S/F ratio for R-XII terminals was 1.2:1 compared to 2.6:1 for T-XII afferents. Asymmetrical membrane densities (Gray Type I) were the predominant form of junctional specialization for S-terminal synapses. Asymmetrical densities with subjunctional dense bodies/bars (S-Taxi) were associated with a higher proportion of T-XII synapses than R-XII synapses. Almost all of the F-terminals from both sources had symmetrical densities (Gray Type II). The average diameter of R-XII terminals was greater than that of T-XII terminals. R-XII-F terminals were the largest terminals. The majority of axon terminals from both sources formed axodendritic synapses. However, R-XII terminals had a higher incidence (10% vs. 3%) of axosomatic contacts. The proportion of R-XII-F-terminals decreased from the central toward the distal dendrites, whereas the opposite was found for T-XII-F and T-XII-S-terminals. In contrast to these findings, R-XII-S-terminals were more uniformly distributed on dendrites of all sizes.(ABSTRACT TRUNCATED AT 250 WORDS)