Quantitative electromicroscope study of the oculomotor parasympathetic neurons projecting to the ciliary ganglion in cats: comparison of the synaptic (axo-somatic and axo-proximal dendritic) organization of anterior-dorsal and ventral cell groups

The synaptic organization of the oculomotor parasympathetic preganglionic neurons (OPNs), labeled retrogradely after a horseradish peroxidase (HRP) injection into the ciliary ganglion, was studied in cats by electron microscopy. We divided the OPNs into two groups, anterior-dorsal (ADG) and ventral (VG) cell groups, based upon physiological studies in cats suggesting that accomodation-related OPNs are predominantly located anterior and dorsal to the somatic nuclei of the oculomotor nuclear complex (i.e., the anteromedian and Edinger Westphal nuclei, and the ventral central gray area), while pupillo-constriction-related OPNs are predominantly located ventral to the somatic nuclei (i.e., the ventral tegmental area). The synaptic organization of these two groups was quantitatively compared, using a nested analysis of variance to determine statistical significance (P<0.05). Partial reconstructions of the labeled somata and proximal dendrites were made from tracings of electron micrographs of every 2nd section in serial ultrathin sections that included the nucleolus or were adjacent to sections that included the nucleolus. The mean number of boutons of apposition on a reconstructed labeled soma of VG was significantly greater than that of ADG (mean ±SD; ADG, 5.3±3.3; VG, 8.6±3.2). The mean synaptic density on a VG soma was significantly greater than on an ADG soma (mean±SD; ADG, 3.74±2.11 counts/100 (m²; VG, 6.30±1.99 counts/100 m²). The mean synaptic covering ratio on a VG soma was significantly greater than on an ADG soma (mean±SD; ADG, 5.21±2.91%; VG, 10.14±3.76%). The mean estimated number of boutons of apposition on a VG soma was significantly greater than on an ADG soma (mean±SD: ADG, 53±36; VG, 100±48). Boutons were classified on the basis of the shape of their synaptic vesicles as S-type (containing spherical clear synaptic vesicles) or P-type (containing both flattened and spherical clear synaptic vesicles). The mean S-type/S+P-type bouton ratio on a VG soma was significantly greater than on an ADG soma (mean±SD; ADG, 0.31±0.20; VG, 0.67±0.18). The differences demonstrated in this study reinforce, morphologically, the assumption of functional localization of OPNs, and further allow us to estimate the relative characteristics of the synaptic organization of accommodation-related OPNs and pupillo-constriction-related OPNs.