A quantitative study of structural features, synapses and nearest-neighbour relationships of small, granule-containing cells in the rat superior cervical sympathetic ganglion at various adult stages.

Groups and sub-groups (clusters) of small granule-containing cells ("small cells") were analysed at 3 and 6 micron intervals and in serial sections, in rats aged 2-13 months. Fully intraganglionic clusters of small cells were all found to receive an incoming ("afferent") innervation, of the order of 3-6 afferent terminals per cell, derived from axons of preganglionic type via multifocal, symmetrical, mainly axosomatic synapses. No evidence was obtained of sharing of preganglionic inputs between small cells and principal neurones. Intraganglionic clusters also regularly gave outgoing ("efferent") synapses of the asymmetrical type, of the order of 2-6 per cell, to intraganglionic nerve elements; 30-50% of these synapses were given from somata, 50-70% from processes of the small cells. Whenever the postsynaptic structure was identifiable these synapses were all found to be given to postganglionic neurones or their dendrites, principally to spine-like processes or slender twigs. In some ganglia a few efferent synapses to other small cells were observed; these were of the symmetrical type. Efferent synapses to nerve profiles resembling chemosensory axon terminals, also of the symmetrical type, were extremely infrequent (fewer than 1% of all efferent synapses) in intraganglionic small cell groups and appeared virtually restricted to glomus-like clusters of small cell, which lay intracapsularly, or in and near the bases of nerves entering or leaving the ganglion. Almost all groups and clusters of small cells were located near to fenestrated capillary vessels, which are not found elsewhere in the ganglion. The implications of possible non-synaptic release of material from small cells via membrane regions not covered by satellite cell cytoplasm, were explored in a nearest-neighbour analysis. These "exposed" regions comprised 1-3% of the small cell surface, a proportion comparable with those engaged in receiving afferent synapses or in giving efferent synapses. The majority of such regions faced toward other nerve profiles (axons and dendrites) ensheathed in satellite cytoplasm (mean 30%), intraganglionic tissue spaces wider than 3 micron (mean, 30%) or other small cells (mean, 14%); 25% faced toward blood vessels, but of these vascularly directed regions, only one fifth (or 5% of the total) on average faced directly toward fenestrated endothelium, the rest being non-fenestrated and/or separated by pericyte processes from the exposed regions of small cell membrane. Thirty-three percent of the small cells in a sample of 242 lay within 2 micron of the nearest blood vessel.(ABSTRACT TRUNCATED AT 400 WORDS)