Chemical synapses, particle arrays, pseudo‐gap junctions and gap junctions of neurons and glia in the buccal ganglion of Helisoma

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Abstract
The nervous system of the snail, Helisoma trivolvis, has been utilized for a wide range of studies of neuronal plasticity; however, the ultrastructural features of this tissue were perviously unknown. The present study examined the nature of synaptic interactions of neurons and glia and considered several plasma membrane specializations of these cells. The symmetrical pair of buccal ganglia consisted of a ring of unipolar neurons surrounding a central neuropil. The neurons were separated by two morphologically distinct types of glia: type I were most numerous and possessed an electron-dense homogenous cytoplasm, whereas type II glia were of lower electron density, possessed a heterogenous cytoplasm, and appeared to be phagocytic. Gap junctions were abundant between glia and were occasionally found between neuronal processes, including those of neurons 19 injected with horseradish peroxidase (HRP). Comparison of neuron and glial gap junction widths (16.4 and 17.6 nm, respectively) in thin sections and their intramembrane particle diameters (13.1 and 13.7 nm, respectively) by freeze fracture, did not elucidate significant differences. A heterogenous population of putative chemical synapses, similar to those reported in other molluscs, was also observed between axonal collaterals in the neuropil. Additionally, examination of freeze-fractured neuropil revealed rhombic arrays of particles localized on neuronal membranes; these arrays do not appear to form intercellular junctions but may represent postsynaptic receptor sites. Freeze fracture also revealed small, square arrays consisting of 7–9 nm diameter particles on glial membranes which may correspond to pentalaminar membrane contact (pseudo-gap junctions) seen in thin sections between glia situated around dilated extracellular spaces (lacunae).