Section 7
Chapter 6,558

Synaptology of the central nervous system of the freshwater snail Lymnaea stagnalis (L.) , with particular reference to neurosecretion

Roubos, E.W.; Moorer-van Delft, C.M.

Cell and Tissue Research 198(2): 217-235


ISSN/ISBN: 0302-766X
PMID: 466668
Accession: 006557186

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In the CNS of the freshwater snail L. stagnalis 3 types of interneuronal contacts can be distinguished by EM: true synapses, synapse-like structures (SLS), and spinules. Use of the EM specimen tilting stage reveals numerous true synapses. Both terminal and en passant contacts occur on neurons and on glial cells. Bigeminal synapses are also present. Complex (combined) convergent and divergent synaptic arrangements are found. On the basis of the morphology of presynaptic vesicles 7 types of true synapses can be discerned. Histochemical data on the contents of the vesicles are lacking. Vesicle morphology suggests that type IV is aminergic and type VII cholinergic. Terminal and en passant SLS may penetrate deeply into neuronal somata and large axons, and into glial cells. A cluster of synaptic vesicles is present in the presynapse-like element. Spinules (spine-coated evagination-invagination specializations of the plasma membranes of 2 adjacent neuronal elements) are observed between somata, between axons and between soma and axon. The neurosecretory Light Green Cells (LGC) and Caudo-Dorsal Cells (CDC) receive complex synaptic input. Type V true synapses, 2 types of SLS, and spinules contact the LGC. The complex morphology of the relationship between type A SLS and LGC, studied in serial sections, reveals that adjacent glial cells are also contacted by type A SLS. Type II true synapses, 3 types of SLS, and spinules are identified on the CDC. The validity of the methods of identification and classification of interneuronal contacts in the CNS of L. stagnalis as well as the role of these contacts in the regulation of the activity of ordinary neurons, neurosecretory cells, and glial cells is discussed.

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