The distribution and connections of serotonin-immunoreactive fibers in the cell body region and neural sheath of the central ganglia of the snail, Helix pomatia, have been examined. The cell body region of the ganglia is supplied by an extremely dense network of varicose serotonin-immunoreactive fibers which surround neuronal perikarya in the ganglia. Immunoreactive processes also run to the neural sheath of both the ganglia and the peripheral nerve roots, forming a dense network. Electron microscopy revealed five different connections of serotonin-immunoreactive varicosities, according to their target: (i) non-specialized contacts with neuronal perikarya; (ii) non-specialized contacts with axon processes on the surface of the peripheral nerve roots; (iii) non-specialized neuromuscular connections with smooth muscle fibers in the neural sheath; (iv) varicosities engulfed by glial processes in both the cell body region and neural sheath; (v) varicosities embedded in the connective tissue elements of the sheath either partly or completely free of glial processes. In all cases of appositions no membrane specializations could be observed on either site of the contacts. These observations provide morphological evidence for non-synaptic regulatory actions of serotonin-containing neurons in Helix central nervous system: (i) modulation of the activity of neuronal perikarya; (ii) involvement in neuromuscular regulation; (iii) neurohormonal modulation of peripheral processes by release through the neural sheath.
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