Secretory activity and postembryonic development of the tentacle sensory system controlling growth hormone-producing neurons in Lymnaea stagnalis

A pattern confirmed and refined--synaptic, nonsynaptic and parasynaptic exocytosis

Neurons are now known to produce a variety of types of chemical transmitters. Classical transmitters are stored within synaptic vesicles which undergo synaptic exocytosis in association with presynaptic thickenings. The larger, dense-cored secretory granules present in most neurons contain neuropeptides and mainly discharge their contents at morphologically undifferentiated (i.e. nonsynaptic) sites. The synaptic character of vesicle discharge enables transmitters to exercise a highly focal action, whereas nonsynaptic release probably relates to the slow rate of degradation of many neuropeptides and their consequent widespread diffusion and sphere of action. However, one variant of the basic pattern, involving the restriction of granule discharge to areas of the terminal plasmalemma situated adjacent to the postsynaptic cells (i.e. a parasynaptic configuration), enables a degree of targeted peptide discharge to be achieved. The diversity of patterns of neural exocytosis adds a further dimension to the complexity of nervous function.