Nervous System: Morphology and Physiology of Cnidarian Conducting Systems

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Cnidaria are the earliest metazoans with recognizable nervecells with typical synapses and synaptic vesicles.Cnidarian neurons are organized into networks called nervenets and nerve plexuses, as well as distinct nerve tracts.Many cnidarians possess two or more parallel, yet interacting,electrical conducting systems, whose impulses may be transmittedthroughout the body of solitary forms and into thepolyps of colonial forms.Cnidarians transmit signals between neurons and effectorcells, and between neurons, by classical chemical synapses.Gap junctions mediate electrical conduction by allowing thepassage of select, small ions between the cytoplasm of twoclosely associated cells. Nonionic electrical interaction,referred to as ephaptic transmission, between excitable cellshas also been described.Uniquely among metazoans, some cnidarians possess nonneural,epithelial conducting systems, capable of producingand conducting action potentials.Nematocysts, muscle cells and the myonemes of epitheliomuscularcells are the effectors of cnidarian coordinating systems.Typical of cnidarian transmitting systems are pacemakers thatare responsible for the periodic contraction of polyps andmedusae.Convincing evidence from many multidisciplinary studieshave shown that both classical fast (acetylcholine, glutamate,γ-amino-butyric acid [GABA], glycine), and slow (catecholaminesand serotonin) neurotransmitters, as well as peptides(FMRFamide-related peptides) and small molecules (nitricoxide, ATP and CO), are fundamental to cnidarian coordinatingsystems

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Diseases of Coral