Crayfish abdominal ganglia are simple systems with 850 neurons innervating a few metameric muscles and producing relatively simple behavioural responses, some of them rather well known such as the tail flip in escape behaviour. Electron microscope studies show a variety of communicatory structures, secretory organelles, structures for transcellular glial transport, as well as a great diversity in neuronal ultrastructure. Chemical synapses, electrical synapses, mixed electrical and chemical synapses and electrotonic septate junctions are the morphologically identifiable loci for neuron-to-neuron signalling. Release of neuroactive substances also occurs at non-synaptic sites, at synapsing terminals, as well as at specific terminals. Neurohormonal release at specific neurosecretory organs has also been described. The ganglia contains a large variety of secretory organelles of several sizes and morphologies: clear oval and round vesicles, but also a diversity of organelles with dense contents. The diversity in vesicles and in structures for cell signaling should be considered when behavioural responses are analyzed. In addition, a collection of structures suggesting specific transport has been described in relation to hemato-neuronal transglial transport. Finally, recent descriptions on neuronal ultrastructure are also reviewed.
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