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1. Four of the 850 neuron cell bodies of the crayfish third abdominal ganglion contain large dense secretory granules. 2. The processes of these cells form a neurohemal organ in the dorsal perineurium/neurilemma in the ganglion. 3. None of the immunocytochemically identified peptides accounts for the observed distribution of granules.

Data reported up to now on neuron-glia relationships, show that neuron cell bodies and glial cells are separated by a narrow intercellular cleft which is considered as the microenvironment of the nervous system, and where neuron-glia exchange occurs. We present here evidence that neuron perikarya and ensheathing glial cells of the abdominal ganglia of the crayfish communicate through gap-like junctions. These junctions could constitute short circuits for ionic exchange between neuron perikarya and glial cells, probably with some degree of electrotonic coupling between neurons and glia. In the preparation described here, the intercellular cleft would play only a secondary role in neuron-glia communication.

In the crayfish neuropile, dense-cored vesicles (DCV) have been found in chemical terminals, mixed in with round or pleomorphic agranular synaptic vesicles, as well as in electrical terminals and neurohemal endings. DCV release their content at unspecialized non-synaptic sites. The simultaneous exocytosis of DCV and synaptic vesicles seems to be the rule in chemical terminals. DCV in specific terminals suggest non-synaptic communication. In chemical and electrical terminals, the content of DCV could have a neuromodulatory function.

Under normal conditions crustacean muscles consist of a mixed population of muscle fibres, some show only delayed rectification upon depolarization, while others display graded or all-or-none action potentials. It is now well documented that these action potentials are due to a tetrodotoxin (TTX)-insensitive voltage-dependent Ca2+ influx. Here we present evidence that several weeks after axonotomy of the motor nerve and removal of an abdominal ganglion in crayfish there was an increase of the number of slow flexor muscle fibres showing action potentials. Some of these action potentials were dependent on Na ions and were TTX-sensitive. These results suggest that after operation voltage-dependent Na+ channels become apparent in the muscle fibre membrane of the crayfish.

1. Glial cells of the crayfish abdominal ganglia have been studied by transmission electron microscopy. Special attention is paid to the interrelationships between neurons and glial cells. Covers and hemocyte-related elements have also been considered. 2. Glial cells are identified by a common ultrastructure and close relationships with neurons. Four glial classes are considered, depending on their morphology, the compartment of neurons they ensheathe and neuron-glia interface. 3. Four ultrastructural classes of neurons are proposed. They differ in geometry and ultrastructure, as well as in glial covers (complexity and evaginations into the neuron somata). The morphology and organization of glial covers is specific for the neuron type they ensheathe. Specific glial covers do not differ in glia-glia communicatory structures. 4. The morphological and metabolical compartments of neurons are separated from the extracellular matrix or blood by specific glial systems. A system of two cells is interposed between neuron somata and hemolymph or the extracellular matrix. 5. Glial processes are crossed by membraneous tubular systems, at neuron perikarya and axons. Frequent gap junctions of varying area, density and number of IMP are found in the covers of neuron somata. 6. Neuron-glia interface bears numerous communicatory structures for both ionic and macromolecular exchange. They include junctions and transient modifications of membranes. Some of them suggest active transport mechanisms. 7. Modified endocytotic mechanisms seem to be responsible for the glia-to-neuron transfer of macromolecules as well as for the neuron-to-glia transfer of lamellar bodies. 8. The neuropil is divided into glomeruli (electrical or chemical) by glial processes and the trabeculae of the extracellular dense matrix. Neuron-glia membrane appositions have been found in electrical glomeruli. In chemical glomeruli, dense cored vesicles can release their content at neuron-neuron or neuron-glia intercellular cleft, at non-synaptic loci. 9. Neurons of type II contain peripheral complex Golgi systems, associated to subsurface cisternae and neuron-glia gap junctions, suggesting a cooperation of glial cells in specific macromolecular synthesis.