Ultrastructure of the electrotonic and chemical components of the lateral‐to‐motor and medial‐to‐motor synapses in crayfish nerve cord

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Abstract
Lateral‐to‐motor and medial‐to‐motor synapses in crayfish nerve cords are composed of an electrical and a chemical component. The presynaptic terminals showed localized clusters of synaptic vesicles, electron‐dense areas, coated pits, and coated vesicles. In thin sections, active zones were defined by electron‐dense regions where synaptic vesicles attached and, in freeze‐fracture replicas, by clusters of intramembrane particles localized in bands with vesicle openings on the sides of these bands. The cytoplasmic surface of the postsynaptic membrane opposite the active zones was coated with electron‐dense material that in freeze‐fracture replicas was seen as an increase in intramembrane particles located in the external leaflet (EF‐face). This specialization of the postsynaptic membrane may correspond to the neurochemical receptor. Also, pre‐ and postsynaptic membranes were separated by a wider extracellular gap than those of adjacent nonsynaptic regions and electrical synapses or gap junctions. Synaptic vesicles were located exclusively at the synaptic regions by means of a cytoskeleton that was different for the electrical and the chemical components. The vesicles associated with the electrical component were anchored to a cytoskeleton composed of a beaded layer of densities located parallel to the membrane. This cytoskeleton maintained the synaptic vesicles separated from the presynaptic membrane by a distance of 13 ± 2 nm. The synaptic vesicles associated with the chemical component were anchored to electron‐dense regions formed by filaments arranged in bundles, anchored to the presynaptic membrane. Vesicles lined both sides of the bundle of filaments in very close proximity to the presynaptic membrane, probably positioned to discharge their contents into the extracellular space and to replace the discharged vesicles.