Fine structure of the electrotonic synapse of the lateral giant axons in a crayfish (Procambarus clarkii).

The electrotonic synapse of lateral giant axons of the crayfish was studied by conventional thin sectioning. The most prominent membrane specialization observed in this synaptic region is the communicating junction. It is characterized by a close apposition of the two axonal membranes which are separated by a 4--5 nm wide gap. Other characteristics of the junction are an array of particles spaced about 20--22 nm apart and a row of vesicles symmetrically arranged at the cytoplasmic leaflets of each membrane. The communicating junction does not cover the entire surface of the electrotonic synapse. Indeed, we have found other specializations such as: finger-like Schwann cell processes extending between synaptic membranes, saccular invaginations of one synaptic membrane into its axon, and coated vesicles continuous with one of the membranes. In addition, large vesicular pieces of the communicating junctions, with their accompanying vesicles, appeared to extend deeply inside the axoplasm. The morphological appearance of the communicating junction is found to be different from the one reported for mammalian maculae communicans such as liver or heart muscle. This is surprising because, regardless of their morphological differences, both junctions seem to transmit electrotonically.