The ultrastructural organization of peripheral nerves in two insect species ( Periplaneta americana and Schistocerca gregaria)

Abstract

The ultrastructural organization of various peripheral nerves, including the crural nerve, has been investigated in the locust and cockroach. In some cases the larger nerves are ensheathed by a fat body layer which is not always complete. However, like many nervous connectives, they do possess a continuous acellular neural lamella and a perineurial cell layer which surround the glial-axonal mass. Adjacent perineurial cells are associated with one another by septate desmosomes, gap junctions and tight junctions. These last may represent the morphological basis of the ‘blood-brain barrier’ observed electrophysiologically in these peripheral nerves in another report. Very small nerves of the cockroach, however, although lying embedded in a neural lamella, do not possess a specialized perineurial layer displaying junctional complexes, unless they contain one or more large axons. If they have only one or more small axons, these small nerves may either appear naked, or display a single glial cell process loosely enveloping them; in either case there is no structural basis for a ‘barrier’ system. Various comparisons have been made between locust crural nerve and the cockroach central nervous connectives in an attempt to correlate some aspects of their ultrastructural organization with relevant electrophysiological information.