Morphological and electrophysiological properties of giant interneurons during the postembryonic development of the cockroach CNS

The giant interneurons (GIN) of the central nervous system of the cockroach undergo two major physiological changes during the postembryonic development period: (A) a gradual decrease in the safety factor for action potential propagation across the GIN in the metathoracic ganglion (T3); and (B) a marked decrease in the number of afferent pathways innervating the GIN in T3 (Spira and Yarom³⁵). Analysis of the morphological structure of the GINs, by intracellular injection of cobalt ions and by cross-sections prepared for light and electron microscope, reveals that despite the extensive growth of the GINs during the postembryonic developmental period, the main structural outline of the fibers is not altered. In adult preparations, however, the GIN diameter narrows 25-26% in ganglion T3, while in early nymphal stages the reduction is only 8-10%. The difference in the extent of narrowing of the fibers in adult and nymphal stages is the major factor that accounts for the development of a low safety factor region for impulse propagation across T3. Analysis of the passive membrane properties of the GIN reveals that the electrotonic lenght of the GIN segment in T3 is identical in adult and nymphal stages. It is concluded that the functional disappearance of afferents innervating the GINs in T3 is a consequence of decreased transmission efficacy along the afferent pathways.