The Resolution of Neuronal Assemblies after Cobalt Injection into Neuropil

Cobalt ions are incorporated by many interneurons of the insect central nervous system after injection of cobalt chloride or cobalt acetate into the neuropil. Entire nerve cells are subsequently revealed by sulphide precipitation which is followed by silver intensification of whole brains and ganglia. The number of neurons resolved depends upon the optimal timing of an injection phase, followed by a period of diffusion. These, and the site of injection, can be adjusted so as to demonstrate arrangements within and between complete populations of morphological classes of interneurons. This novel use of cobalt reveals cytoarchitectural features of neuropil that are refractory to other histological procedures and shows that in columnar visual neuropil the nerve cell arrangements are limited to a few basic modes of assembly.