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Cockroach giant interneurones stained by cobalt-backfilling of dissected axons.

Detailed neuroanatomical information is an essential prerequisite for functional analysis of specific neurones in the central nervous system. In this investigation we have combined the dissection of single axons with cobalt-backfilling to reveal the structural organisation of a class of interganglionic giant interneurones in the central nervous system of the cockroach, Periplaneta americana (L). Each giant interneurone possesses a cell body, neurite and dendrites within the sixth abdominal ganglion and also has a giant axon which ascends the ventral nerve cord (Harris & Smyth, 1971). Cereal sensory neurones provide monosynaptic input to at least some giant inter-neurones (Callec et ah, 1974). Furthermore the cereal afferent, giant interneurone pathway has proved to be a useful system for quantitative pharmacology of putative A preparation consisting of the cereal nerves, the abdominal ganglia and their interganglionic connectives was excised from an adult male cockroach and transferred to a drop of saline on a microscope slide. A single giant axon was located and dissected under a binocular microscope (x 150 magnification), as previously described (Pichon & Callec, 1970; Hue, Pelhate & Chanelet, 1978), from one of the paired connectives linking either the fourth and fifth abdominal ganglia or the fifth and sixth abdominal ganglia. The dissected preparation was removed to a perspex chamber in which the cereal nerves and sixth abdominal ganglion were located in a compartment containing physiological saline of the following composition The isolated axon was immersed in a paraffin-oil channel which separated the saline from a compartment containing 2-5 % (w/v) cobaltous chloride in which the remainder of the abdominal nerve cord was submerged. The rate of cobalt penetration into those elements of a giant interneurone located in the sixth abdominal ganglion was influenced by temperature and applied current. For example, backfilling in preparations dissected midway between the fifth and sixth