Conduction Processes in the Nerve Cord of the Moth Manduca Sexta In Relation to its Ultrastructure and Haemolymph Ionic Composition

Abstract

1. Connectives of the ventral nerve cord of Manduca sexta consist of glia-ensheathed axons surrounded by a perineuriurn and an acellular neural lamella, which is greatly expanded on the dorsal surface. The glial cells are linked to one another by desmosomea and tight junctions; the latter also occur between adjacent perineurial cells. There no continuous circum-neural fat-body sheath. 2. A ten-fold change in the external potassium concentration results in a 43 mV change in the resting potential of de-sheathed connectives. Action potentials of such exposed axons are rapidly blocked in low-sodium or sodium-free saline and under these conditions neither calcium nor magnesium is able to maintain conduction. Spikes from de-sheathed preparations are rapidly abolished on exposure to 10-6M tetrodotoxin. These iindmgs indicate a conventional ionic basis of excitation for the axonal membrane of this insect. 3. Analyses of the haemolymph reveal a mean sodium concentration of 25.4 (s.E. ± 0.98) mM/1 and a mean potassium concentration of 25.1 (s.E. ± 1.74) mM/.1 4. Action potentials recorded from sheathed connectives are maintained for extended periods in sodium-free saline. 5. Exposure of most sheathed connectives to elevated potassium concentrations results in a two-stage depolarization. A rapid, single-stage, apparently extraneuronal potential change is, however, observed in some preparations. 6. These results on sheathed connectives indicate the presence of some peripheral barrier to the movements of sodium and potassium; the tight junctions between adjacent perineurial cells are considered to be possible sites of this restriction.