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

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Abstract
Connectives of the ventral nerve cord of Manduca sexta consist of glia-ensheathed axons surrounded by a perineurium and an acellular neural lamella, which is greatly expanded on the dorsal surface. The glial cells are linked to one another by desmosomes and tight junctions; the latter also occur between adjacent perineurial cells. There no continuous circum-neural fat-body sheath. 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 10−6 M tetrodotoxin. These findings indicate a conventional ionic basis of excitation for the axonal membrane of this insect. 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) HIM/1. 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. 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.