Membrane potentials in the central nervous system of a phytophagous insect (Carausius morosus).

The extremely specialized ionic composition of the haemolymph in some phytophagous insect species presents certain difficulties in the interpretation of axonal conduction in terms of the classical membrane theory which has been found to be applicable in most animal groups. In particular the very low sodium level, together with the relatively high concentration of magnesium ions, represent conditions which are very different from the conventional ones encountered in the blood of most vertebrate and invertebrate species. Some recent experiments have indicated that there is some regulation of the sodium concentration in the extracellular fluid of the central nervous tissues of Carausius morosus (Treherne, 1965 a). These preliminary studies were, however, carried out using extracellular electrodes. There have, in fact, been no studies carried out on the axons of phytophagous species using intracellular electrode techniques. The present investigation was, therefore, carried out to examine the membrane potentials in axons from the central nervous system of an insect in which the haemolymph is characterized by low sodium and high magnesium levels. These experiments have been largely directed towards an investigation of the electrical events occurring in the intact nervous system as a necessary step prior to further work employing isolated preparations of the central nervous system of this species. Particular attention has been paid to the influence of the neural fat-body sheath, which has been recently described in this species (Maddrell & Treherne, 1966), on the resting and action potentials of the larger axons in the nerve cord.