Membrane Potentials in the Central Nervous System of a Phytophagous Insect (Carausius Morosus)

1. The nerve cord of the stick insect is surrounded by a fat-body sheath. This sheath encloses an extraneural space and thus interposes an additional fluid compartment between the neural lamella and the haemolymph. The axons in the thoracic connectives were found to be relatively small, the largest ones averaging 7--II µ in diameter. 2. The apparent resting potentials of axons, impaled with glass capillary microelectrodes, were found to be relatively small, averaging only 25.1 mV., with an overshoot of 59.3 mV. in action potentials in intact preparations. In the absence of the neural fat-body sheath the resting potentials were increased to a mean value of 40.3 mV., there being no significant alteration in the total amplitude of the action potentials. This effect appears to result from the interpolation of a positive potential of some 15-20 mV. between the indifferent and recording electrodes. 3. The positive potential was abolished, in intact preparations, when the nerve cords were bathed with solutions of elevated chloride concentration. Positive potentials were also obtained when gradients of chloride ions were maintained across the isolated fatbody sheath. It is suggested that the positive potentials may result from a chloride diffusion potential across the neural fat-body sheath. 4. The results are discussed in relation to the ability of the axons of this species to function in ganglia and connectives bathed with solutions of low sodium concentration.