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Notes: Summary 1. The role of cations in nervous conduction in the central nervous system of the herbivorous insect Carausius morosus was studied with external electrodes. 2. The size of the compound action potential in axons (totally desheathed connectives) depends on the sodium concentration of the bathing medium (Fig. 2), the relationship roughly approximating that predicted by the Nernst formula (Table 4). 3. Sodium-free bathing medium reversibly blocks conduction in axons (totally desheathed connectives) within 1 min (Fig. 2). 4. At low concentrations in the perfusion solution, both tetrodotoxin and procaine reversibly block conduction in axons within a few minutes. 5. Lithium ions replace sodium ions in the maintenance of action potential size in axons for about 10 min, but then action potentials gradually decline. The action potential size is restored when the nerve cord is perfused once again with a medium containing the initial sodium concentration (Fig. 3). 6. External sodium concentrations either higher than 180 mM or lower than 150 mM significantly decrease the viability of axons (Fig. 4); we concluded that the sodium concentration of the extracellular fluid of the nerve cord is probably between 150 and 180 mM. 7. The order of effectiveness of potassium, rubidium and cesium ions in suppressing compound action potentials in axons is K+= Rb+〉Cs+ (Fig. 5). 8. The action potentials in axons perfused with a solution containing 27 mM K+ are reversibly suppressed by approximately 37%; we concluded that the potassium concentration in the extracellular fluid of the ventral nerve cord is less than 27 mM. 9. When the calcium concentration in the external medium is reduced from 7.5 to 1 mM, repetitive firing and progressive conduction blockage are produced after nearly 4 hr in connectives in the absence of a fat-body sheath (Fig. 6). 10. When manganous ions (16 mM) are added to the bathing medium, there is little change in the level of axonal electrical activity (Fig. 7). Since manganous ions have been shown to suppress the increase in membrane conductance to calcium during the so-called calcium spikes of some irritable tissues, we concluded that calcium ions do not contribute significantly to the inward current during the action potential. 11. Axonal viability increases with the external magnesium concentration in the range from 0 to 50 mM Mg++ (Fig. 8). However, the actual magnesium concentration of the extracellular fluid may be between 10 and 25 mM. 12. When axons are perfused with magnesium-free solution, the action potentials are reduced to one-half of their initial size only after 2.36±0.42 hr; we concluded that magnesium ions are not carriers of inward current during action potentials. 13. Comparing our data with published data obtained from other animals, we concluded that nervous conduction in Carausius conforms to the classical membrane theory despite the unusual cationic levels in the hemolymph and that a yet undefined homeostatic mechanism maintains extracellular cationic concentrations at favorable levels in the nerve cord. 14. Viability studies indicate that an active transport mechanism is probably located in the fat-body sheath and that this mechanism probably maintains the extracellular sodium concentration at a high level (Table 2). These studies also indicate that the neural lamella and perineurium form a diffusion barrier which is only slightly permeable to the common cations (Tables 2, 3 and Fig. 6). 15. A model (Fig. 9), explaining the regulation of cationic concentrations in the extracellular fluid of Carausius nerve cord, is proposed from available data.