Some Effects of the Ionic Composition of the Extracellular Fluid on the Electrical Activity of the Cockroach Abdominal Nerve Cord

The chemical composition of the fluid surrounding the cells of the central nervous system of Periplaneta has been shown to differ from that of the haemolymph, due to a Donnan equilibrium which maintains an excess of inorganic cations in the extracellular fluid (Treherne, 1962 a, b). In view of the demonstrated permeability of the nerve sheath to ions and molecules (Treherne, 1961 a, b) it was suggested that the enhanced rates of potassium depolarization obtained in desheathed preparations (Hoyle, 1953; Twarog & Roeder, 1956) might be a result, in part at least, of the changed extracellular environment resulting from the disruption of the Donnan equilibrium with the haemolymph (Treherne, 19626). The present investigation represents an attempt to determine the part which such ionic disturbances play in the electrical changes resulting from the removal of the nerve sheath in solutions of varying potassium concentration. In these experiments the rates of potassium depolarization of desheathed preparations irrigated with various solutions have been followed and compared with those obtained in salines in which the cation levels were elevated to correspond to their extracellular concentrations as calculated from the Donnan equilibrium. The synaptic transmission in the desheathed terminal abdominal ganglion in high concentrations of acetylcholine has also been studied in an attempt to throw some light on the factors involved in the development of conduction block under these conditions.