A quantitative study of potassium movements in the central nervous system of Periplaneta americana.

Our understanding of the ionic exchange taking place in insect central nervous tissues is largely derived from radioisotope experiments carried out on isolated connectives or abdominal nerve cords of the cockroach (cf. Treherne, 1961a, b, 1962; Eldefrawi & O'Brien, 1967). These studies have indicated that the exchange of small, water-soluble inorganic ions takes place relatively rapidly across the superficial connective tissue sheath and can be represented as an essentially two-stage process. As with some other central nervous tissues (e.g. Ames & Nesbett, 1966; Abbott, 1970) the initial rapidly exchanging fraction was tentatively identified as an extracellular one, the secondary, ouabain-sensitive fraction being identified as the intracellular ion fraction (Treherne, 1962). However, the gross exchanges of inorganic ions between the blood, or bathing medium, and the complicated series of compartments represented by the neurones, glial elements and the complex extracellular spaces are extremely difficult to interpret in terms of the diifusion processes involved in the movements of ions to the extra-axonal fluid. In particular, the role of the narrow and tortuous intercellular channels formed by the closely applied glial and neuronal membranes and the extended mesaxon clefts remains obscure. For these reasons the present investigation was undertaken in an attempt to elucidate the processes involved in the movements of inorganic ions from the blood or bathing medium to the extraneuronal fluid. The available evidence indicates that cockroach giant axons in desheathed preparations respond to changes in the external potassium concentration in a predictable manner (cf. Yamasaki & Narahashi, 1959), similar potential changes also being observed in intact preparations (Twarog & Roeder, 1956; Pichon & Boistel, 1965 a, b). This cation was therefore chosen for the present study which attempts to link ultrastuctural observations with those made using electrophysiological techniques.