Exchanges of sodium ions in the central nervous system of Anodonta cygnea.

Our understanding of the ionic basis of axonal conduction has been largely derived from studies carried out on vertebrate and invertebrate animals which possess conventional body fluids containing relatively high concentrations of sodium and other inorganic ions. Apart from some recent studies on a phytophagous insect (Treherne, 1965 a, b; Treherne & Maddrell, 1967 a, b) we have very little knowledge of the functioning of the nervous system in animals which possess body fluids of specialized chemical composition. One such animal is the freshwater bivalve, Anodonta cygnea, which possesses the most dilute blood so far described in any animal species. The total concentration of the blood of this mollusc averages only 44-0 m-osmoles, with a sodium content of around 15 mM/kg. (Potts, 1954). In a recent paper from this laboratory (Treherne, Mellon & Carlson, 1969) it was shown that, despite the specialized nature of the blood, the axons were conventional in possessing sodium-dependent action potentials. Furthermore, it was shown that both the small, slowly conducting fibres, and the larger (2-O-6-O fi) rapidly conducting ones, appeared to be accessible to water-soluble ions and molecules contained in the bathing medium. The rapidly conducting fibres differed from the slowly conducting ones in their response to the treatment of preparations with sodium-free solutions, for whereas the latter showed a rapid decline in activity in these circumstances the former continued to function for extended periods in sodium-free Ringers, in sucrose and in dextran solutions. The ability of the rapidly conducting fibres to function in sucrose solution presents something of a paradox, for a recent electron microscopic investigation has shown that the extracellular space surrounding the axons in the connective of Anodonta appears to be freely accessible to ions and molecules in the bathing medium. Apart from the neural lamella no visible structures were interposed between the axon surfaces and the bathing medium (Gupta, Mellon & Treherne, 1969). In this respect the results from the previous investigation (Treherne et al. 1969) agree with these electron microscope pictures of the central nervous tissues. The difficulty arises in the explanation of the ability of the rapidly conducting fibres to function in the absence of external sodium ions, for electron microscope investigation revealed that there is a paucity of glial elements in the central nervous connectives. It is, for example, not possible to postulate any local dynamic extra-axonal sodium regulation of the kind suggested in the central nervous system of the stick insect, Carausius morosus (Treherne & Maddrell,