The ionic basis of axonal conduction in the central nervous system of Anodonta cygnea (Mollusca: eulamellibranchia).

The bivalve mollusc Anodonta cygnea appears to be unique in possessing the most dilute body fluids so far described in the Animal Kingdom. The osmotic concentration of the blood of this species has been found to amount to only about 44-0 m-osmole, with a sodium concentration which averages 15-6 mM/kg. (Potts, 1954). It is difficult to explain the ionic basis of electrical activity of excitable tissues with such specialized body fluids. In particular, the relatively low sodium concentration of the blood presents difficulties in the interpretation of neuronal activity in terms of the conventional membrane theory, for the propagation of the action potential requires a steep concentration gradient of sodium ions across the nerve cell membrane. The condition of a specialized blood composition with very low concentrations of sodium ions is, however, not confined to freshwater molluscs. A somewhat similar situation is encountered in several phytophagous insect species, although in these arthropods the total osmotic concentration of the blood does not approach the extreme condition found in Anodonta. Recent investigations on axonal function in the stick insect, Carausius morosus, which has a blood containing only about 15-0 mia/l. of sodium, have indicated that electrical activity is maintained by a local regulation of the extra-axonal sodium concentration (Treherne & Maddrell, 19676; Treherne, 1967). In this latter species it appears most likely that the regulation in the sodium concentration in the fluid immediately surrounding the axons is achieved by the activity of the glial cells. It is envisaged that there might be an extrusion of sodium ions into the narrow extracellular channels formed at the axon surfaces by the closely applied glial cell membranes. The postulation of a glial involvement in the extra-axonal sodium regulation is in accord with electron microscopic study of the central nervous tissues of this insect, which shows a very close glial-axon association in which each axon is surrounded by an appreciable number of glial folds (Treherne & Maddrell, 19676). The organization of the central nervous tissues in the insect species described above contrasts strikingly with that observed in the central nervous connectives of Anodonta cygnea (Gupta, Mellon & Treherne, 1969). In this mollusc the central nervous tissues contain extensive extracellular spaces surrounding the axons, with only very sparsely