Architecture of gastropod central nervous tissues in relation to ionic movements

Abstract

Potential ultrastructural barriers to the free diffusion of ions through the central nervous tissues of Viviparus contectus and Limnaea stagnalis are described. The nerve sheath in both species comprises an amorphous matrix containing fibrils and scattered cells which are not specialized for ion transport. The peripheral glial/ secretory cell layer of Viviparus connectives and the more deeply situated glial processes enveloping the nerve cell bodies in Limnaea ganglia constitute the most likely sites of restriction of ion movements. Electrophysiological evidence, however, reveals an unrestricted access to the neuronal surface of potassium and sodium ions in the bathing medium. Passive diffusion through the open extracellular channels appears to be the mechanism for the exchange of ions between the blood and the extra-neuronal fluid of these gastropod molluses.