The Extracellular Space of A Simple Molluscan Nervous System and Its Permeability to Potassium

Abstract

ABSTRACT The movement of ions through the extracellular space in the tissues surrounding the giant neuron (G cell) of the nudibranch mollusc, Anisodoris nobilis, was studied with anatomical and physiological techniques. The diffusion pathways in the gastro-oesophageal ganglion and nerve were identified anatomically in electron micrographs obtained from preparations which were first incubated in sea water containing lanthanum chloride and subsequently fixed in a basic solution of glutaraldehyde. A lanthanum precipitate was found extracellularly in the connective sheath, in the extracellular clefts between the glial cells and in the space between the glia and the G cell. The rate of diffusion of potassium was estimated from the rate of change of the G cell membrane potential following a change in the potassium concentration of the artificial sea water bathing the preparation. The average half-time for diffusion corresponds to an equivalent diffusion pathway of about 200 μ m. This value is sufficiently close to the average length (70–95 μ m) of the pathways identified with lanthanum to suggest that the restriction to diffusion is minor. The permeability of the extracellular space to potassium is high (5· 7× 10−1 cm/sec), and our calculations show that a difference larger than 2 mM, between the potassium concentration of the external solution and that of the fluid layer adjacent to the G cell membrane, cannot be maintained under most conditions.