Extracellular ionic homeostasis in an insect central nervous system involves a peripheral intercellular diffusion barrier, an extracellular matrix and neuroglial cation transport. The peripheral location of the barrier in the superficial neuroglia is confirmed by intracellular recording from glial cells identified by peroxidase injection. This barrier protects the underlying neurones from large changes in ionic composition of the blood-plasma, but renders them more susceptible to fluctuations in ion composition resulting from neuronal signalling within the very restricted extracellular system. Because of the peripheral intercellular barrier, sodium movements between the blood and the extracellular fluid are largely transcellular and are mediated by ion pumps on the perineurial and underlying glial membranes. It is suggested that the homeostatic role of the neuroglial ion pumps is augmented by an anion matrix which functions as an extracellular sodium reservoir. It is proposed that during depletion of extracellular sodium, this cation is released by the matrix to maintain the sodium activity in the fluid at the axon surfaces.

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