Printed in Great Britain © The Company of Biologists Limited 1990 REGIONAL VARIATION IN THE CURRENT FLOW ACROSS

One notable characteristic of the insect and vertebrate central nervous system is the presence of a clearly defined blood-brain barrier. In the insect this barrier is made up of the perineurial glial cells, and shows a heterogeneity in structure and possibly function between the connectives and ganglia. In this paper we have used a two-dimensional vibrating probe to investigate the net flow of electrical current across the barrier at various locations in the abdominal nervous system. The results show clear differences between different areas. There is a strong and consistent outward current flow (3.16^Acm~2) perpendicular to the ganglion surface over the equatorial plane. Current returns through the peripheral nerves and the connectives. A detailed study of the latter shows that the net inward flow is principally over the initial section, immediately adjacent to the ganglia (2.11|UAcm~2). The different current polarities can be correlated with structural differences in the underlying glial cells