Potassium-sensitive double-barrelled microelectrodes were used to measure the potassium content of extracellular spaces in leech ganglia, both intact and with the ganglion capsule opened. When the ganglion capsule was opened, the extracellular concentrations of potassium in the ganglion were similar to that of the bathing medium (4 mM). With intact ganglia the extracellular potassium concentration in the neuropile averaged 6.3 +/− 0.7 mM and in the nerve cell body region 5.8 +/− 0.6 mM. The potential measured in these parts of the ganglion was between +2 and −8 mV, averaging −1.9 mV. The change of potassium concentration in the extracellular spaces following increase or decrease in the concentration of potassium ions in the bath declined exponentially. This rate of change, which would be expected of a first-order diffusion process, was found in both the neuropile and the nerve cell body region. In a medium containing 5 × 10(−4) M ouabain, the potassium concentration in both parts of the ganglion increased transiently by an average of 3.8 +/− 1.0 mM in the neuropile and 1.2 +/− 0.4 mM in the nerve cell body region. Negatively charged polyelectrolytes in extracellular spaces of leech ganglia could affect the distribution of potassium ions to give a Donnan distribution. It is also possible, that the endothelial layer influences the extracellular potassium concentration in a ganglion under resting conditions.

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