Extracellular potassium activity during frequency-dependent conduction block of giant axons in the metathoracic ganglion of the cockroach

In the metathoracic ganglion (T₃) of the cockroach, extracellular potassium activity (aK) was measured with ion-sensitive microelectrodes and intracellular recordings were simultaneously made from giant axons (GAs) during high frequency stimulation of the connectives. Blockade of spike conduction through T₃ was associated with intraganglionic aK rises of 0.2-0.5 mM, which were only 10% of the periaxonal aK rises suggested from GA depolarizations. When aK in the bath was increased 10-fold, GA conduction block during 1 Hz stimulation did not occur until much higher levels of aK and GA depolarization were achieved. The results suggest that glial sheaths surrounding GAs significantly impede K⁺ movement, and may thus prevent non-specific axonal interactions, and that stimulus-induced conduction block is not primarily due to K⁺-induced depolarization and consequent Na⁺-inactivation.