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.