Mechanisms involved in differential conduction of potentials at high frequency in a branching axon.

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Abstract
The ionic mechanisms involved in block of conduction of action potentials following high frequency stimulation were studied in a branching axon of the lobster Panulirus penicillatus. A 2-3 mM increase in extracellular K concentration (normal concentration 12 mM) produced a conduction block in both daughter branches. While the conduction block induced by high frequency stimulation occurs first in the large daughter branch and only later in the smaller one, propagation in both branches is blocked simultaneously by increased extracellular K concentration. Increasing extracellular K by 2-3 mM resulted in membrane depolarization, reduction in membrane resistance and reduced excitability. The latter 2 effects were larger than expected from the small depolarization. An increase of extracellular K may have direct effects on membrane excitability. The conduction block occurring after high frequency stimulation may result from accumulation of K in the extracellular space. To account for differential conduction block in the 2 branches, differential buildup of extracellular K concentration around the 2 branches during high frequency stimulation must be assumed. Ultrastructural studies using La and horseradish peroxidase as extracellular markers show that the space around the 2 branches is similar and is open to the extracellular space. Differences in periaxonal volume cannot account for differential buildup of K around the 2 branches. The lobster axon has a Na+/K+ electrogenic pump. After blocking this pump with ouabain, stimulation at high frequency resulted in a conduction block in the 2 branches at almost the same time. Injection of Ca2+ intracellularly into the thick branch prevents or delays the appearance of the conduction block after high frequency stimulation. A mechanism is suggested to explain the differential conduction block seen after high frequency stimulation in a branching axon with almost ideal impedance matching.