Differential conduction block in branches of a bifurcating axon.

1. Propagation of action potentials at high frequency was studied in a branching axon of the lobster by means of simultaneous intracellular recording both before and after the branch point. 2. Although the branching axon studied has a geometrical ratio close to one (perfect impedance matching) conduction across the branch point failed at stimulation frequencies above 30 Hz. 3. The block of conduction after high frequency stimulation occurred at the branch point per se. The parent axon and daughter branches continued to conduct action potentials. 4. Conduction block after high frequency stimulation appeared first in the thicker daughter branch and only later in the thin branch. 5. With high frequency stimulation there was a 10-15% reduction in amplitude of the action potential in the parent axon, a corresponding decrease in the rate of rise of the action potential, a 25-30% decrease in conduction velocity, marked increase in threshold and prolongation of the refractory period. In addition the membrane was depolarized by 1-3 mV. 6. Measurements of the membrane current using the patch clamp technique showed a large decrease in the phase of inward current associated with the action potential, before the branching point. 7. The small membrane depolarization seen after high frequency stimulation is not the sole cause of the conduction block. Imposed prolonged membrane depolarization (8 mV for 120 sec) was insufficient to produce conduction block. 8. In vivo chronic extracellular recordings from the main nerve bundle (which contains the parent axon) and the large daughter branch revealed that: (a) the duration and frequency of trains of action potentials along the axons exceeded those used in the isolated nerve experiments and (b) conduction failure in the large daughter branch could be induced in the whole animal by electrical stimulation of the main branch as in the isolated preparation. 9. Possible mechanisms underlying block of conduction after high frequency stimulation in a branching axon are discussed.

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

1. 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. 2. A 2-3 mM increase in extracellular K concentration (normal concentration 12 mM) produced block of conduction into both daughter branches. 3. While conduction block induced by high frequency stimulation occurs first into the large daughter branch and only later into the smaller one, propagation into both branches is blocked simultaneously by increased extracellular K concentration. 4. Increasing extracellular K by 2-3 mM resulted in membrane depolarization, reduction in membrane resistance and reduced excitability. The latter two effects were larger than expected from the small depolarization. It appears that increase of extracellular K has direct effects on membrane excitability. 5. It is suggested that block of conduction after high frequency stimulation results from accumulation of K in the extracellular space. However, in order to account for differential conduction block in the two branches one must assume differential buildup of extracellular K concentration around the two branches during high frequency stimulation. 6. Ultrastructural studies using La and horseradish peroxidase as extracellular markers show that the space around the two branches is similar and is open to the extracellular space. Therefore differences in periaxonal volume cannot account for differential buildup of K around the two branches. 7. It is demonstrated that 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 two branches almost at the same time. 8. Injection of Ca2+ intracellularly into the thick branch prevents or delays the appearance of conduction block after high frequency stimulation. 9. A mechanism based on these findings is suggested to explain the differential conduction block seen after high frequency stimulation in a branching axon with almost ideal impedance matching.