Figure 5.

Block of single action potentials also blocks repetitive firing. (A) Schematic of experiment: current was injected on the left-hand side, the axon was raised to a temperature sufficient to induce block of a single propagating action potential, and potentials were recorded at the points labeled 0 and 1. (B) Repetitive firing (80 Hz) due to a steady depolarizing pulse (2000 nA; duration of the current is indicated by the dark green line above the action potentials) is completely blocked by the same conditions that block a single action potential. Action potentials are measured prior to the block region (black line, at 25% of the length of the axon; point labeled 0 on the schematic diagram), and membrane potential is measured at the end of the axon (dashed green line, at 90% of the length of the axon, point labeled 1 on the schematic diagram). Small residual filtered versions of the original action potential appear at the end of the axon, but the action potential does not propagate past the block region (see inset, which shows an expanded view of the dashed green line contained within the box; the potential measured at the end of the axon is a small passive response). (C) Repetitive firing (53.33 Hz) due to individual current pulses (2000 nA, 1 ms duration, 15 ms period; small dark green lines above each action potential indicate time of current injection) is completely blocked by the same conditions that block a single action potential. Once again, the action potentials propagate normally until the block region (black line, measured at the point labeled 0 in the schematic), and only small residual passive versions appear at the end of the axon (dashed green line, measured at the point labeled 1 in the schematic), but the action potential does not propagate past the block region.