Figure 1. Threshold electrotonus in motor axons.

A, threshold electrotonus recorded from motor axons using 1.0 ms test stimuli (mean data for the 7 normal subjects in the present study), showing fast and slow phases (F, S1, S2). B, the threshold changes were induced by conditioning polarizing currents of 100 ms duration, set to +40%, +20%, 0%, −20% and −40% of the threshold current for the test potential. The changes in excitability and the responsible conditioning currents are plotted against the conditioning–test interval, delayed by 10 ms so that the onset of threshold electrotonus can be clearly seen. At the onset and offset of the polarizing current there are rapid changes in threshold for the test potential (the ‘F’ phase), reflecting polarization of the node of Ranvier. This phase is followed by a slower increase in excitability with depolarization or decrease in excitability with hyperpolarization (‘S1’) as the membrane potential of the internodal region begins to change due to spread of current from the node. A second slow phase in the opposite direction follows as the axon accommodates to the polarization (‘S2’ with depolarizing currents; ‘S3’ with hyperpolarizing currents, the latter not apparent in the figure). Accommodation to the depolarizing current in the S2 phase results from the activation of outwardly rectifying slow K⁺ channels to limit the extent of depolarization (Bostock et al. 1998; Schwarz et al. 2006).