Figure 1.
Multiple excitability measurements recorded from normal subjects: motor axons in the median nerve were tested at the wrist and compound muscle action potentials recorded from the abductor pollicis brevis muscle.
A–C: Mean +/− SD for all 38 subjects. D–F: Comparisons between means of Lower K (grey) and Higher K (black) groups. A, D: Threshold electrotonus, i.e., threshold changes during and after polarizing currents set to +40 (top), +20, −20 and −40% (bottom) of threshold. B, E: Recovery cycle showing successive phases of refractoriness, superexcitability, and late subexcitability. C,F: Current-threshold (I/V) relationship.
Figure 2.
Examples of nerve excitability measures showing significant relationship to serum potassium levels.
A: Superexcitability, B: TEd20(90–100 ms) threshold decrease at end of 20% depolarizing current, C,D: TEh20(90–100 ms) and TEh40(90–100 ms) threshold decrease at end of 20% and 40% hyperpolarizing current (NB Negative threshold decrease indicates threshold were increased by hyperpolarization).
Table 1.
Mean values of excitability parameters derived from the multiple measures of nerve excitability performed on the median nerve in 38 normal subjects.
Figure 3.
Threshold electrotonus (top row) and recovery cycle (bottom row) waveforms generated by Models 1–3 for values of extracellular potassium corresponding to the Lower K (grey) and Higher K (black) groups.
Figure 4.
Potassium dependence of 2 nerve excitability measurements predicted by Models 1 (red line), 2 (green line) and 3 (blue line) compared with mean measurements for Higher K (▴), Medium K (•) and Lower K (▪) groups, and ellipse representing 1 SD limits for 9 patients with chronic renal failure (reproduced from Kiernan et al.).9
Only Model 3 predicts an appropriate drop in superexcitability with increasing potassium level.
Table 2.
Comparison between the three models in their ability to account for the effects of changes in serum potassium levels on multiple measures of nerve excitability.