Action potential fatigue in nonmyelinated nerve fibers: garfish olfactory and rabbit vagus nerve

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Abstract
A decrease in amplitude and conduction speed in the compound action potential is observed with time in gar olfactory and rabbit vagus nerve when it is stimulated between 4 and 15 Hz at 26 °C in vitro. The amplitude decays exponentially for 1–3 min before reaching a steady state. Recovery also occurs exponentially. The fatigue exponential time constant is 15 s for gar olfactory nerve and 50 s for rabbit vagus nerve. The steady state values are 14% and 36% of the original amplitude, respectively, and conduction speeds are reduced by 25% in both nerves. The effect results from competition between ion flow during the action potential and active transport. The accumulation of K+ ions and depletion of Na+ ions in the restricted extracellular space contributes to the amplitude fatigue while the depletion of Na+ ions alone causes the decrease in conduction speed. Ouabain increases the fatigue rate in both preparations, but more so for gar. The fatigue and recovery measurements may provide a useful method to investigate active pumping, including both the total pumping rate and the electrogenic component.