[Stimulus effect of submaximal trains of impulses on nerves]

Although the occurrence of rate-dependent conduction block upon exposure to supramaximal stimuli is a well-known phenomenon, changes in the stimulus effect of submaximal trains of impulses on nerves remain unknown. To investigate changes in the stimulus effect, we evaluated median nerve action potentials using various frequencies of impulse trains. The subjects were 12 healthy controls and 9 patients with diabetic polyneuropathy. A tungsten microelectrode was inserted into the median nerve trunk at the elbow. Weak stimuli that produced 10 microV compound nerve action potentials were repeated. Each value recorded is the average of 20 trials. Repetition of 1- to 5-Hz stimuli yielded the same average wave, but above 7-Hz, the stimuli produced a diminution in amplitude and slight prolongation of the latency of each peak. This was most prominent with 50-Hz or 100-Hz repeated stimuli. The potentials changed amplitude with a waxing and waning pattern, and gradually stabilized at a lower level. The averaged wave corresponded to the record of reduced stimuli at 0.6 mA, maximally. Paired stimuli at an interval of less than 5 msec were equivalent to a relative refractory period, whereas at an interval of 4-18 msec they were equivalent to a supernormal period, and at 12-90 msec, to a subnormal period. The 'jumping' (unexpected shortening of the latency of a potential in response to increased stimulus intensity) of a single nerve unit was provoked or released corresponding to these periods. No differences were found between healthy individuals and patients with diabetic polyneuropathy. As a consequence of electrogenic Na+/K+ pumping, a three-Na+ ion efflux occurred instead of a two-K+ ion influx. Thus, repeated high-frequency impulses induced membrane hyperpolarization that reduced the stimulus effect on nerves. With trains of impulses at a frequency of 100-Hz, which corresponds to a stimulus every 10 msec, the second response was greater than the first, reflecting the supernormal period, but impulse trains provoked hyperpolarization, as mentioned above, and reduced the amplitude of nerve action potentials. The results of this study show that the stimulus effect on nerves decreased at submaximal stimuli greater than 7-Hz, which reduced the amplitude of compound nerve action potentials. Therefore, averaging should be done at a stimulus frequency below 6-Hz.