[Stimulus effect and jumping of delayed potentials--investigation of human single nerve action potentials]

The jumping phenomenon is defined as an unexpected reduction in latency of evoked nerve responses with augmentation of stimulus intensity. This phenomenon may occur in the presence of segmental demyelination. To determine the severity of neuropathies, we recorded median nerve action potentials at the elbow of 60 patients of neuropathy by means of intraneural microneurography. Submaximal stimuli were delivered at the wrist or at one finger. In 53 of the 60 patients evaluated, delayed single nerve action potentials were recorded by submaximal stimuli that produced less than 10 microV compound nerve action potentials. In three delayed single unit potentials, latencies were shortened unexpectedly by several milliseconds when the intensity of stimulus was gradually increased. With the boundary stimulus intensity, we observed a small amplitude of the corresponding potentials at the delayed and the jumped latencies. Instead, jumping was suppressed with rapid repetitive stimuli. At the stimulus intensity sufficient to provoke jumping, we administered repetitive stimuli at 2Hz to 50Hz. At the rate of 20 to 30Hz, the corresponding potentials again returned to the delayed latencies. Rapid repetitive impulses may evoke cathodal hyperpolarization which raises the excitation threshold of the nerve. Thus, the stimulus effect can be decreased and jumping will be released. In addition, a diminution in amplitude and slight prolongation of the latency of each peak occurred in proportion to the increase in stimulus rate. However, paired stimuli with an interval of 4 to 13 msec was equivalent to a supernormal period and provoked the jumping of a single nerve unit. Conditioning stimuli may have increased the excitability of the corresponding fiber.(ABSTRACT TRUNCATED AT 250 WORDS)