The effect of frequency of excitation on the thermal response of medullated nerve

Reversible conduction block in peripheral nerve using electrical waveforms

INTRODUCTION

Electrical nerve block uses electrical waveforms to block action potential propagation.

MATERIALS & METHODS

Two key features that distinguish electrical nerve block from other nonelectrical means of nerve block: block occurs instantly, typically within 1 s; and block is fully and rapidly reversible (within seconds).

RESULTS

Approaches for achieving electrical nerve block are reviewed, including kilohertz frequency alternating current and charge-balanced polarizing current. We conclude with a discussion of the future directions of electrical nerve block.

CONCLUSION

Electrical nerve block is an emerging technique that has many significant advantages over other methods of nerve block. This field is still in its infancy, but a significant expansion in the clinical application of this technique is expected in the coming years.

Nerve excitation by alternating current

When an alternating current is passed through a nerve a relation exists between the intensity I necessary for steady threshold excitation, once to each cycle at the electrode considered, and the frequencyn. For a pure sine-wave current applied to a nerve lying on two non-polarizable electrodes, and taking account of both time-constants (kof the local excitatory disturbance,λof accommodation), this relation (Hill, 1936a, p. 343) should be:— I/I0= √—(1 + 4π2k2n2) (1 + 1/4π2λ2n2), (1) where I0is the “ true rheobase ” for steady excitation by a regular series of constant-current pulses, each of sufficient duration to attain the minimum threshold. The theory, in its present form, takes no account of the phenomena of absolute and relative refractoriness after an effective stimulus, so that equation (1) need not be expected to apply rigorously at frequencies so high that successive waves fall within the refractory periods, absolute and relative, of their predecessors. Nor can it allow for the fact that, owing to effects of electrical capacity at surfaces or membranes in the nerve, the current distribution in the nerve must change with change of frequency. It was important, however, to examine the relation between I andnover the wider range, and experiments were made at frequencies from nearly zero to 10,000 cycles per second. Those at the higher frequencies are referred to in § XI below; in the rest of the paper we deal only with the lower range (up to 300-1000 cycles per second, according to the tempera­ture).

Adaptation in the compound action potential response of the guinea pig VIIIth nerve to electric stimulation

An experimental study, carried out in guinea pigs, was designed to investigate whether forward masking measured psychophysically in 3M-House cochlear implant users might have a correlate in VIIIth nerve activity. The study was based on electrically evoked VIIIth nerve compound action potentials (ECAPs), using a masking paradigm comparable to the one used in the psychophysical study. Trains of 50 maskers with inter-masker-intervals of 509 ms appeared to induce a long-term fatigue effect that could influence the recovery from adaptation measurements. Fatigue stabilized within about 1 to 3 min when masker trains were repeated with intervening silent intervals of 10.5 s. The change in amplitude of probe-evoked ECAPs with increasing masker-probe delays was determined within the steady fatigue state. The recovery-from-adaptation functions obtained from these measurements resembled the forward masking functions found in 3M-House cochlear implant users. No correlate of psychophysical backward masking was found at the VIIIth nerve level. To examine whether hair cells were involved in fatigue and recovery from adaptation, the measurements described above were carried out in intact cochleas and in cochleas without hair cells. Results were essentially the same in the different preparations. The results suggest that processes at the level of the VIIIth nerve could, at least partly, account for forward masking found in 3M-House cochlear implant users. Backward masking must be attributed to mechanisms located centrally to the VIIIth nerve.