Abstract
There is multiple evidence in the literature that a sub-threshold pre-pulse, delivered immediately prior to an electrical stimulation pulse, can alter the activation threshold of nerve fibers and motor unit recruitment characteristics. So far, previously published works combined monophasic stimuli with sub-threshold depolarizing pre-pulses (DPPs) with inconsistent findings—in some studies, the DPPs decreased the activation threshold, while in others it was increased. This work aimed to evaluate the effect of DPPs during biphasic transcutaneous electrical stimulation and to study the possible mechanism underlying those differences. Sub-threshold DPPs between 0.5 and 15 ms immediately followed by biphasic or monophasic pulses were administered to the tibial nerve; the electrophysiological muscular responses (motor-wave, M-wave) were monitored via electromyogram (EMG) recording from the soleus muscle. The data show that, under the specific studied conditions, DPPs tend to lower the threshold for nerve fiber activation rather than elevating it. DPPs with the same polarity as the leading phase of biphasic stimuli are more effective to increase the sensitivity. This work assesses for the first time the effect of DPPs on biphasic pulses, which are required to achieve charge-balanced stimulation, and it provides guidance on the effect of polarity and intensity to take full advantage of this feature.Graphical abstractIn this work, the effect of sub-threshold depolarizing pre-pulses (DPP) is investigated in a setup with transcutaneous electrical stimulation. We found that, within the tested 0–15 ms DPP duration range, the DPPs administered immediately before biphasic pulses proportionally increase the nerve excitability as visible in the M-waves recorded from the soleus muscle. Interestingly, these findings oppose published results, where DPPs, administered immediately before monophasic stimuli via implanted electrodes, led to decrease of nerve excitability.
Highlights
Clinical and research environments use electrical stimulation (ES) widely
The results show that a depolarizing pre-pulses (DPPs) prior to the stimulation pulse increases the H-reflex, except by DPPs with the inverse polarity on monophasic pulses (Table 2)
Further assessments should be done to validate such result in other electrode setups. This is the first work that assesses the effect of depolarizing pre-pulses with biphasic pulses
Summary
Clinical and research environments use electrical stimulation (ES) widely. ES induces an artificial electrical field that depolarizes the nerve fibers near the active electrodes and, if the depolarization is large enough, it evokes action potentials in such fibers [1]. ES can serve as a powerful tool for restoration of movement by non-invasive treatments and neuroprostheses applications, reducing potential risks as associated with implant-based invasive solutions. A significant limitation of electrical stimulation is the difficulty to selectively activate target population fibers. Studies with ex vivo experiments [7], implanted electrodes [3] and computer simulations [3, 5] have given substantial evidence that DPPs inhibit activation of the larger diameter fibers as well as fibers in close distance to stimulation electrodes. Grill and Mortimer proposed that the non-linear properties of the cell membrane
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