Synaptic Effect of Aδ-Fibers by Pulse-Train Electrical Stimulation.

Electrical stimulation of specific small fibers (Aδ- and C-fibers) is used in basic studies on nociception and neuropathic pain and to diagnose neuropathies. For selective stimulation of small fibers, the optimal stimulation waveform parameters are an important aspect together with the study of electrode design. However, determining an optimal stimulation condition is challenging, as it requires the characterization of the response of the small fibers to electrical stimulation. The perception thresholds are generally characterized using single-pulse stimulation based on the strength-duration curve. However, this does not account for the temporal effects of the different waveforms used in practical applications. In this study, we designed an experiment to characterize the effects of multiple pulse stimulation and proposed a computational model that considers electrostimulation of fibers and synaptic effects in a multiscale model. The measurements of perception thresholds showed that the pulse dependency of the threshold was an exponential decay with a maximum reduction of 55%. In addition, the frequency dependence of the threshold showed a U-shaped response with a reduction of 25% at 30 Hz. Moreover, the computational model explained the synaptic effects, which were also confirmed by evoked potential recordings. This study further characterized the activation of small fibers and clarified the synaptic effects, demonstrating the importance of waveform selection.

Efficacy of Silicone Conduit in the Rat Sciatic Nerve Repair Model: Journey of a Thousand Miles.

BACKGROUND: A lot of options have been tried for bridging the two ends of the injured nerves. Researchers have used decellularized nerve grafts, artificial materials and even nerve growth factors to augment functional recovery. These materials are either costly or inaccessible in developing world. OBJECTIVE: The study aimed to evaluate the efficacy of the silicone conduit in a rat sciatic nerve injury model. MATERIALS AND METHODS: 24 healthy Sprague-Dawley (SD) rats (250-300 grams; 8-10 weeks) were used and right sciatic nerve was exposed; transected and re-anastomosed by two different methods in 16 rats. In control group, n = 8 (Group I) the sciatic nerve was untouched; Group II (reverse nerve anastomosis, n = 8): 1-centimeter of nerve was cut and re-anastomosed by using 10-0 monofilament suture; Group III (silicone conduit, n = 8) 1-centimeter nerve segment was cut, replaced by silicone conduit and supplemented by fibrin glue]. Evaluation of nerve recovery was done functionally (pain threshold and sciatic functional index) over 3 months and histologically and electron microscopically. RESULTS: Functional results showed a trend of clinical improvement in Group III and II but recovery was poor and never reached up to normal. Histopathological and electron microscopic results showed an incomplete axonal regeneration in Groups II and III. Psychological analyses showed that no outwards signs of stress were present and none of the rats showed paw biting and teeth chattering. CONCLUSION: The silicone conduit graft may be an economical and effective alternative to presently available interposition grafts, however for short segments only.

Dosimetry Analysis in Non-brain Tissues During TMS Exposure of Broca's and M1 Areas.

For human protection, the internal electric field is used as a dosimetric quantity for electromagnetic fields lower than 5-10 MHz. According to international standards, in this frequency range, electrostimulation is the main adverse effect against which protection is needed. One of the topics to be investigated is the quantification of the internal electric field threshold levels of perception and pain. Pain has been reported as a side effect during transcranial magnetic stimulation (TMS), especially during stimulation of the Broca's (speech) area of the brain. In this study, we designed an experiment to conduct a dosimetry analysis to quantify the internal electric field corresponding to perception and pain thresholds when targeting the Broca's and M1 areas from magnetic stimulator exposure. Dosimetry analysis was conducted using a multi-scale analysis in an individualized head model to investigate electrostimulation in an axonal model. The main finding is that the stimulation on the primary motor cortex has higher perception and pain thresholds when compared to Broca's area. Also, TMS-induced electric field applied to Broca's area exhibited dependence on the coil orientation at lower electric field threshold which was found to be related to the location and thickness of pain fibers. The derived dosimetry quantities provide a scientific rationale for the development of human protection guidelines and the estimation of possible side effects of magnetic stimulation in clinical applications.

Evaluation of Peripheral Electrostimulation Thresholds in Human Model for Uniform Magnetic Field Exposure.

The external field strength according to the international guidelines and standards for human protection are derived to prevent peripheral nerve system pain at frequencies from 300-750 Hz to 1 MHz. In this frequency range, the stimulation is attributable to axon electrostimulation. One limitation in the current international guidelines is the lack of respective stimulation thresholds in the brain and peripheral nervous system from in vivo human measurements over a wide frequency range. This study investigates peripheral stimulation thresholds using a multi-scale computation based on a human anatomical model for uniform exposure. The nerve parameters are first adjusted from the measured data to fit the peripheral nerve in the trunk. From the parameters, the external magnetic field strength to stimulate the nerve was estimated. Here, the conservativeness of protection limits of the international guidelines and standards for peripheral stimulation was confirmed. The results showed a margin factor of 4-6 and 10-24 times between internal and external protection limits of Institute of Electrical and Electronics Engineers standard (IEEE C95.1) and International Commission on Non-Ionizing Radiation Protection guidelines, with the computed pain thresholds.