Effect of posture and body weight loading on spinal posterior root reflex responses

The posterior root muscle response (PRM) is a monosynaptic reflex that is evoked by single pulse transcutaneous spinal cord stimulation (tSCS). The main aim of this work was to analyse how body weight loading influences PRM reflex threshold measured from several lower limb muscles in healthy participants. PRM reflex responses were evoked with 1-ms rectangular monophasic pulses applied at an interval of 6 s via a self-adhesive electrode (9 × 5 cm) at the T11-T12 vertebral level. Surface electromyographic activity of lower limb muscles was recorded during four different conditions, one in decubitus supine (DS) and the other three involving standing at 100%, 50%, and 0% body weight loading (BW). PRM threshold intensity, peak-to-peak amplitude, and latency for each muscle were analysed in different conditions study. PRM reflex threshold increased with body weight unloading compared with DS, and the largest change was observed between DS and 0% BW for the proximal muscles and between DS and 50% BW for distal muscles. Peak-to-peak amplitude analysis showed only a significant mean decrease of 34.6% (SD 10.4, p = 0.028) in TA and 53.6% (SD 15.1, p = 0.019) in GM muscles between DS and 50% BW. No significant differences were observed for PRM latency. This study has shown that sensorimotor networks can be activated with tSCS in various conditions of body weight unloading. Higher stimulus intensities are necessary to evoke reflex response during standing at 50% body weight loading. These results have practical implications for gait rehabilitation training programmes that include body weight support.

Infrared neurostimulation inex-vivorat sciatic nerve using 1470 nm wavelength

Objective.To design and implement a setup forex-vivooptical stimulation for exploring the effect of several key parameters (optical power and pulse duration), activation features (threshold, spatial selectivity) and recovery characteristics (repeated stimuli) in peripheral nerves.Approach.A nerve chamber allowing ex-vivo electrical and optical stimulation was designed and built. A 1470 nm light source was chosen to stimulate the nerve. A photodiode module was implemented for synchronization of the electrical and optical channels.Main results. Compound neural action potentials (CNAPs) were successfully generated with infrared light pulses of 200-2000µs duration and power in the range of 3-10 W. These parameters determine a radiant exposure for stimulation in the range 1.59-4.78 J cm^-2. Recruitment curves were obtained by increasing durations at a constant power level. Neural activation threshold is reached at a mean radiant exposure of 3.16 ± 0.68 J cm^-2and mean pulse energy of 3.79 ± 0.72 mJ. Repetition rates of 2-10 Hz have been explored. In eight out of ten sciatic nerves (SNs), repeated light stimuli induced a sensitization effect in that the CNAP amplitude progressively grows, representing an increasing number of recruited fibres. In two out of ten SNs, CNAPs were composed of a succession of peaks corresponding to different conduction velocities.Significance.The reported sensitization effect could shed light on the mechanism underlying infrared neurostimulation. Our results suggest that, in sharp contrast with electrical stimuli, optical pulses could recruit slow fibres early on. This more physiological order of recruitment opens the perspective for specific neuromodulation of fibre population who remained poorly accessible until now. Short high-power light pulses at wavelengths below 1.5µm offer interesting perspectives for neurostimulation.

Estimation of Intraoperative Stimulation Threshold of the Facial Nerve in Patients Undergoing Microvascular Decompression

Introduction  Facial weakness can result from surgical manipulation of the facial nerve. Intraoperative neuromonitoring reduces functional impairment but no clear guidelines exist regarding interpretation of intraoperative electrophysiological results. Most studies describe subjects with facial nerves encumbered by tumors or those with various grades of facial nerve weakness. We sought to obtain the neurophysiological parameters and stimulation threshold following intraoperative facial nerve triggered electromyography (t-EMG) stimulation during microvascular decompression for trigeminal neuralgia to characterize the response of normal facial nerves via t-EMG. Methods  Facial nerve t-EMG stimulation was performed in seven patients undergoing microvascular decompression for trigeminal neuralgia. Using constant current stimulation, single stimulation pulses of 0.025 to 0.2 mA intensity were applied to the proximal facial nerve. Compound muscle action potentials, duration to onset, and termination of t-EMG responses were recorded for the orbicularis oculi and mentalis muscles. Patients were evaluated for facial weakness following the surgical procedure. Results  Quantifiable t-EMG responses were generated in response to all tested stimulation currents of 0.025, 0.05, 0.1, and 0.2 mA in both muscles, indicating effective nerve conduction. No patients developed facial weakness postoperatively. Conclusions  The presence of t-EMG amplitudes in response to 0.025 mA suggests that facial nerve conduction can take place at lower stimulation intensities than previously reported in patients with tumor burden. Proximal facial nerve stimulation that yields responses with thresholds less than 0.05 mA may be a preferred reference baseline for surgical procedures within the cerebellopontine angle to prevent iatrogenic injury.

[Basics of electrical cardiac stimulation]

An essential part of a pacemaker system is the lead with a stimulation and sensing electrode. Both funtions, which are completely different processes, are performed by the same electrode. Requirements for a pacemaker lead are: outstanding bio-compatibility, low pacing threshold, reliable sensing, high pacing impedance, low polarisation and corresponding longterm stability. This article summarizes the up-to-date knowledge of the effects of cardiac pacing on the cell and the electrode-myocardium-inferface and its consequences on the design of modern pacemaker leads.    The underlying mechanisms for the electrical stimulation of the myocardial cell are determined by the membrane. The myocardial answer of the cell on an adequate stimulus is the generation of a self-regenerating depolarisation wave called excitability. The stimulation impulse generates an electrical field, which causes a decrease in the baseline membrane potential (hypopolarisation) by an ionic current. As a consequence further sodium channels are opened until the threshold potential is reached and the depolarisation of the cell is initiated. The minimal amount of energy required for this process is called stimulation threshold, which depends on the electrode surface, the stimulation impedance, the alignment of the myocardial fibers in the electrical field, the distance to excitable tissue and the polarisation current. The relationship between pacing current and impulse duration is described by rheobase and chronaxy. Implantable pacemaker devices work with biphasic stimulation impulses, which allow a recharge of the delivered impulse energy from the heart to the stimulator and therefore avoid an electrolysis.    The improved understanding of the basic principles of cardiac pacing led to the development of modern stimulation electrodes. Steroid eluting leads avoid an increase in pacing threshold in the initial period. The geometrical surface of electrodes was reduced with the effect of lower pacing thresholds. The introduction of new electrodes with a porous or fractal surface led to a reduction in polarisation and therefore to an improved stimulation with regard to energy delivery. The new pacemaker generation allows a monitoring or analysis of stimulation. With the help of certain algorithms the efficacy of every single ventricular stimulus is monitored (autocapture function) and the pacing amplitude automatically adjusted to the actual situation. This prolongs the pacemaker's life and simplifies follow-up.