Properties of implanted electrodes for functional electrical stimulation

Implanted wire electrodes are increasingly being used for the functional electrical stimulation of muscles in partially paralysed patients, yet many of their basic characteristics are poorly understood. In this study we investigated the selectivity, recruitment characteristics and range of control of several types of electrode in triceps surae and plantaris muscles of anaesthetized cats. We found that nerve cuffs are more efficient and selective (i.e., cause less stimulus spread to surrounding muscles) than intramuscular electrodes. Bipolar intramuscular stimulation was more efficient and selective than monopolar stimulation, but only if the nerve entry point was between the electrodes. Monopolar electrodes are efficient and selective if located close to the nerve entry point, but their performance declines with distance from it. Nonetheless, for a variety of reasons monopolar stimulation provides the best compromise in many current applications. Short duration pulses offer the best efficiency (least charge per pulse to elicit force) but high peak currents, increasing the risk of electrode corrosion and tissue damage. Electrode size has little effect on recruitment and should therefore be maximised because this minimises current density.     

Comparison of neural damage induced by electrical stimulation with faradaic and capacitor electrodes

Arrays of platinum (faradaic) and anodized, sintered tantalum pentoxide (capacitor) electrodes were implanted bilaterally in the subdural space of the parietal cortex of the cat. Two weeks after implantation both types of electrodes were pulsed for seven hours with identical waveforms consisting of controlled-current, chargebalanced, symmetric, anodic-first pulse pairs, 400 μsec/phase and a charge density of 80–100 μC/cm² (microcoulombs per square cm) at 50 pps (pulses per second). One group of animals was sacrificed immediately following stimulation and a second smaller group one week after stimulation. Tissues beneath both types of pulsed electrodes were damaged, but the difference in damage for the two electrode types was not statistically significant. Tissue beneath unpulsed electrodes was normal. At the ultrastructural level, in animals killed immediately after stimulation, shrunken and hyperchromic neurons were intermixed with neurons showing early intracellular edema. Glial cells appeared essentially normal. In animals killed one week after stimulation most of the damaged neurons had recovered, but the presence of shrunken, vacuolated and degenerating neurons showed that some of the cells were damaged irreversibly. It is concluded that most of the neural damage from stimulations of the brain surface at the level used in this study derives from processes associated with passage of the stimulus current through tissue, such as neuronal hyperactivity rather than electrochemical reactions associated with current injection across the electrode-tissue interface, since such reactions occur only with the faradaic electrodes.     

Acute and chronic implantation of coiled wire intraneural electrodes during cyclical electrical stimulation

The posterior tibial nerves of 18 rabbits were intraneurally implanted with coiled wire electrodes for up to 9 weeks to evaluate their usefulness for neuromuscular electrical stimulation. In one group an electrode was implanted and removed in one leg while the other leg was chronically implanted. A second group was chronically implanted without electrical stimulation in one leg and implanted with cyclical electrical stimulation applied through the electrode in the other leg. No significant changes in nerve conduction velocities between the time of implantation and up to 9 weeks post-implantation were observed in either the stimulated or the non-stimulated nerves. Little change in motor current threshold was observed beyond 10 days postimplantation. The nerves showed little or no histologic demyelination or denervation in most specimens, although in about 40% of the nerves, a bulbous formation of connective tissue was observed at electrode entry and exit sites with some demyelination in these regions. The spinal cords showed no histologic abnormalities in either group. The gastrocenemius and soleus muscles showed only occasional signs of denervation. One cat was implanted in both the posterior tibial and peroneal nerves of each leg for a 4-year period. Threshold current showed very little change during the implantation period. The nerves showed minimal focal demyelination at the electrode site and the muscles showed normal fibers.     

Comparison of motor effects following subcortical electrical stimulation through electrodes in the globus pallidus internus and cortical transcranial magnetic stimulation

Current concepts of transcranial magnetic stimulation (TMS) over the primary motor cortex are still under debate as to whether inhibitory motor effects are exclusively of cortical origin. To further elucidate a potential subcortical influence on motor effects, we combined TMS and unilateral subcortical electrical stimulation (SES) of the corticospinal tract. SES was performed through implanted depth electrodes in eight patients treated with deep brain stimulation (DBS) for severe dystonia. Chronaxie, conduction velocity (CV) of the stimulated fibres and poststimulus time histograms of single motor unit recordings were calculated to provide evidence of an activation of large diameter myelinated fibres by SES. Excitatory and inhibitory motor effects recorded bilaterally from the first dorsal interosseus muscle were measured after SES and focal TMS of the motor cortex. This allowed us to compare motor effects of subcortical (direct) and cortical (mainly indirect) activation of corticospinal neurons. SES activated a fast conducting monosynaptic pathway to the alpha motoneuron. Motor responses elicited by SES had significantly shorter onset latency and shorter duration of the contralateral silent period compared to TMS induced motor effects. Spinal excitability as assessed by H-reflex was significantly reduced during the silent period after SES. No ipsilateral motor effects could be elicited by SES while TMS was followed by an ipsilateral inhibition. The results suggest that SES activated the corticospinal neurons at the level of the internal capsule. Comparison of SES and TMS induced motor effects reveals that the first part of the TMS induced contralateral silent period should be of spinal origin while its later part is due to cortical inhibitory mechanisms. Furthermore, the present results suggest that the ipsilateral inhibition is predominantly mediated via transcallosal pathways.This paper is dedicated to Bernd-Ulrich Meyer, who died in a plane accident     

Reduced fatigue in electrically stimulated muscle using dual channel intrafascicular electrodes with interleaved stimulation

Pairs of intrafascicular electrodes were implanted within single fascicles of the nerve innervating the gastrocnemius muscle in cats. Measurements were made of fatigue induced in the muscle by single and dual channel tetanic stimulation. The level and rate of fatigue induced by concurrent dual channel stimulation (pulses presented simultaneously to the two electrodes) did not differ significantly from that induced by single channel stimulation. However, a significant reduction in the level and rate of muscle fatigue was found with interleaved dual channel stimulation. The extent of reduction in fatigue was found to be inversely related to the amount of overlap in the axonal populations activated by each of the two electrodes.     

Reduction in inflammation-induced sensitization of dorsal horn neurons by transcutaneous electrical nerve stimulation in anesthetized rats

Transcutaneous electrical nerve stimulation (TENS) is utilized to treat a variety of painful conditions. Inflamed animals present with an increased response to noxious stimuli, i.e., hyperalgesia, at the site of injury (primary hyperalgesia) and outside the site of injury (secondary hyperalgesia). Further, following acute inflammation, dorsal horn neurons show an increased responsiveness to peripherally applied stimuli, which has been termed sensitization. Previous studies demonstrate a reduction in dorsal horn neuron activity following TENS treatment in normal animals and a reduction in primary and secondary hyperalgesia in acutely inflamed animals. The purpose of this study was to examine the effects of TENS on dorsal horn neurons sensitized by acute inflammation. Extracellular recordings from wide dynamic range (WDR), high threshold (HT) and low threshold (LT) dorsal horn neurons in anesthetized rats were assessed for spontaneous activity, responses to innocuous and noxious mechanical stimulation and receptive field size. Responses were measured before and 3 h after induction of inflammation, and immediately and 1 h after application of either high (100 Hz) or low (4 Hz) frequency TENS (motor intensity, pulse duration = 100 microseconds). TENS was applied to the inflamed paw to encompass the receptive field of the neuron for 20 min. WDR and HT dorsal horn neurons sensitized to mechanical stimulation after induction of inflammation. Application of either high or low frequency TENS to the inflamed paw reduced both innocuous and noxious evoked responses of WDR and HT dorsal horn neurons immediately and 1 h after treatment with TENS. Comparison of responses after TENS with baseline responses showed that the evoked responses in the majority of WDR and HT cells returned to or fell below baseline responses. TENS had no effect on responses of LT neurons. In summary, central neuron sensitization is reduced by TENS and may underlie the reduction in hyperalgesia observed after treatment with TENS.     

Magnetic and electrical stimulation of undulating nerve fibres: a simulation study

Mathematical models of myelinated nerve fibres are highly stylized abstractions of real nerve fibres. For example, nerve fibres are usually assumed to be perfectly straight. Such idealizations can cause discrepancies between theoretical predictions and experimental results. One well-known discrepancy is that the currently used models predict (contradictory to experimental findings) that an activation of nerve fibres is not possible with a pure transverse electric field. This situation occurs when a magnetic coil is placed symmetrically above a straight nerve fibre for magnetic nerve stimulation, or when an anode and a cathode are placed equidistantly on a line perpendicular to the fibre in the case of electrical stimulation. It is shown that this discrepancy does not occur if the physiological undulation of peripheral nerve fibres is included in the models. Even for small undulation amplitudes (e.g. 0.02 mm), it is possible to activate the fibre in these positions. For physiological undulations, as found in the literature, and favourable (off-centre) positions, the typical reduction of the thresholds is in a range between one and five, compared with perfectly straight fibres.     

Magnetic and electrical stimulation of undulating nerve fibres: a simulation study

Mathematical models of myelinated nerve fibres are highly stylised abstractions of real nerve fibres. For example, nerve fibres are usually assumed to be perfectly straight. Such idealisations can cause discrepancies between theoretical predictions and experimental results. One well-known discrepancy is that the currently used models predict (contradictory to experimental findings) that an activation of nerve fibres is not possible with a pure transverse electric field. This situation occurs when a magnetic coil is placed symmetrically above a straight nerve fibre for magnetic nerve stimulation, or when an anode and a cathode are placed equidistantly on a line perpendicular to the fibre in the case of electrical stimulation. It is shown that this discrepancy does not occur if the physiological undulation of peripheral nerve fibres is included in the models. Even for small undulation amplitudes (e.g. 0.02 mm), it is possible to activate the fibre in these positions. For physiological undulations, as found in the literature, and favourable (off-centre) positions, the typical reduction of the thresholds is in a range between one and five, compared with perfectly straight fibres.     

Selective activation of muscles using peripheral nerve electrodes

The feasibility of two methods for selectively activating muscles with peripheral nerve electrodes has been investigated. One method for achieving selectivity is to place a cuff electrode around the nerves to each group of synergistic muscles to be stimulated. A second method is to stimulate through pairs of electrodes selected from a multielectrode array placed around a common nerve trunk. Both methods have been tesed in experiments conducted on four dogs. It was shown that the first method, cuff electrodes placed on individual motor branches, is an effective technique for selective activation, Thresholds of motor bibres lying outside of, but adjacent to, cuff electrodes are much greater than the stimulus amplitudes required to maximally stimulate motor fibres contained within the cuff electrode. Good results were obtained with a multielectrode array in two animals, but results, were poor in a third dog. Electrode position and contact with the nerve were found to be important factors in achieving good selectivity.     

术中直接皮层电刺激判断大脑功能区在胶质瘤切除术中的应用

目的:试用术中直接电刺激判断大脑功能区的位置和范围的方法,以求术中最大程度切除肿瘤,减少神经系统损伤。方法:对唤醒麻醉下手术切除优势半球语言区胶质瘤患者26例以及邻近或累及脑运动功能区胶质瘤患者26例,在术中直接行皮层刺激,判断功能区的位置和肿瘤的关系。结果:患者在唤醒麻醉下,利用直接皮质电刺激可以准确定位初级运动皮质区和语言功能区,术后患者Karnofsky生活状态(KPS)评分结果较术前提高。结论:在唤醒麻醉下,可检测到患者的运动和语言的脑功能区,并可判断与肿瘤位置的关系。术中采用直接皮质电刺激可定位脑重要功能区,能最大限度地切除病变,最大限度地保护脑功能区。     

电刺激效应在血管新生中的作用

人体生命活动的不断延续,往往伴随着生物电现象的产生,一旦这种电生理环境发生改变,机体易于陷入疾病状态。那么基于这种电生理环境的改变以及电刺激技术的发展,借助电刺激技术改变疾病状态。从而修复机体结构和功能的治疗手段得到大量研究者的证实。血管新生是一个多因素参与、多步骤演变的过程。多种病变情况下如创伤愈合、组织再生修复和肿瘤生长、转移均牵涉到血管的新生,同时也伴随着生物电现象的变化。就近年来电刺激技术作用于血管新生的研究进展作一综述。     

Effects of waveform parameters on comfort during transcutaneous neuromuscular electrical stimulation

Twenty-three females between the ages of 19 and 35 were studied in order to compare the effects of variations in pulse duration, waveform symmetry, and source regulation on comfort during quadriceps surface stimulation at amplitudes necessary to produce 27 Nm torque. Stimulation parameters compared were: 1) 50 and 300 μs pulse durations, 2) asymmetrical and symmetrical biphasic waveforms, and 3) current and voltage source regulation. Subjects overwhelmingly preferred the 300 μs pulse duration regardless of waveform or source regulation, strongly preferred the symmetrical biphasic waveform, and had inconsistent preference for either regulated voltage or regulated current sources.     

Contraction characteristics of the human quadriceps muscle during percutaneous electrical stimulation

Percutaneous electrical stimulation was used to study the force response of the quadriceps muscle. The normal frequency dependence of force was investigated in muscles at rest and after fatiguing contractions. A comparison between force response during fatigue induced by electrical stimulation at different frequencies and by voluntary work suggested equal changes in contractility, irrespective of the fatigue-inducing procedure. In fresh muscle we found a linear relation between stimulation period (10–100 ms) and force. At fatigue the relation changes with maximal deviation from linearity at a 50-ms period (20 Hz). There is a rapid recovery of high frequency force whereas the low frequency response remains low even after 30 min rest. At very low frequencies there is initially unexpectedly high force in fatigued muscle. This could be a result of increased fusion of twitches with initially prolonged relaxation time. To study the twitch summation we compared experimental results in a wide frequency range with computer-simulated twitch summations and present the frequency dependence of summation processes in human quadriceps muscle.     

Multielectrode intrafascicular and extraneural stimulation

The relationship between nerve stimulation, pulse amplitude and isometric muscle force was measured to investigate recruitment of motor units. Force addition experiments were performed to obtain insight in the intersection of motor unit groups recruited by different electrodes. Intrafascicular and extraneural multielectrode configurations were used for nerve stimulation. Experiments were performed on rats. The common peroneal nerve was stimulated and the forces of the tibial anterior and extensor digitorum longus muscles were measured isometrically. Recruitment was more stable for intrafascicular electrodes than for extraneural electrodes. Especially for intrafascicular electrodes no strict inverse recruitment was observed. Force addition experiments indicated that small overlap of recruited motor unit groups occurred more often for intrafascicular than for extraneural electrodes.     

Attenuation of pathological tremors by functional electrical stimulation I: Method

In this study we explored the possibility of suppressing pathological tremors using closed-loop functional electrical stimulation (FES) to activate the tremorogenic muscles out-of-phase. A displacement signal monitored with a transducer was filtered so as to be “tuned” to the tremor frequency at the wrist or elbow. The filtered signal was used to amplitude-modulate the electrical stimulation. The design process was based on measurements of the open-loop frequency response characteristics of the forearm and hand to stimulation of the elbow and wrist flexors and extensors in a number of subjects. These data allowed us to identify closed-loop configurations, which attenuated 2–5 Hz tremors substantially, while only minimally attenuating functional movements in the 0–1 Hz range. There was a fairly delicate balance between efficacy and the risk of instability. However, designs were identified that offered enough tremor suppression and adequate immunity to muscle/load variations for the technique to be considered seriously for clinical application.     

Production of elastic electrodes for nerve stimulation

For better evoking and monitoring electrical activities of visceral nerves, a elastic electrode made of conductive plastic and silicone gum is developed.     

Stump nerve signals during transcranial magnetic motor cortex stimulation recorded in an amputee via longitudinal intrafascicular electrodes

Do central and peripheral motor pathways associated with an amputated limb retain at least some functions over periods of years? This problem could be addressed by evaluating the response patterns of nerve signals from peripheral motor fibers during transcranial magnetic stimulation (TMS) of corticospinal tracts. The aim of this study was to record for the first time TMS-related responses from the nerves of a left arm stump of an amputee via intrafascicular longitudinal flexible multi-electrodes (tfLIFE4) implanted for a prosthetic hand control. After tfLIFE4 implant in the stump median and ulnar nerves, TMS impulses of increasing intensity were delivered to the contralateral motor cortex while tfLIFE4 recordings were carried out. Combining TMS of increasing intensity and tfLIFE4 electrodes recordings, motor nerve activity possibly related to the missing limb motor control and selectively triggered by brain stimulation without significant electromyographic contamination was identified. These findings are entirely original and indicate that tfLIFE4 signals are clearly driven from M1 stimulation, therefore witnessing the presence in the stump nerves of viable motor signals from the CNS possibly useful for artificial prosthesis control.     

Three-dimensional current density distribution under surface stimulation electrodes

Overflow to non-target tissue during FNS can be reduced by controlling current density distribution under surface stimulating electrodes. A method is introduced for the acquisition of 3-D current density distributions under complex surface stimulating FNS electrode geometries. The method makes use of a phantom model in which a conventional homogeneous model has been improved by adding a layer to simulate skin impedance properties, based on specific FNS parameters. Signal acquisition and processing circuits have been developed to simulate the process by which excitable tissue responds to external stimulation. In addition, a data analysis method has been introduced to allow for the characterisation of stimulation current intensity, electrode geometry and pulse waveform required to achieve target muscle activation, with minimal overflow and to avoid pain or burning. Measurements of integrated differential voltage corresponding to current density distribution acquired under electrodes of various geometries are presented in terms of 3-D attenuation coefficient maps as examples of the applicability of the method.     

Estimation of the distribution of intramuscular current during electrical stimulation of the quadriceps muscle

Electrical stimulation is commonly used for strengthening muscle but little evidence exists as to the optimal electrode size, waveform, or frequency to apply. Three male and three female subjects (22–40 years old) were examined during electrical stimulation of the quadriceps muscle. Two self adhesive electrode sizes were examined, 2 cm × 2 cm and 2 cm × 4 cm. Electrical stimulation was applied with square and sine waveforms, currents of 5, 10 and 15 mA, and pulse widths of 100–500 μs above the quadriceps muscle. Frequencies of stimulation were 20, 30, and 50 Hz. Current on the skin above the quadriceps muscle was measured with surface electrodes at five positions and at three positions with needle electrodes in the same muscle. Altering pulse width in the range of 100–500 μs, the frequency over a range of 20–50 Hz, or current from 5 to 15 mA had no effect on current dispersion either in the skin or within muscle. In contrast, the distance separating the electrodes caused large changes in current dispersion on the skin or into muscle. The most significant finding in the present investigation was that, while on the surface of the skin current dispersion was not different between sine and square wave stimulation, significantly more current was transferred deep in the muscle with sine versus square wave stimulation. The use of sine wave stimulation with electrode separation distances of less then 15 cm is recommended for electrical stimulation with a sine wave to achieve deep muscle stimulation.