Orderly stimulation of skeletal muscle motor units with tripolar nerve cuff electrode

Different Transcutaneous Auricular Vagus Nerve Stimulation Parameters Modulate the Anti-Inflammatory Effects on Lipopolysaccharide-Induced Acute Inflammation in Mice

Vagus nerve stimulation (VNS) is considered a potential method for anti-inflammation due to the involvement of the VN in the cholinergic anti-inflammatory pathway (CAP) formation of a connection between the central nervous system and peripheral immune cells that help relieve inflammation. However, whether a non-invasive transcutaneous auricular VNS (taVNS) modulates the inflammation levels via altering the parameter of taVNS is poorly understood. This study aimed to determine the differential inhibitory effects of taVNS on lipopolysaccharide (LPS)-induced systemic inflammation using electrical stimulation parameters such as pulse frequency and time. The taVNS-promoted CAP activity significantly recovered LPS-induced tissue injuries (lung, spleen, and intestine) and decreased inflammatory cytokine levels and tissue-infiltrated immune cells. Interestingly, the anti-inflammatory capacity of taVNS with 15 Hz was much higher than that of taVNS with 25 Hz. When a cytokine array was used to investigate the changes of inflammation and immune response-related cytokines/chemokines expression in taVNS with 15 Hz or 25 Hz treatment in LPS-induced endotoxemia in mice, most of the expression of cytokines/chemokines associated with pro-inflammation was severely decreased in taVNS with 15 Hz compared to 25 Hz. This study demonstrated that the taVNS parameter could differentially modulate the inflammation levels of animals, suggesting the importance of taVNS parameter selection for use in feasible interventions for acute inflammation treatment.

Kilohertz-frequency stimulation of the nervous system: A review of underlying mechanisms

BACKGROUND

Electrical stimulation in the kilohertz-frequency range has gained interest in the field of neuroscience. The mechanisms underlying stimulation in this frequency range, however, are poorly characterized to date.

OBJECTIVE/HYPOTHESIS

To summarize the manifold biological effects elicited by kilohertz-frequency stimulation in the context of the currently existing literature and provide a mechanistic framework for the neural responses observed in this frequency range.

METHODS

A comprehensive search of the peer-reviewed literature was conducted across electronic databases. Relevant computational, clinical, and mechanistic studies were selected for review.

RESULTS

The effects of kilohertz-frequency stimulation on neural tissue are diverse and yield effects that are distinct from conventional stimulation. Broadly, these can be divided into 1) subthreshold, 2) suprathreshold, 3) synaptic and 4) thermal effects. While facilitation is the dominating mechanism at the subthreshold level, desynchronization, spike-rate adaptation, conduction block, and non-monotonic activation can be observed during suprathreshold kilohertz-frequency stimulation. At the synaptic level, kilohertz-frequency stimulation has been associated with the transient depletion of the available neurotransmitter pool - also known as synaptic fatigue. Finally, thermal effects associated with extrinsic (environmental) and intrinsic (associated with kilohertz-frequency stimulation) temperature changes have been suggested to alter the neural response to stimulation paradigms.

CONCLUSION

The diverse spectrum of neural responses to stimulation in the kilohertz-frequency range is distinct from that associated with conventional stimulation. This offers the potential for new therapeutic avenues across stimulation modalities. However, stimulation in the kilohertz-frequency range is associated with distinct challenges and caveats that need to be considered in experimental paradigms.

Interleaved intramuscular stimulation with minimally overlapping electrodes evokes smooth and fatigue resistant forces

OBJECTIVE

It is known that multi-site interleaved stimulation generates less muscle fatigue compared to single-site synchronous stimulation. However, in the limited number of studies in which intramuscular electrodes were used, the fatigue reduction associated with interleaved stimulation could not consistently be achieved. We hypothesize that this could be due to the inability to place the intramuscular electrodes used in interleaved stimulation in locations that minimize overlap amongst the motor units activated by the electrodes. Our objective in the present study was to use independent intramuscular electrodes to compare fatigue induced by interleaved stimulation with that generated by synchronous stimulation at the same initial force and ripple.

APPROACH

In the medial gastrocnemius muscle of an anesthetized rabbit (n = 3), ten intramuscular hook wire electrodes were inserted at different distances from the nerve entry. Overlap was measured using the refractory technique and only three electrodes were found to be highly independent. After ensuring that forces obtained by both stimulation modalities had the same ripple and magnitude, fatigue induced during interleaved stimulation across three independent distal electrodes was compared to that obtained by synchronously delivering pulses to a single proximal electrode.

MAIN RESULTS

Contractions evoked by interleaved stimulation exhibited less fatigue than those evoked by synchronous stimulation. Twitch force recruitment curves collected from each of the ten intramuscular electrodes showed frequent intermediate plateaus and the force value at these plateaus decreased as the distance between the electrode and nerve entry increased.

SIGNIFICANCE

The results indicate that interleaved intramuscular stimulation is preferred over synchronous intramuscular stimulation when fatigue-resistant and smooth forces are desired. In addition, the results suggest that the large muscle compartments innervated by the primary intramuscular nerve branches give rise to progressively smaller independent compartments in subsequent nerve divisions.

Specific vagus nerve stimulation parameters alter serum cytokine levels in the absence of inflammation

Background

Electrical stimulation of peripheral nerves is a widely used technique to treat a variety of conditions including chronic pain, motor impairment, headaches, and epilepsy. Nerve stimulation to achieve efficacious symptomatic relief depends on the proper selection of electrical stimulation parameters to recruit the appropriate fibers within a nerve. Recently, electrical stimulation of the vagus nerve has shown promise for controlling inflammation and clinical trials have demonstrated efficacy for the treatment of inflammatory disorders. This application of vagus nerve stimulation activates the inflammatory reflex, reducing levels of inflammatory cytokines during inflammation.

Methods

Here, we wanted to test whether altering the parameters of electrical vagus nerve stimulation would change circulating cytokine levels of normal healthy animals in the absence of increased inflammation. To examine this, we systematically tested a set of electrical stimulation parameters and measured serum cytokine levels in healthy mice.

Results

Surprisingly, we found that specific combinations of pulse width, pulse amplitude, and frequency produced significant increases of the pro-inflammatory cytokine tumor necrosis factor (TNF), while other parameters selectively lowered serum TNF levels, as compared to sham-stimulated mice. In addition, serum levels of the anti-inflammatory cytokine interleukin-10 (IL-10) were significantly increased by select parameters of electrical stimulation but remained unchanged with others.

Conclusions

These results indicate that electrical stimulation parameter selection is critically important for the modulation of cytokines via the cervical vagus nerve and that specific cytokines can be increased by electrical stimulation in the absence of inflammation. As the next generation of bioelectronic therapies and devices are developed to capitalize on the neural regulation of inflammation, the selection of nerve stimulation parameters will be a critically important variable for achieving cytokine-specific changes.

Selective stimulation of human intrinsic laryngeal muscles: Analysis in a mathematical three-dimensional space

OBJECTIVES/HYPOTHESIS

Standard stimulating methods using square waves do not appropriately restore physiological control of individual intrinsic laryngeal muscles (ILMs). To further explore our earlier study of evoked orderly recruitment by quasitrapezoidal (QT) currents, we integrated the contribution of the cricothyroideus (CT) with attention to mutual activation in an additional patient, based on recent studies of appropriate responses via strict recurrent laryngeal nerve (RLN) stimulation.

STUDY DESIGN

Basic science study.

METHODS

The patient received functional electrical stimulation (FES) with QT pulses at 5 Hz, 60 to 2,000 μAmp, 100 to 500 μs pulse width, 0 to 500 μs decay. Ipsilateral electromyography (EMG) responses were calculated using the average maximum amplitude, area under the curve, and the root mean square of the rectified amplitude waveforms. The thyroarytenoideus (TA), posterior cricoarytenoideus (PCA), lateral cricothyroideus (LCA), and the CT were each interrogated via two monopolar electrodes, values were recorded in MATLAB, exported to Excel, and analyzed. Individual and mutual recruitment configurations and activation delays with stimulation were explored using multiple regression and exploration factor analyses.

RESULTS

A total of 868 EMG data points based on 18 trials and up to 11 subtrials were captured from each of the four ILMs. Various combinations of pulse amplitude, pulse width, and exponential decay were found to produce significant (P ≤ .001) individual ILM responses. CT mirrored the LCA, whereas the TA and PCA exhibited separate interactions along shared trajectories in a three-dimensional space.

CONCLUSIONS

FES calibrated to individual and coupled ILMs offers promise for restoring normal and pathological contraction patterns via strict RLN stimulation.

LEVEL OF EVIDENCE

5 Laryngoscope, 130:967-973, 2020.

Measurement of block thresholds in kiloHertz frequency alternating current peripheral nerve block

BACKGROUND

Kilohertz frequency alternating currents (KHFAC) produce rapid nerve conduction block of mammalian peripheral nerve and have potential clinical applications in reducing peripheral nerve hyperactivity. The experimental investigation of KHFAC nerve block requires a robust output measure and this has proven to be the block threshold (BT), the lowest current or voltage at which the axons of interest are completely blocked. All significant literature in KHFAC nerve block, both simulations and experimental, were reviewed to determine the block threshold method that was used. The two common methods used are the High-Low method experimentally and the Binary search method for simulations.

NEW METHOD

Four methods to measure the block threshold (High-Low, High-Low-High, Binary and Random) at three frequencies (10, 20 and 30 kHz) were compared through randomized repeated experiments in the in-vivo rodent sciatic nerve-gastrocnemius model.

RESULTS

The literature review showed that more than 50% of publications did not measure the block threshold. The experimental results showed no statistical difference in the BT value between the four methods.

COMPARISON WITH EXISTING METHOD(S)

However, there were differences in the number of significant onset responses, depending on the method. The run time for the BT determination was the shortest for the High-Low method.

CONCLUSIONS

It is recommended that all research in electrical nerve block, including KHFAC, should include measurement of the BT. The High-Low method is recommended for most experimental situations but the Binary method could also be a viable option, especially where onset responses are minimal.

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.

Artificial Voice Modulation in Dogs by Recurrent Laryngeal Nerve Stimulation: Electrophysiological Confirmation of Anatomic Data

Objectives:

We hypothesized that voice may be artificially manipulated to ameliorate dystonias considered to be a failure in dynamic integration between competing neuromuscular systems.

Methods:

Orderly intrinsic laryngeal muscle recruitment by anodal block via the recurrent laryngeal and vagus nerves has allowed us to define specific values based on differential excitabilities, but has precluded voice fluency because of focused breaks during stimulation and the need to treat several neural conduits. Such problems may be obviated by a circuit capable of stimulating some axons while simultaneously blocking others in the recurrent laryngeal nerve, which carries innervation to all intrinsic laryngeal muscles, including the arguably intrinsic cricothyroideus. In 5 dogs, both recurrent laryngeal nerves received 40-Hz quasi-trapezoidal pulses (0 to 2,000 μA, 0 to 2,000 μs, 0 to 500 μs decay) via tripolar electrodes. Electromyograms were matched with audio intensities and fundamental frequencies recorded under a constant flow of humidified air. Data were digitized and evaluated for potential correlations.

Results:

Orderly recruitment of the thyroarytenoideus, posterior cricoarytenoideus, and cricothyroideus was correlated with stimulating intensities (p < .001), and posterior cricoarytenoideus opposition to the thyroarytenoideus and cricothyroideus was instrumental in manipulating audio intensities and fundamental frequencies.

Conclusions:

Manipulation of canine voice parameters appears feasible via the sole recurrent laryngeal nerve within appropriate stimulation envelopes, and offers promise in human laryngeal dystonias.

Differential responses to high-frequency electrical stimulation in ON and OFF retinal ganglion cells

OBJECTIVE

The field of retinal prosthetics for artificial vision has advanced considerably in recent years, however clinical outcomes remain inconsistent. The performance of retinal prostheses is likely limited by the inability of electrical stimuli to preferentially activate different types of retinal ganglion cell (RGC).

APPROACH

Here we examine the response of rabbit RGCs to high-frequency stimulation, using biphasic pulses applied at 2000 pulses per second. Responses were recorded using cell-attached patch clamp methods, and stimulation was applied epiretinally via a small cone electrode.

MAIN RESULTS

When prolonged stimulus trains were applied to OFF-brisk transient (BT) RGCs, the cells exhibited a non-monotonic relationship between response strength and stimulus amplitude; this response pattern was different from those elicited previously by other electrical stimuli. When the amplitude of the stimulus was modulated transiently from a non-zero baseline amplitude, ON-BT and OFF-BT cells exhibited different activity patterns: ON cells showed an increase in activity while OFF cells exhibited a decrease in activity. Using a different envelope to modulate the amplitude of the stimulus, we observed the opposite effect: ON cells exhibited a decrease in activity while OFF cells show an increase in activity.

SIGNIFICANCE

As ON and OFF RGCs often exhibit opposing activity patterns in response to light stimulation, this work suggests that high-frequency electrical stimulation of RGCs may be able to elicit responses that are more physiological than traditional pulsatile stimuli. Additionally, the prospect of an electrical stimulus capable of cell-type specific selective activation has broad applications throughout the fields of neural stimulation and neuroprostheses.

Diameter-dependent excitation of peripheral nerve fibers by multipolar electrodes during electrical stimulation

Neural prostheses are technical devices that interface the nervous system to restore lost body functions by means of electrical stimulation, and to increase the activities of daily living of disabled persons--at least to some extent. One of the major reasons for the limited performance of neural prostheses is caused by the inverse recruitment of axons via excitation, by means of electrical stimulation. Due to their biophysical properties, electrical stimulation excites axons according to their diameter, starting with large-diameter fibers. This is the inverse order with respect to physiologic recruitment. It causes muscle fatigue, dyssynergia and limited muscle force control, respectively. A new electrode arrangement, with longitudinal multipolar stimulation, allows the selection of fiber diameters by its design with rectangular stimulation pulses and a reasonable amount of charge per phase. This report discusses the impact of the new approach in comparison with other stimulation paradigms to obtain fiber-selective nerve activation and its opportunities for neural prostheses in context with miniaturization trends in implant technology.

Reversible nerve conduction block using kilohertz frequency alternating current

OBJECTIVES

The features and clinical applications of balanced-charge kilohertz frequency alternating currents (KHFAC) are reviewed. Preclinical studies of KHFAC block have demonstrated that it can produce an extremely rapid and reversible block of nerve conduction. Recent systematic analysis and experimentation utilizing KHFAC block have resulted in a significant increase in interest in KHFAC block, both scientifically and clinically.

MATERIALS AND METHODS

We review the history and characteristics of KHFAC block, the methods used to investigate this type of block, the experimental evaluation of block, and the electrical parameters and electrode designs needed to achieve successful block. We then analyze the existing clinical applications of high-frequency currents, comparing the early results with the known features of KHFAC block.

RESULTS

Although many features of KHFAC block have been characterized, there is still much that is unknown regarding the response of neural structures to rapidly fluctuating electrical fields. The clinical reports to date do not provide sufficient information to properly evaluate the mechanisms that result in successful or unsuccessful treatment.

CONCLUSIONS

KHFAC nerve block has significant potential as a means of controlling nerve activity for the purpose of treating disease. However, early clinical studies in the use of high-frequency currents for the treatment of pain have not been designed to elucidate mechanisms or allow direct comparisons to preclinical data. We strongly encourage the careful reporting of the parameters utilized in these clinical studies, as well as the development of outcome measures that could illuminate the mechanisms of this modality.

Post stimulus effects of high frequency biphasic electrical current on a fibre's conductibility in isolated frog nerves

OBJECTIVE

High frequency biphasic (HFB) electrical currents are widely used in nerve blocking studies. Their safety margins largely remain unknown and need to be investigated.

APPROACH

This study, exploring the post stimulus effects of HFB electrical currents on a nerve's conductibility, was performed on bullfrog sciatic nerves. Both compound action potentials (CAPs) and differential CAPs (DCAPs, i.e. control CAPs subtracted by CAPs following HFB currents) were obtained, and N1 and N2 components, which were the first and second upward components of DCAPs, were used for analyses of the effects introduced by HFB electrical stimulation.

MAIN RESULTS

First, HFB currents of 10 kHz at a completely blocking threshold were applied for 5 s. The maximum amplitudes and conducting velocities of the CAPs were significantly (P < 0.02) decreased within the observed period (60 s) following HFB currents. The DCAPs displayed clear N1 and N2 components, demonstrating respectively the losses of the fibres' normal conductibility and the appearances of new delayed conductions. Decreases of N1 amplitudes along time, regarded as the recovery of the nerve's conductibility, exhibited two distinct phases: a fast one lasting several seconds and a slow one lasting longer than 5 min. Further tests showed a linear relationship between the HFB stimulation durations and recovering periods of N1 amplitudes. Supra-threshold blocking did not cause higher N1 amplitudes.

SIGNIFICANCE

This study indicates that HFB electrical currents lead to long lasting post stimulus reduction of a nerve's conductibility, which might relate to potential nerve injuries. A possible mechanism, focusing on changes in intracellular and periaxonal ionic concentrations, was proposed to underlie the reduction of the nerve's conductibility and potential nerve injuries. Greater caution and stimulation protocols with greater safety margins should be explored when utilizing HFB electrical current to block nerve conductions.

High-frequency stimulation selectively blocks different types of fibers in frog sciatic nerve

Conduction block using high-frequency alternating current (HFAC) stimulation has been shown to reversibly block conduction through various nerves. However, unlike simulations and experiments on myelinated fibers, prior experimental work in our lab on the sea-slug, Aplysia, found a nonmonotonic relationship between frequency and blocking thresholds in the unmyelinated fibers. To resolve this discrepancy, we investigated the effect of HFAC waveforms on the compound action potential of the sciatic nerve of frogs. Maximal stimulation of the nerve produces a compound action potential consisting of the A-fiber and C-fiber components corresponding to the myelinated and unmyelinated fibers' response. In our study, HFAC waveforms were found to induce reversible block in the A-fibers and C-fibers for frequencies in the range of 5-50 kHz and for amplitudes from 0.1-1 mA. Although the A-fibers demonstrated the monotonically increasing threshold behavior observed in published literature, the C-fibers displayed a nonmonotonic relationship, analogous to that observed in the unmyelinated fibers of Aplysia. This differential blocking behavior observed in myelinated and unmyelinated fibers during application of HFAC waveforms has diverse implications for the fields of selective stimulation and pain management.

EMG analysis tuned for determining the timing and level of activation in different motor units

Recruitment patterns and activation dynamics of different motor units greatly influence the temporal pattern and magnitude of muscle force development, yet these features are not often considered in muscle models. The purpose of this study was to characterize the recruitment and activation dynamics of slow and fast motor units from electromyographic (EMG) recordings and twitch force profiles recorded directly from animal muscles. EMG and force data from the gastrocnemius muscles of seven goats were recorded during in vivo tendon-tap reflex and in situ nerve stimulation experiments. These experiments elicited EMG signals with significant differences in frequency content (p<0.001). The frequency content was characterized using wavelet and principal components analysis, and optimized wavelets with centre frequencies, 149.94 Hz and 323.13 Hz, were obtained. The optimized wavelets were used to calculate the EMG intensities and, with the reconstructed twitch force profiles, to derive transfer functions for slow and fast motor units that estimate the activation state of the muscle from the EMG signal. The resulting activation-deactivation time constants gave r values of 0.98-0.99 between the activation state and the force profiles. This work establishes a framework for developing improved muscle models that consider the intrinsic properties of slow and fast fibres within a mixed muscle, and that can more accurately predict muscle force output from EMG.

Electrode design for high frequency block: effect of bipolar separation on block thresholds and the onset response

The delivery of high frequency alternating currents (HFAC) to peripheral nerves has been shown to produce a rapid and reversible nerve conduction block at the site of the electrode, and holds therapeutic promise for diseases associated with undesired or pathological neural activity. It has been known since 1939 that the configuration of an electrode used for nerve block can impact the quality of the block, but to date no formal study of the impact of electrode design on high frequency nerve block has been performed. Using a mammalian small animal model, it is demonstrated that the contact separation distance for a bipolar nerve cuff electrode can impact two important factors related to high frequency nerve block: the amplitude of HFAC required to block the nerve (block threshold), and the degree to which the transient "onset response" which always occurs when HFAC is first applied to peripheral nerves, is present. This study suggests that a bipolar electrode with a separation distance of 1.0 mm minimizes current delivery while producing high frequency block with a minimal onset response in the rat sciatic nerve.

Effect of bipolar cuff electrode design on block thresholds in high-frequency electrical neural conduction block

Many medical conditions are characterized by undesired or pathological peripheral neurological activity. The local delivery of high-frequency alternating currents (HFAC) has been shown to be a fast acting and quickly reversible method of blocking neural conduction and may provide a treatment alternative for eliminating pathological neural activity in these conditions. This work represents the first formal study of electrode design for high-frequency nerve block, and demonstrates that the interpolar separation distance for a bipolar electrode influences the current amplitudes required to achieve conduction block in both computer simulations and mammalian whole nerve experiments. The minimal current required to achieve block is also dependent on the diameter of the fibers being blocked and the electrode-fiber distance. Single fiber simulations suggest that minimizing the block threshold can be achieved by maximizing both the bipolar activating function (by adjusting the bipolar electrode contact separation distance) and a synergistic addition of membrane sodium currents generated by each of the two bipolar electrode contacts. For a rat sciatic nerve, 1.0-2.0 mm represented the optimal interpolar distance for minimizing current delivery.

Pulse charge and not waveform affects M-wave properties during progressive motor unit activation

The aim of this study was to investigate changes in experimentally recorded M-waves with progressive motor unit (MU) activation induced by transcutaneous electrical stimulation with different pulse waveforms. In 10 subjects, surface electromyographic signals were detected with a linear electrode array during electrically elicited contractions of the biceps brachii muscle. Three different monophasic waveforms of 304-micros duration were applied to the stimulation electrode on the main muscle motor point: triangular, square, and sinusoidal. For each waveform, increasing stimulation current intensities were applied in 10 s (frequency: 20 Hz). It was found that: (a) the degree of MU activation, as indicated by M-wave average rectified value, was a function of the injected charge and not of the stimulation waveform, and (b) MUs tended to be recruited in order of increasing conduction velocity with increasing charge of transcutaneous stimulation. Moreover, the subjects reported lower discomfort during the contractions elicited by the triangular waveform with respect to the others. Since subject tolerance to the stimulation protocol must be considered as important as MU recruitment in determining the effectiveness of neuromuscular electrical stimulation (NMES), we suggest that both charge and waveform of the stimulation pulses should be considered relevant parameters for optimizing NMES protocols.

Self-consistent analyses for potential conduction block in nerves by an ultrashort high-intensity electric pulse

Simulation studies are presented that probe the possibility of using high-field (> 100 kV/cm) , short-duration ( approximately 50 ns) electrical pulses for nonthermal and reversible cessation of biological electrical signaling pathways. This would have obvious applications in neurophysiology, clinical research, neuromuscular stimulation therapies, and even nonlethal bioweapons development. The concept is based on the creation of a sufficiently high density of pores on the nerve membrane by an electric pulse. This modulates membrane conductance and presents an effective "electrical short" to an incident voltage wave traveling across a nerve. Net blocking of action potential propagation can then result. A continuum approach based on the Smoluchowski equation is used to treat electroporation. This is self-consistently coupled with a distributed circuit representation of the nerve dynamics. Our results indicate that poration at a single neural segment would be sufficient to produce an observable, yet reversible, effect.

Mechanism of nerve conduction block induced by high-frequency biphasic electrical currents

The mechanisms of nerve conduction block induced by high-frequency biphasic electrical currents were investigated using a lumped circuit model of the myelinated axon based on Frankenhaeuser-Huxley (FH) model or Chiu-Ritchie-Rogart-Stagg-Sweeney (CRRSS) model. The FH model revealed that the constant activation of potassium channels at the node under the block electrode, rather than inactivation of sodium channels, is the likely mechanism underlying conduction block of myelinated axons induced by high-frequency biphasic stimulation. However, the CRRSS model revealed a different blocking mechanism where the complete inactivation of sodium channels at the nodes next to the block electrode caused the nerve conduction block. The stimulation frequencies to observe conduction block in FH model agree with the observations from animal experiments (greater than 6 kHz), but much higher frequencies are required in CRRSS model (greater than 15 kHz). This frequency difference indicated that the constant activation of potassium channels might be the underlying mechanism of conduction block observed in animal experiments. Using the FH model, this study also showed that the axons could recover from conduction block within 1 ms after termination of the blocking stimulation, which also agrees very well with the animal experiments where nerve block could be reversed immediately once the blocking stimulation was removed. This simulation study, which revealed two possible mechanisms of nerve conduction block in myelinated axons induced by high-frequency biphasic stimulation, can guide future animal experiments as well as optimize stimulation waveforms for electrical nerve block in clinical applications.

Simulation analysis of conduction block in myelinated axons induced by high-frequency biphasic rectangular pulses

Nerve conduction block induced by high-frequency biphasic rectangular pulses was analyzed using a lumped circuit model of the myelinated axon based on Frankenhaeuser-Huxley (FH) equations. At the temperature of 37 degrees C, axons of different diameters (2-20 microm) can be blocked completely at supra-threshold intensities when the stimulation frequency is above 10 kHz. However, at stimulation frequencies between 6 kHz and 9 kHz, both nerve block and repetitive firing of action potentials can be observed at different stimulation intensities. When the stimulation frequency is below 6 kHz, nerve block does not occur regardless of stimulation intensity. Larger diameter axons have a lower threshold intensity to induce conduction block. When temperature is reduced from 37 degrees C to 20 degrees C, the lowest frequency to completely block large axons (diameters 10-20 microm) decreased from 8 kHz to 4 kHz. This simulation study can guide future animal experiments as well as optimize stimulation waveforms for electrical nerve block in clinical applications.

High-frequency electrical conduction block of mammalian peripheral motor nerve

A quick-acting, quick-reversing method for blocking action potentials in peripheral nerves could be used in the treatment of muscle spasticity and pain. A high-frequency alternating-current (HFAC) sinusoidal waveform is one possible means for providing this type of block. HFAC was used to block peripheral motor nerve activity in an in vivo mammalian model. Frequencies from 10 to 30 kHZ at amplitudes of between 2 and 10 V were investigated. A complete and reversible motor block was obtained at all frequencies. The block threshold amplitudes showed a linear relationship with frequency, the lowest threshold being at 10 kHZ. HFAC block has three phases: an onset response; a period of asynchronous firing; and a steady state of complete or partial block. The onset response and the asynchronous firing can be minimized by using an optimal frequency-amplitude combination. In general, the onset response was lowest for the combination of 30 kHZ and 10 V.

Electrode Array for Reversing the Recruitment Order of Peripheral Nerve Stimulation: Experimental Studies

One of the most challenging problems in peripheral nerve stimulation is the ability to activate selectively small axons without large ones. Electrical stimulation of peripheral nerve activates large diameter fibers before small ones. Currently available techniques for selective activation of small axons without large ones require long-duration stimulation pulses (>500 micros) and large stimulation amplitude, which shorten battery life of the implanted stimulator and could lead to electrode corrosion. In the current study, the hypothesis that small axons can be recruited before large ones with narrow pulse width (50 micros) using an electrode array was tested in both simulations simulation and experiments in the cat lateral gastrocnemius (LG) model. The LG nerve innervates both LG and soleus muscle groups with axons within 10-13 and 8-12 microm diameter ranges, respectively. A finite element model of LG nerve was constructed and simulations showed that, when activating 40% of LG, a conventional tripolar electrode activated only 9% of soleus whereas the electrode arrays of 5, 7, and 11 contacts activated 39, 46, and 60% of soleus respectively, suggesting that the arrays could activate small axons before fully recruiting large axons. In animal experiments, peak twitch force of LG and soleus were plotted as a function of stimulation amplitude to indicate the recruitment curve. At 40% activation of LG, a conventional tripolar electrode activated only 7% of soleus whereas the electrode arrays of 5, 7, and 11 contacts activated 43, 48, and 72% of soleus respectively. The electrode arrays also decreased significantly the recruitment curve slopes to only 10-20% of the value obtained for the tripolar electrode in both computer simulations and experiments. In conclusion, the 5-, 7-, and 11-contact arrays can be used to reverse the recruitment order of peripheral nerve stimulation with a narrow pulse.

Simulation of nerve block by high-frequency sinusoidal electrical current based on the Hodgkin-Huxley model

Nerve conduction block induced by high-frequency sinusoidal electrical current was simulated using a lumped circuit model of the unmyelinated axon based on Hodgkin-Huxley equations. Axons of different diameters (1-20 microm) can be blocked when the stimulation frequency is above 4 kHz. At higher frequency, a higher stimulation intensity is needed to block nerve conduction. Larger diameter axons have a lower threshold intensity for conduction block. High-frequency sinusoidal electrical currents are less effective in blocking nerve conduction than biphasic square pulses of the same frequency. The activation of potassium channels, rather than inactivation of sodium channels, is the possible mechanism underlying the nerve conduction block of the unmyelinated axon induced by high-frequency biphasic (sinusoidal or square pulse) stimulation. This simulation study, which provides more information about the axonal conduction block induced by high-frequency sinusoidal currents, can guide future animal experiments, as well as optimize stimulation waveforms for electrical nerve block in possible clinical applications.

Simulation analysis of conduction block in unmyelinated axons induced by high-frequency biphasic electrical currents

Nerve conduction block induced by high-frequency biphasic electrical currents is analyzed using a lumped circuit model of the unmyelinated axon based on Hodgkin-Huxley equations. Axons of different diameters (5-20 microm) can not be blocked completely when the stimulation frequency is between 2 kHz and 4 kHz. However, when the stimulation frequency is above 4 kHz, all axons can be blocked. At high-frequency a higher stimulation intensity is needed to block nerve conduction. The larger diameter axon has a lower threshold intensity for conduction block. The stimulation waveform in which the pulsewidth changes with frequency is more effective in blocking nerve conduction than the waveform in which the pulsewidth is fixed. The activation of potassium channels, rather than inactivation of sodium channels, is the possible mechanism underlying the nerve conduction block of the unmyelinated axon. This simulation study further increases our understanding of axonal conduction block induced by high-frequency biphasic currents, and can guide future animal experiments as well as optimize stimulation waveforms that might be used for electrical nerve block in clinical applications.

Selective activation of small-diameter motor fibres using exponentially rising waveforms: A theoretical study

The present study investigated the possibility of using exponentially rising waveforms for selectively activating small motor fibres in a nerve bundle enclosed by a cuff electrode. Exponentially rising waveforms were studied using models of motor fibres and a volume conductor model. With an exponentially rising waveform (duration: 2 ms, time constant: 1 ms) large (15.5 microm) and small (8 microm) nerve fibres located at the edge of the nerve bundle had a current threshold of 125 microA and 53 microA, respectively. These reversals in the recruitment order of large and small nerve fibres located at the edge of the nerve bundle were observed for exponentially rising waveforms of 2, 4, and 6 ms in duration with time constants of 0.9, 0.6 and 0.6 ms, respectively. Reversals of the same nerve fibres located at the centre of the nerve bundle were observed for exponentially rising waveforms of 4 and 6 ms in duration, with a time constant of 0.6 ms for both waveforms. The underlying mechanism for selective activation of small nerve fibres with exponentially rising waveforms was found to be a combination of a decrease in the size of the local excitations in the centre node due to sodium channel inactivation and blocking of action potentials in large nerve fibres due to their larger difference in the membrane potential of adjacent nodes. The exponentially rising waveforms were compared with both rectangular prepulses and ramp prepulses. The rectangular prepulses were found to be unable selectively to activate small nerve fibres with the volume conductor model and criteria used in the present study, whereas the ramp prepulses performed as well as the exponentially rising waveforms. In conclusion, a novel stimulation paradigm has been proposed that may provide smooth, gradual control of muscle force with minimum fatigue.

Observations on force enhancement in submaximal voluntary contractions of human adductor pollicis muscle

It has been observed consistently and is well accepted that the steady-state isometric force after active muscle stretch is greater than the corresponding isometric force for electrically stimulated muscles and maximal voluntary contractions. However, this so-called force enhancement has not been studied for submaximal voluntary efforts; therefore, it is not known whether this property affects everyday movements. The purpose of this study was to determine whether there was force enhancement during submaximal voluntary contractions. Human adductor pollicis muscles ( n = 17) were studied using a custom-built dynamometer, and both force and activation were measured while muscle activation and force were controlled at a level of 30% of maximal voluntary contraction. The steady-state isometric force and activation after active stretch were compared with the corresponding values obtained during isometric reference contractions. There was consistent and reliable force enhancement in 8 of the 17 subjects, whereas there was no force enhancement in the remaining subjects. Subjects with force enhancement had greater postactivation potentiation and a smaller resistance to fatigue in the adductor pollicis. We conclude from these results that force enhancement exists during submaximal voluntary contractions in a subset of the populations and suggest that it may affect everyday voluntary movements in this subset. On the basis of follow-up testing, it appears that force enhancement during voluntary contractions is linked to potentiation and fatigue resistance and therefore possibly to the fiber-type distribution in the adductor pollicis muscle.

Orderly activation of human motor neurons using electrical ramp prepulses

OBJECTIVE

Conventional electrical stimulation (rectangular pulses) recruits large before small diameter motor neurons and motor neurons close to the electrode before more distant motor neurons. The present study investigated the possibility for changing the recruitment order of electrical stimuli with sub-threshold ramp prepulses.

METHODS

The median nerve was stimulated using surface electrodes at the wrist and elbow. Compound motor action potentials were recorded from abductor pollicis brevis and flexor carpi radialis. Stimulus-response curves, nerve conduction velocity and excitation thresholds of abductor pollicis brevis and flexor carpi radials, with and without ramp prepulses, were recorded in order to study the effect of ramp prepulses on axonal excitability.

RESULTS

The conduction velocity of the initial response (10% of the maximal response) was decreased by 4.3+/-0.83m/s with ramp prepulses (500ms, 80% of the excitation threshold). The ramp prepulses also had a differential effect on the excitation thresholds of abductor pollicis brevis and flexor carpi radialis. In addition, ramp prepulses increased the threshold of 10% of the maximal response more than the threshold of 90% of the maximal response.

CONCLUSIONS

These results demonstrate that large diameter and motor neurons close to the electrode accommodate more to ramp prepulses than small diameter and distant motor neurons, which suggests that ramp prepulses may be used to change the recruitment order of rectangular pulses.

SIGNIFICANCE

This technique of ramp prepulses allows stimulation of alternate subsets of motor nerves.

Nerve conduction block utilising high-frequency alternating current

High-frequency alternating current (AC) waveforms have been shown to produce a quickly reversible nerve block in animal models, but the parameters and mechanism of this block are not well understood. A frog sciatic nerve/gastrocnemius muscle preparation was used to examine the parameters for nerve conduction block in vivo, and a computer simulation of the nerve membrane was used to identify the mechanism for block. The results indicated that a 100% block of motor activity can be accomplished with a variety of waveform parameters, including sinusoidal and rectangular waveforms at frequencies from 2 kHz to 20 kHz. A complete and reversible block was achieved in 34 out of 34 nerve preparations tested. The most efficient waveform for conduction block was a 3-5 kHz constant-current biphasic sinusoid, where block could be achieved with stimulus levels as low as 0.01 microCphase(-1). It was demonstrated that the block was not produced indirectly through fatigue. Computer simulation of high-frequency AC demonstrated a steady-state depolarisation of the nerve membrane, and it is hypothesised that the conduction block was due to this tonic depolarisation. The precise relationship between the steady-state depolarisation and the conduction block requires further analysis. The results of this study demonstrated that high-frequency AC can be used to produce a fast-acting, and quickly reversible nerve conduction block that may have multiple applications in the treatment of unwanted neural activity.

High frequency block of selected axons using an implantable microstimulator

Currently, the majority of neural stimulation studies are limited to acute animal experiments due to lack of suitable implantable microstimulation devices. As an initial step to observe the long-term effects of neural stimulation, a system consisting of an external wireless controller and an implantable dual-channel microcontroller-based microstimulator for tripolar high frequency blocking was developed. The system is not only small in size, and thus suitable for short-term implantation, but also has sufficient current output parameter ranges to meet the demand for high frequency blocking experiments. Using this implantable microstimulator, a series of experiments were conducted on New Zealand rabbit's tibial nerve, including frequency and amplitude selection in driving stimulus and blocking effect tests, which were designed to assess the feasibility and efficiency of the device via torque measurements. Our results showed that the implantable microstimulator system gave a satisfactory performance and could be utilized to achieve selective stimulation and blocking on various sizes of nerve fibers. Our implantable microstimulation system is not only a novel tool for neuromuscular control studies but could also provide a basis for developing various types of sophisticated neural prostheses.

Noninvasive measurement of torque development in the rat foot: measurement setup and results from stimulation of the sciatic nerve with polyimide-based cuff electrodes

In neural rehabilitation, selective activation of muscles after electrical stimulation is mandatory for control of paralyzed limbs. For an evaluation of electrode selectivity, a setup to noninvasively measure the force development after electrical stimulation in the rat foot was developed. The setup was designed in accordance to the anatomical features of the rat model to test the isometric torque development at given ankle positions in an intact leg. In this paper, the setup design and development is presented and discussed. In a first study, the selectivity of small nerve cuffs with 12 electrodes implanted around the rat sciatic nerve was investigated. Special attention was drawn to the performance of the torque measurement setup in comparison to electrophysiological data obtained from compound muscle action potential recordings. Using one cuff around the nerve, electrical stimulation on different electrode tripoles led to plantarflexion and dorsiflexion of the foot without an a priori alignment of the cuff.

Electronic analysis of intrinsic laryngeal muscles in canine sound production

This study explores the relationship between voice production and intrinsic laryngeal muscle (ILM) activities as expressed by orderly recruitment of their specific motor units. In 5 dogs, both the recurrent laryngeal nerve (RLN) and the vagus nerve (cranial nerve X) were stimulated via tripolar electrodes with stimulating frequencies (Fs) of 10 to 60 Hz and 0 to 7 mA during application of symmetric 600 Hz, 7 to 0 mA blocking currents. The fundamental frequency (Fo) and the intensity (I) of sounds generated by tracheal insufflation of humidified air were recorded while electromyograms of the cricothyroideus (CT), thyroarytenoideus (TA), and posterior cricoarytenoideus (PCA) were obtained via surface electrodes. Contractions of the CT were concurrently induced by stimulating the superior laryngeal nerve (SLN). The recruitment rates were highly specific and were affected by which nerve was stimulated. For the RLN, PCA ramping was lowest for Fs of < or =50 Hz. For Fs of 10 to 30 Hz, the recruitment rate of the TA was significantly steeper than that for the other ILMs, and the CT had the highest rate for Fs of 40 to 50 Hz. Conversely, for the vagus nerve, PCA recruitment was highest for Fs of > or =30 Hz. The average Fo was significantly higher with the RLN than with the vagus nerve. When the TA recruited faster than the CT (ie, via the RLN, but not the vagus nerve), the Fo was higher. While only CT ramping was significantly related to changes in sound intensity, there was a trend toward a decrease when PCA ramping was higher than CT ramping, as occurred when only the vagus nerve was stimulated. Stimulation of the SLN always increased Fo and loudness. We conclude that changes in Fo occur mainly through RLN-mediated CT and TA contraction. Loudness is controlled by the CT. The PCA exerts reciprocal coupling on both functions via the vagus nerve, and they are boosted across the board by the SLN. These findings may allow artificial manipulation of voice.

A nerve stimulation method to selectively recruit smaller motor-units in rat skeletal muscle

Electrical stimulation of peripheral nerve results in a motor-unit recruitment order opposite to that attained by natural neural control, i.e. from large, fast-fatiguing to progressively smaller, fatigue-resistant motor-units. Yet animal studies involving physiological exercise protocols of low intensity and long duration require minimal fatigue. The present study sought to apply a nerve stimulation method to selectively recruit smaller motor-units in rat skeletal muscle. Two pulse generators were used, independently supplying short supramaximal cathodal stimulating pulses (0.5 ms) and long subthreshold cathodal inactivating pulses (1.5 s) to the sciatic nerve. Propagation of action potentials was selectively blocked in nerve fibres of different diameter by adjusting the strength of the inactivating current. A tensile-testing machine was used to gauge isometric muscle force of the plantaris and both heads of the gastrocnemius muscle. The order of motor-unit recruitment was estimated from twitch characteristics, i.e. peak force and relaxation time. The results showed prolonged relaxation at lower twitch peak forces as the intensity of the inactivating current increased, indicating a reduction of the number of large motor-units to force production. It is shown that the nerve stimulation method described is effective in mimicking physiological muscle control.

Closed-loop control of muscle length through motor unit recruitment in load-moving conditions

Neuroprostheses aimed at restoring lost movement in the limbs of spinal cord injured individuals are being developed in this laboratory. As part of this program, we have designed a digital proportional-integral-derivative controller integrated with a stimulation system which effects recruitment of motor units according to the size principle. This system is intended to control muscle length while shortening against fixed loads. Feline sciatic nerves were exposed and stimulated with ramp, triangular, sinusoidal, staircase and random signals as test inputs. Changes in muscle length and effective time delay under different conditions were measured and analyzed. Differences of tracking quality between open- and closed-loop conditions were examined through analysis of variance as well as the differences between small (250g) and large (1kg) loads. The results showed that parameters used to compare muscle length output to the input signals were dramatically improved in the closed-loop trials as compared to the open-loop condition. Mean squared correlation coefficients between input and output signals for ramp signals increased by 0.019, and for triangular signals by 0.12. Mean peak cross correlation between input and output signals for sinusoidal waveforms increased by 0.06, with decreases in time to peak cross correlation (effective time delay) from 195 to 38ms. In slow random signals (power up to 0.5Hz), peak cross correlation went from 0.74 to 0.89, and time-to-peak cross correlation decreased from 205 to 55ms. In fast random signals (power up to 1Hz), peak cross correlation went from 0.82 to 0.89, and time-to-peak cross correlation from 200 to 65ms. For staircase signals, both rise times and mean steady-state errors decreased. It was found that, once the length range was set, the load weight had no effect on tracking performance. Analysis of mean square error demonstrated that for all signals tested, the feedback decreased the tracking error significantly, whereas, again, load had no effect. The results suggest that tracking is vastly improved by using a closed-loop system to control muscle length, and that load does not affect the quality of signal tracking as measured by standard control system analysis methods.

Urinary bladder control by electrical stimulation: review of electrical stimulation techniques in spinal cord injury

Evacuation of urine in paraplegics without the need for catheters would be possible when voiding could be induced by eliciting a bladder contraction. A challenging option to obtain detrusor contraction is electrical stimulation of the detrusor muscle or its motor nerves. This article reviews the 4 possible stimulation sites where stimulation would result in a detrusor contraction: the bladder wall, the pelvic nerves, the sacral roots, and the spinal cord. With respect to electrode application, sacral root stimulation is most attractive. However, in general, sacral root stimulation results in simultaneous activation of both the detrusor muscle and the urethral sphincter, leading to little or no voiding. Several methods are available to overcome the stimulation-induced detrusor-sphincter dyssynergia and allow urine evacuation. These methods, including poststimulus voiding, fatiguing of the sphincter, blocking pudendal nerve transmission, and selective stimulation techniques that allow selective detrusor activation by sacral root stimulation, are reviewed in this paper.

Restoration of bladder function in spastic neuropathic bladder using sacral deafferentation and different techniques of neurostimulation

PURPOSE

Conventional sacral anterior root stimulation (SARS) results in simultaneous activation of both the detrusor muscle and the external urethral sphincter. We evaluated the possibilities of different neurostimulation techniques to overcome stimulation induced detrusor-sphincter-dyssynergia and to achieve a physiological voiding.

MATERIAL AND METHODS

The literature was reviewed on different techniques of sacral anterior root stimulation of the bladder and the significance of posterior rhizotomy in patients with supraconal spinal cord injury suffering from the loss of voluntary bladder control, detrusor hyperreflexia and sphincter spasm.

RESULTS

The achievement of selective detrusor activation would improve current sacral neurostimulation of the bladder, including the principle of "poststimulus voiding". This is possible with the application of selective neurostimulation in techniques of anodal block, high frequency block, depolarizing prepulses and cold block. Nowadays, sacral deafferentation is a standard therapy in combination with neurostimulation of the bladder because in conclusion advantages of complete rhizotomy predominate.

CONCLUSIONS

The combination of sacral anterior root stimulation and sacral deafferentation is a successful procedure for restoration of bladder function in patients with supraconal spinal cord injury. Anodal block technique and cryotechnique are excellent methods for selective bladder activation to avoid detrusor-sphincter-dyssynergia and thus improve stimulation induced voiding.

Endoneural selective stimulating using wire-microelectrode arrays

In acute experiments eight 5- to 24-wire-microelectrode arrays were inserted into the common peroneal nerve of the rat, to investigate whether the electrodes could selectively stimulate motor units of the extensor digitorum longus (EDL) muscle. Twitch-force-recruitment curves were measured from the EDL for each array electrode. The curves were plotted on a double-logarithmic scale and parameterized by the low-force slope (which represents the power p in the power-law relationship of force F versus stimulus current I, or F approximately I(p)) and the threshold current. The slopes and threshold currents measured with array electrodes did not differ significantly from those obtained with randomly inserted single wire-microelectrodes. This indicates that, although involving a more invasive insertion procedure, electrode arrays provide neural contacts with low-force recruitment properties similar to those of single wires. Array results revealed partial blocking of neural conduction, similar to that reported with microneurographic insertion with single needles. The efficiency of the array was defined as the fraction of array electrodes selectively contacting a motor unit and evoking the corresponding threshold force. Efficiency thus expresses the practical value of the used electrode array in terms of the total number of distinct threshold forces that can be stimulated by selecting the appropriate electrodes. The eight arrays were capable of evoking threshold forces selectively with an average efficiency of 0.81 (or 81%).

Reduction of bladder outlet resistance by selective sacral root stimulation using high-frequency blockade in dogs: an acute study

Detrusor-sphincter dyssynergia is a main problem in supra-sacral spinal cord injured patients. The problem of high pressure voiding is also encountered in most electrically induced micturition because of the mixed somatic and autonomic fiber components of the ventral sacral root. We studied the effect of selective high-frequency blockade at the sacral nerve root in an acute spinalized canine model to prevent the deleterious consequences associated with the elevated bladder outlet resistance. A new functional electrical stimulation system which can generate one signal composed of two independent adjustable current waveforms delivered via a single bipolar electrode was used in 11 dogs. The selectivity resulted from the inhibition of the sphincteric somatic innervation by a high frequency pulse while the low frequency stimuli activated the bladder autonomic fibers. Bladder and urethral pressure as well as electromyograms of external urethral sphincter were recorded to determine whether selective high-frequency blockade occurred. Our experiments showed that we were able to achieve selective blockade of the external urethral sphincter during the simultaneous detrusor stimulation, thus obtaining a more physiologic voiding.

Reduction of bladder outlet resistance by selective sacral root stimulation using high-frequency blockade in dogs: an acute study

Detrusor-sphincter dyssynergia is a main problem in supra-sacral spinal cord injured patients. The problem of high pressure voiding is also encountered in most electrically induced micturition because of the mixed somatic and autonomic fiber components of the ventral sacral root. We studied the effect of selective high-frequency blockade at the sacral nerve root in an acute spinalized canine model to prevent the deleterious consequences associated with the elevated bladder outlet resistance. A new functional electrical stimulation system which can generate one signal composed of two independent adjustable current waveforms delivered via a single bipolar electrode was used in 11 dogs. The selectivity resulted from the inhibition of the sphincteric somatic innervation by a high frequency pulse while the low frequency stimuli activated the bladder autonomic fibers. Bladder and urethral pressure as well as electromyograms of external urethral sphincter were recorded to determine whether selective high-frequency blockade occurred. Our experiments showed that we were able to achieve selective blockade of the external urethral sphincter during the simultaneous detrusor stimulation, thus obtaining a more physiologic voiding.

Force-current relationships in intraneural stimulation: role of extraneural medium and motor fibre clustering

Animal experiments and model simulations of monopolar, intrafascicular nerve stimulation are presented to study force-current relationships (recruitment curves). The conductivity of the extraneural medium is of prime importance to the resulting recruitment cures: an insulating extraneural medium generally leads to steeper curves with lower threshold currents than a well-conducting extraneural medium. Extensive statistical comparison of experimental and model results suggests the occurrence of clustering of alpha-motoneurons within the fascicle, manifesting itself mainly by an increased spread in threshold currents, as opposed to the situation where the fibres are distributed uniformly throughout the entire fascicle.

Force feedback control of motor unit recruitment in isometric muscle

The use of simple force feedback in an isometric muscle control system utilizing orderly recruitment of motor units is explored. Cat medial gastrocnemius motor units were stimulated with and without simple force feedback gain ranging from 0.7 to 0.9. Ramp, triangular, staircase, sinusoidal and bandwidth-limited pseudo-random input recruitment signals were used to study tracking accuracy through linear correlation in ramp and triangular signals, cross correlation in sinusoidal and random signals, and rise time and steady state error in staircase signals. Dramatic improvements were found in most tested tracking variables with the use of feedback; squared correlation coefficients increased from a mean of 0.93 to 0.99 for ramp signals and from 0.76 to 0.98 in triangular signals. Mean peak cross-correlations improved from 0.85 to 0.98 in random signals and from 0.93 to 0.98 for sinusoidal inputs, and mean time to peak cross-correlations decreased from 144 to 24 ms in random signals and from 156 to 25 ms in sine waves. Rise times in staircase signals decreased from a mean of 520 to 175 ms, and mean steady state error decreased from 12 to 3%. Significant effects of the triangle cycle time, sinusoidal frequency and staircase step were found on the performance of the muscle force control system. In addition, the possible effects of intrinsic feedback mechanisms on the control system were examined by repeating the closed loop part of the study but with the sciatic nerve cut proximally. The tracking results were essentially and statistically the same as in the closed loop condition. It was concluded that a simple feedback configuration provided superior tracking performance for a practical application in which orderly recruitment is used to control muscles; furthermore, it was concluded that this type of system would be virtually immune to external disturbances such as spasticity resulting from intact spinal neural feedback mechanisms found in paralyzed individuals.

Long-term excitability and fine tuning of nerve pedicles reinnervating strap muscles in the dog

Contraction of paralyzed striated muscles has been restored by stimulating reinnervating pedicles with currents of low intensity. In order to allow clinical application, stable, long-term excitability must emulate the parameters necessary for the stimulation of normal motor nerves. In 6 dogs, the ansa hypoglossi nerve was implanted into the contralateral denervated sternohyoid muscle and surrounded with a bipolar cuff electrode. Three of the reinnervating pedicles were chronically paced with a Medtronic Itrel II Multiprogrammable Pulse Generator (0.5 V, 0.2 second on [30 pulses per second, 0.21-millisecond pulse width], 2.9 seconds off). At reexploration after 8 months (6 months for 1 dog), frank contraction confirmed by electromyography tracings occurred in all animals with currents in the range of 0.1 to 0.5 mA. Muscle force was further manipulated by selective release of blocking currents (600 Hz, 1.7 to 0.4 mA) superimposed over regular stimulation (50 Hz, 0.3 to 1.7 mA). Nerve and muscle vitality were histologically confirmed. Long-term, low-intensity conduction capabilities, fine tuning, good tolerance of implanted electrodes, and lack of fatigue suggest that reinnervating pedicles may be successfully used for pacing when clinically indicated.