Laser-Microfabricated Polymer Multielectrodes for Intraspinal Microstimulation.

OBJECTIVE: The overall goal of this study was to design, fabricate, and characterize a new polymer-based multielectrode for the spinal cord for the application of intraspinal microstimulation (ISMS). METHODS: Three-channel multielectrodes were fabricated from modified poly(dimethylsiloxane) (PDMS) and platinum-iridium (Pt-Ir) foil using nanosecond laser microfabrication techniques. These devices were compared against traditional 50 µm diameter Pt-Ir microwire electrodes mechanically and electrochemically in bench environments, and were assessed electrochemically and functionally in vivo in a domestic pig model. RESULTS: Polymer-based multielectrodes were significantly more flexible than microwire electrodes (p < 0.05) and had greater charge storage capacities in phosphate buffered saline (p < 0.05). In a domestic pig model, multielectrodes had significantly greater charge injection limits than microwire electrodes (p < 0.05). When stimulating within the quadriceps motor pool in the spinal cord, multielectrodes generated strong knee extensor joint torques of up to 4.4 ± 0.3 Nm and were able to extend the knee by up to 26 ± 1°. However, histological analyses showed that polymer-based multielectrodes, implanted with half-needle insertion aids, produced greater acute tissue damage compared to microwire electrodes (p < 0.05). Alternative insertion methods for these flexible electrodes should be explored to reduce acute tissue damage. CONCLUSION: The PDMS-based three-channel multielectrodes demonstrated improved flexibility and charge injection capabilities over traditional microwire electrodes, and were able to produce functional responses in vivo. SIGNIFICANCE: Polymer-based multielectrodes demonstrate improved functionality over microwire electrodes while remaining more flexible than silicon multielectrode designs. These features may in the future permit polymer-based multielectrodes to implement ISMS with greater efficacy and biocompatibility compared to traditional technologies.

A speed-adaptive intraspinal microstimulation controller to restore weight-bearing stepping in a spinal cord hemisection model.

OBJECTIVE: The goal of this study was to develop control strategies to produce alternating, weight-bearing stepping in a cat model of hemisection spinal cord injury (SCI) using intraspinal microstimulation (ISMS). APPROACH: Six cats were anesthetized and the functional consequences of a hemisection SCI were simulated by manually moving one hind-limb through the gait cycle over a moving treadmill belt. ISMS activated the muscles in the other leg by stimulating motor networks in the lumbosacral enlargement using low levels of current (<110 µA). The control strategy used signals from ground reaction forces and angular velocity from the manually-moved limb to anticipate states of the gait cycle, and controlled ISMS to move the other hind-limb into the opposite state. Adaptive control strategies were developed to ensure weight-bearing at different stepping speeds. The step period was predicted using generalizations obtained through four supervised machine learning algorithms and used to adapt the control strategy for faster steps. MAIN RESULTS: At a single speed, 100% of the steps had sufficient weight-bearing; at faster speeds without adaptation, 97.6% of steps were weight-bearing (significantly less than that for single speed; p = 0.002). By adapting the control strategy for faster steps using the predicted step period, weight-bearing was achieved in more than 99% of the steps in three of four methods (significantly more than without adaptation p < 0.04). Overall, a multivariate model tree increased the number of weight-bearing steps, restored step symmetry, and maintained alternation at faster stepping speeds. SIGNIFICANCE: Through the adaptive control strategies guided by supervised machine learning, we were able to restore weight-bearing and maintain alternation and step symmetry at varying stepping speeds.

Non-gait-specific intervention for the rehabilitation of walking after SCI: role of the arms.

Arm movements modulate leg activity and improve gait efficiency; however, current rehabilitation interventions focus on improving walking through gait-specific training and do not actively involve the arms. The goal of this project was to assess the effect of a rehabilitation strategy involving simultaneous arm and leg cycling on improving walking after incomplete spinal cord injury (iSCI). We investigated the effect of 1) non-gait-specific training and 2) active arm involvement during training on changes in over ground walking capacity. Participants with iSCI were assigned to simultaneous arm-leg cycling (A&L) or legs only cycling (Leg) training paradigms, and cycling movements were assisted with electrical stimulation. Overground walking speed significantly increased by 0.092 ± 0.022 m/s in the Leg group and 0.27 ± 0.072m/s in the A&L group after training. Whereas the increases in the Leg group were similar to those seen after current locomotor training strategies, increases in the A&L group were significantly larger than those in the Leg group. Walking distance also significantly increased by 32.12 ± 8.74 m in the Leg and 91.58 ± 36.24 m in the A&L group. Muscle strength, sensation, and balance improved in both groups; however, the A&L group had significant improvements in most gait measures and had more regulated joint kinematics and muscle activity after training compared with the Leg group. We conclude that electrical stimulation-assisted cycling training can produce significant improvements in walking after SCI. Furthermore, active arm involvement during training can produce greater improvements in walking performance. This strategy may also be effective in people with other neural disorders or diseases. NEW & NOTEWORTHY This work challenges concepts of task-specific training for the rehabilitation of walking and encourages coordinated training of the arms and legs after spinal cord injury. Cycling of the legs produced significant improvements in walking that were similar in magnitude to those reported with gait-specific training. Moreover, active engagement of the arms simultaneously with the legs generated nearly double the improvements obtained by leg training only. The cervico-lumbar networks are critical for the improvement of walking.

Mechanically Stable Intraspinal Microstimulation Implants for Human Translation.

The goal of this study was to develop stable intraspinal microstimulation (ISMS) implants for use in humans to restore standing and walking after spinal cord injury. ISMS electrically activates locomotor networks within the lumbar region of the spinal cord. In animals, ISMS produced better functional outcomes than those obtained by other interventions, and recent efforts have focused on translating this approach to humans. This study used domestic pigs to: (1) quantify the movements and length changes of the implant region of the spinal cord during spine flexion and extension movements; and (2) measure the forces leading to the dislodgement of the ISMS electrodes. The displacement of the spinal cord implant region was 5.66 ± 0.57 mm relative to the implant fixation point on the spine. The overall length change of the spinal cord implant region was 5.64 ± 0.59 mm. The electrode dislodgment forces were 60.9 ± 35.5 mN. Based on these results, six different coil types were fabricated and their strain relief capacity assessed. When interposed between the electrodes and the stimulator, five coil types successfully prevented the dislodgement of the electrodes. The results of this study will guide the design of mechanically stable ISMS implants for ultimate human use.

Functional electrical stimulation and spinal cord injury.

Spinal cord injuries (SCI) can disrupt communications between the brain and the body, resulting in loss of control over otherwise intact neuromuscular systems. Functional electrical stimulation (FES) of the central and peripheral nervous system can use these intact neuromuscular systems to provide therapeutic exercise options to allow functional restoration and to manage medical complications following SCI. The use of FES for the restoration of muscular and organ functions may significantly decrease the morbidity and mortality following SCI. Many FES devices are commercially available and should be considered as part of the lifelong rehabilitation care plan for all eligible persons with SCI.

Prevention of deep tissue injury through muscle contractions induced by intermittent electrical stimulation after spinal cord injury in pigs.

Deep tissue injury (DTI) is a severe medical complication that commonly affects those with spinal cord injury. It is caused by prolonged external loading of the muscles, entrapping them between a bony prominence and the support surface. The entrapment causes excessive mechanical deformation and increases in interstitial pressure, leading to muscle breakdown deep around the bony prominences. We proposed the use of intermittent electrical stimulation (IES) as a novel prophylactic method for the prevention of DTI. In this study, we assessed the long-term effectiveness of this technique in pigs that had received a partial spinal cord injury that paralyzed one hindlimb. The pigs recovered for 2 wk postsurgery, and subsequently, their paralyzed limbs were loaded to 25% of their body weights 4 h/day for 4 consecutive days each week for 1 mo. One group of pigs (n = 3) received IES during the loading, whereas another group (n = 3) did not. DTI was quantified using magnetic resonance imaging (MRI) and postmortem histology. In the group that did not receive IES, MRI assessments revealed signs of tissue damage in 48% of the volume of the loaded muscle. In the group that did receive IES, only 8% of the loaded muscle volume showed signs of tissue damage. Similar findings were found through postmortem histology. This study demonstrates, for the first time, that IES may be an effective technique for preventing the formation of DTI in loaded muscles after spinal cord injury.

Spinal cord stimulation: therapeutic benefits and movement generation after spinal cord injury.

Spinal cord injury (SCI) is a devastating neurological condition that leads to loss of motor and sensory function. It commonly causes impairments in limb movements, respiration, bowel and bladder function, as well as secondary complications including pain, spasticity, and pressure ulcers. Numerous interventions such as neuroprotection, regeneration, pharmacology, rehabilitation training, and functional electrical stimulation are under investigation for improving function after SCI. This chapter discusses the use of spinal cord stimulation (epidural and intraspinal electrical stimulation) for alleviating pain and spasticity, and restoring standing and walking. Epidural stimulation is effective in reducing the intensity of intractable pain, but its effectiveness in the treatment of spasticity remains unclear. It can induce rhythmic, locomotor-like movements in the legs, presumably due to the activation of afferent pathways. Intraspinal microstimulation is a new electrical stimulation approach that activates locomotor-related networks within the ventral regions of the lumbosacral spinal cord. In animals, this approach is capable of producing prolonged, fatigue-resistant standing and stepping of the hindlegs. While the results in animals have been very encouraging, technical advancements are necessary prior to its implementation in humans with SCI. Taken collectively, spinal cord stimulation holds substantial promise in restoring function after neural injury or disease.

Distribution of internal pressure around bony prominences: implications to deep tissue injury and effectiveness of intermittent electrical stimulation.

The overall goal of this project is to develop interventions for the prevention of deep tissue injury (DTI), a form of pressure ulcers that originates in deep tissue around bony prominences. The present study focused on: (1) obtaining detailed measures of the distribution of pressure experienced by tissue around the ischial tuberosities, and (2) investigating the effectiveness of intermittent electrical stimulation (IES), a novel strategy for the prevention of DTI, in alleviating pressure in regions at risk of breakdown due to sustained loading. The experiments were conducted in adult pigs. Five animals had intact spinal cords and healthy muscles and one had a spinal cord injury that led to substantial muscle atrophy at the time of the experiment. A force-controlled servomotor was used to load the region of the buttocks to levels corresponding to 25%, 50% or 75% of each animal's body weight. A pressure transducer embedded in a catheter was advanced into the tissue to measure pressure along a three dimensional grid around the ischial tuberosity of one hind leg. For all levels of external loading in intact animals, average peak internal pressure was 2.01 ± 0.08 times larger than the maximal interfacial pressure measured at the level of the skin. In the animal with spinal cord injury, similar absolute values of internal pressure as that in intact animals were recorded, but the substantial muscle atrophy produced larger maximal interfacial pressures. Average peak internal pressure in this animal was 1.43 ± 0.055 times larger than the maximal interfacial pressure. Peak internal pressure was localized within a ±2 cm region medio-laterally and dorso-ventrally from the bone in intact animals and ±1 cm in the animal with spinal cord injury. IES significantly redistributed internal pressure, shifting the peak values away from the bone in spinally intact and injured animals. These findings provide critical information regarding the relationship between internal and interfacial pressure around the ischial tuberosities during loading levels equivalent to those experienced while sitting. The information could guide future computer models investigating the etiology of DTI, as well as inform the design and prescription of seating cushions for people with reduced mobility. The findings also suggest that IES may be an effective strategy for the prevention of DTI.

An implantable neural stimulator for intraspinal microstimulation.

This paper reports on a wireless stimulator device for use in animal experiments as part of an ongoing investigation into intraspinal stimulation (ISMS) for restoration of walking in humans with spinal cord injury. The principle behind using ISMS is the activation of residual motor-control neural networks within the spinal cord ventral horn below the level of lesion following a spinal cord injury. The attractiveness to this technique is that a small number of electrodes can be used to induce bilateral walking patterns in the lower limbs. In combination with advanced feedback algorithms, ISMS has the potential to restore walking for distances that exceed that produced by other types of functional electrical stimulation. Recent acute animal experiments have demonstrated the feasibility of using ISMS to produce the coordinated walking patterns. Here we described a wireless implantable stimulation system to be used in chronic animal experiments and for providing the basis for a system suitable for use in humans. Electrical operation of the wireless system is described, including a demonstration of reverse telemetry for monitoring the stimulating electrode voltages.

Intraspinal microstimulation for the recovery of function following spinal cord injury.

Spinal cord injury is a devastating neurological trauma, often resulting in the impairment of bladder, bowel, and sexual function as well as the loss of voluntary control of muscles innervated by spinal cord segments below the lesion site. Research is ongoing into several classes of therapies to restore lost function. These include the encouragement of neural sparing and regeneration of the affected tissue, and the intervention with pharmacological and rehabilitative means to improve function. This review will focus on the application of electrical current in the spinal cord in order to reactivate extant circuitry which coordinates and controls smooth and skeletal muscle below the injury. We first present a brief historical review of intraspinal microstimulation (ISMS) focusing on its use for restoring bladder function after spinal cord injury as well as its utilization as a research tool for mapping spinal cord circuits that coordinate movements. We then present a review of our own results related to the use of ISMS for restoring standing and walking movements after spinal cord injury. We discuss the mechanisms of action of ISMS and how they relate to observed functional outcomes in animal models. These include the activation of fibers-in-passage which lead to the transsynaptic spread of activation through the spinal cord and the ability of ISMS to produce fatigue-resistant, weight-bearing movements. We present our thoughts on the clinical potential for ISMS with regard to implantation techniques, stability, and damage induced by mechanical and electrical factors. We conclude by suggesting improvements in materials and techniques that are needed in preparation for a clinical proof-of-principle and review our current attempts to achieve these.

The effects of intermittent electrical stimulation on the prevention of deep tissue injury: varying loads and stimulation paradigms.

A pressure ulcer is a medical complication that arises in persons with decreased mobility and/or sensation. Deep pressure ulcers starting at the bone-muscle interface are the most dangerous, as they can cause extensive damage before showing any signs at the skin surface. We previously proposed a novel intervention called intermittent electrical stimulation (IES) for the prevention of deep tissue injury (DTI). In this study, we tested the effects of four paradigms of IES and one conventional pressure relief paradigm in preventing the formation of deep pressure ulcers in rats. Loading equivalent to 18, 28, or 38% of the body weight (BW) of each rat was applied to the triceps surae muscle in one hind limb. Treatment groups received IES every 10 min for either (i) 5 or 10 s with moderate or maximal contraction, or (ii) complete pressure removal every 10 min for 10 s (conventional pressure relief). The results showed that conventional pressure relief, emulating a wheelchair push-up every 10 min, was inadequate for the prevention of DTI. In contrast, all IES paradigms were equally effective in significantly reducing the extent of deep muscle damage caused by 28 or 38% BW pressure application. These findings suggest that, in conjunction with existing techniques, IES may be an effective intervention for the prophylactic prevention of DTI.

Effects of intermittent electrical stimulation on superficial pressure, tissue oxygenation, and discomfort levels for the prevention of deep tissue injury.

The overall goal of this project is to develop effective methods for the prevention of deep tissue injury (DTI). DTI is a severe type of pressure ulcer that originates at deep bone-muscle interfaces as a result of the prolonged compression of tissue. It afflicts individuals with reduced mobility and sensation, particularly those with spinal cord injury. We previously proposed using a novel electrical stimulation paradigm called intermittent electrical stimulation (IES) for the prophylactic prevention of DTI. IES-induced contractions mimic the natural repositioning performed by intact individuals, who subconsciously reposition themselves as a result of discomfort due to prolonged sitting. In this study, we investigated the effectiveness of various IES paradigms in reducing pressure around the ischial tuberosities, increasing tissue oxygenation throughout the gluteus muscles, and reducing sitting discomfort in able-bodied volunteers. The results were compared to the effects of voluntary muscle contractions and conventional pressure relief maneuvers (wheelchair push-ups). IES significantly reduced pressure around the tuberosities, produced significant and long-lasting elevations in tissue oxygenation, and significantly reduced discomfort produced by prolonged sitting. IES performed as well or better than both voluntary contractions and chair push-ups. The results suggest that IES may be an effective means for the prevention of DTI.

Restoring stepping after spinal cord injury using intraspinal microstimulation and novel control strategies.

The overall objective of this project is to develop a feedback-driven intraspinal microstimulation (ISMS) system. We hypothesize that ISMS will enhance the functionality of stepping by reducing muscle fatigue and producing synergistic movements by activating neural networks in the spinal cord. In the present pilot study, the controller was tested with ISMS and external sensors (force plates, gyroscopes, and accelerometers). Cats were partially supported in a sling and bi-laterally stepped overground on a 4-m instrumented walkway. The walkway had variable friction. Limb angle was controlled to within 10° even in the presence of variable friction. Peak ground reaction forces in each limb were approximately 12% of body weight (12.5% was full load bearing in this experimental setup); rarely, the total supportive force briefly decreased to as low as 4.1%. Magnetic resonance images were acquired of the excised spinal cord and the implanted array. The majority of electrodes (75%) were implanted successfully into their target regions. This represents the first successful application of ISMS for overground walking.

New functional electrical stimulation approaches to standing and walking.

Spinal cord injury (SCI) is a devastating neurological trauma that is prevalent predominantly in young individuals. Several interventions in the areas of neuroregeneration, pharmacology and rehabilitation engineering/neuroscience are currently under investigation for restoring function after SCI. In this paper, we focus on the use of neuroprosthetic devices for restoring standing and ambulation as well as improving general health and wellness after SCI. Four neuroprosthetic approaches are discussed along with their demonstrated advantages and their future needs for improved clinical applicability. We first introduce surface functional electrical stimulation (FES) devices for restoring ambulation and highlight the importance of these devices for facilitating exercise activities and systemic physiological activation. Implanted muscle-based FES devices for restoring standing and walking that are currently undergoing clinical trials are then presented. The use of implanted peripheral nerve intraneural arrays of multi-site microelectrodes for providing fine and graded control of force during sit-to-stand maneuvers is subsequently demonstrated. Finally, intraspinal microstimulation (ISMS) of the lumbosacral spinal cord for restoring standing and walking is introduced and its results to date are presented. We conclude with a general discussion of the common needs of the neuroprosthetic devices presented in this paper and the improvements that may be incorporated in the future to advance their clinical utility and user satisfaction.

Suppressing the excitability of spinal motoneurons by extracellularly applied electrical fields: insights from computer simulations.

The effect of extracellularly applied electrical fields on neuronal excitability and firing behavior is attributed to the interaction between neuronal morphology and the spatial distribution and level of differential polarization induced by the applied field in different elements of the neuron. The presence of voltage-gated ion channels that mediate persistent inward currents (PICs) on the dendrites of spinal motoneurons enhances the influence of electrical fields on the motoneuronal firing behavior. The goal of the present study was to investigate, with a realistic motoneuron computer model, the effects of extracellularly applied electrical fields on the excitability of spinal motoneurons with the aim of reducing the increased motoneuronal excitability after spinal cord injury (SCI). Our results suggest that electrical fields could suppress the excitability of motoneurons and reduce their firing rate significantly by modulating the magnitude of their dendritic PIC. This effect was achieved at different field directions, intensities, and polarities. The reduction in motoneuronal firing rate resulted from the reduction in the magnitude of the dendritic PIC reaching the soma by the effect of the applied electrical field. This reduction in PIC was attributed to the dendritic field-induced differential polarization and the nonlinear current-voltage relationship of the dendritic PIC-mediating channels. Because of the location of the motoneuronal somata and initial segment with respect to the dendrites, these structures were minimally polarized by the applied field compared with the extended dendrites. In conclusion, electrical fields could be used for suppressing the hyperexcitability of spinal motoneurons after SCI and reducing the level of spasticity.

Strategies for generating prolonged functional standing using intramuscular stimulation or intraspinal microstimulation.

Spinal cord injury (SCI) often results in the loss of the ability to stand. The goal of this study was to implement a functional electrical stimulation (FES) system for restoring prolonged periods of standing after SCI. For this purpose, we tested two control strategies: open-loop and closed-loop control, and two stimulation paradigms: non-interleaved intramuscular stimulation (IM-S) and interleaved intraspinal microstimulation (ISMS). The experiments were conducted in anesthetized cats. Stimulation was applied to the muscles through IM-S electrodes implanted in the main knee and ankle extensor muscles, or to the spinal cord through ultra-fine ISMS wires implanted within the ventral horn of the lumbosacral enlargement. The cats were partially supported over parallel force plates and accelerometers were secured to the hindlimbs above and below the ankle joint. Ground reaction forces and knee and ankle joint angles were measured by the force plates and accelerometers, respectively. The closed-loop controller used these feedback signals to modulate the amplitude of stimulation applied to the extensor muscles. The open-loop controller applied constant levels of stimulation which were determined before the onset of each trial. The duration of standing achieved using closed-loop control of IM-S was significantly longer than that achieved with open-loop control (approximately 2 times longer). The increase in the duration of standing corresponded with a decrease in the rate of force decay and a lower average injected current during closed-loop control. Standing was further improved with the use of ISMS. Closed-loop control of interleaved ISMS resulted in a period of standing > 3 times longer than the best trial generated using non-interleaved IM-S. There was also a significant improvement in the balance of force between the two hindlimbs. The results suggest that a system which uses closed-loop control in conjunction with interleaved ISMS could achieve prolonged FES standing in people with SCI.

Prevention of pressure-induced deep tissue injury using intermittent electrical stimulation.

Pressure ulcers develop due to morphological and biochemical changes triggered by the combined effects of mechanical deformation, ischemia, and reperfusion that occur during extended periods of immobility. The goal of this study was to test the effectiveness of a novel electrical stimulation technique in the prevention of deep tissue injury (DTI). We propose that contractions elicited by intermittent electrical stimulation (IES) in muscles subjected to constant pressure would induce periodic relief in internal pressure; additionally, each contraction would also restore blood flow to the tissue. The application of constant pressure to the quadriceps muscles of rats generated a DTI that affected 60 +/- 15% of the compressed muscle as assessed by magnetic resonance imaging. In contrast, in the groups of rats that received IES at 10- and 5-min intervals, DTI of the muscle was limited to 16 +/- 16 and 25 +/- 13%, respectively. Injury to the muscle was corroborated by histology. In an experiment with a human volunteer, compression of the buttocks reduced the oxygenation level of the muscles by approximately 4%; after IES, oxygenation levels increased by approximately 6% beyond baseline. Concurrently, the surface pressure profiles of the loaded muscles were redistributed and the high-pressure points were reduced during each IES-induced contraction. The results of this study indicate that IES significantly reduces the amount of DTI by increasing the oxygen available to the tissue and by modifying the pressure profiles of the loaded muscles. This presents a promising technique for the prevention of pressure ulcers in immobilized and/or insensate individuals.

Modulation of motoneuronal firing behavior after spinal cord injury using intraspinal microstimulation current pulses: a modeling study.

We simulated the effects of delivering focal electrical stimuli to the central nervous system to modulate the firing rate of neurons and alleviate motor disorders. Application of these stimuli to the spinal cord to reduce the increased excitability of motoneurons and resulting spasticity after spinal cord injury (SCI) was examined by means of a morphologically detailed computer model of a spinal motoneuron. High-frequency sinusoidal and rectangular pulses as well as biphasic charge-balanced and charge-imbalanced pulses were examined. Our results suggest that suprathreshold high-frequency sinusoidal or rectangular current pulses could inactivate the Na+ channels in the soma and initial segment, and block action potentials from propagating through the axon. Subthreshold biphasic charge-imbalanced pulses reduced the motoneuronal firing rate significantly (up to approximately 25% reduction). The reduction in firing rate was achieved through stimulation-induced hyperpolarization generated in the first node of Ranvier. Because of their low net DC current, these pulses could be tolerated safely by the tissue. To deliver charge-imbalanced pulses with the lowest net DC current and induce the largest reduction in motoneuronal firing rate, we studied the effect of various charge-imbalanced pulse parameters. Short pulse durations were found to induce the largest reduction in firing rate for the same net DC level. Subthreshold high-frequency sinusoidal and rectangular current pulses and low-frequency biphasic charge-balanced pulses, on the other hand, were ineffective in reducing the motoneuronal firing rate. In conclusion, the proposed electrical stimulation paradigms could provide potential rehabilitation interventions for suppressing the excitability of neurons to reduce the severity of motor disorders after injury to the central nervous system.

Could cortical signals control intraspinal stimulators? A theoretical evaluation.

In this paper, we examine the control signals that are required to generate stepping using two different intraspinal microstimulation (ISMS) paradigms and discuss the theoretical feasibility of controlling ISMS-evoked stepping using a brain computer interface. Tonic (constant amplitude) and phasic (modulated amplitude) ISMS protocols were used to produce stepping in the hind limbs of paralyzed cats. Low-amplitude tonic ISMS activated a spinal locomotor-like network that resulted in bilateral stepping of the hind limbs. Phasic ISMS generated coordinated stepping by simultaneously activating flexor synergies in one limb coupled with extensor synergies in the other. Using these ISMS paradigms, we propose that one or two independent cortical signals will be adequate for controlling ISMS-induced stepping after SCI.

Intraspinal microstimulation using cylindrical multielectrodes.

A cylindrical multielectrode system specifically designed for intraspinal microstimulation was mechanically and electrically evaluated in the ventral horn of the feline lumbo-sacral spinal cord. Electrode insertions proved to be straight as evaluated from radiographs. Impedances were measured in situ and force recruitment curves from quadriceps muscles were collected over a wide range of stimulus parameters. For a given charge, higher current amplitudes produced greater forces than proportionally longer pulse durations, indicating that charge is not the sole indicator of evoked force in applications utilizing electrical stimulation. Overlap measurements for calculating current-distance constants were collected at a variety of current amplitudes, electrode pair separations, and pair orientations in the spinal grey matter. Forces obtained in the majority of these trials demonstrated an order effect, presumably due to asymmetric neuronal connectivity within the spinal cord. In the cases showing no order effect, the dorso-ventral electrode pair orientation yielded a higher average current-distance constant (278 microA/mm2) than either the medio-lateral or rostro-caudal electrode pair orientations (197 microA/mm2). Specifications of an array of cylindrical multielectrodes for use in future intraspinal microstimulation prostheses were updated.