Generation of locomotor‑like activity using monopolar intraspinal electrical microstimulation in rats

Severe spinal cord injury (SCI) affects the ability of functional standing and walking. As the locomotor central pattern generator (CPG) in the lumbosacral spinal cord can generate a regulatory signal for movement, it is feasible to activate CPG neural network using intra-spinal micro-stimulation (ISMS) to induce alternating patterns. The present study identified two special sites with the ability to activate the CPG neural network that are symmetrical about the posterior median sulcus in the lumbosacral spinal cord by ISMS in adult rats. A reversal of flexion and extension can occur in an attempt to generate a stepping movement of the bilateral hindlimb by either reversing the pulse polarity of the stimulus or changing the special site. Therefore, locomotor-like activity can be restored with monopolar intraspinal electrical stimulation on either special site. To verify the motor function regeneration of the paralyzed hindlimbs, a four-week locomotor training with ISMS applied to the special site in the SCI + ISMS group (n=12) was performed. Evaluations of motor function recovery using behavior, kinematics and physiological analyses, were used to assess hindlimb function and the results showed the stimulation at one special site can promote significant functional recovery of the bilateral hindlimbs (P<0.05). The present study suggested that motor function of paralyzed bilateral hindlimbs can be restored with monopolar ISMS.

Materials approaches for modulating neural tissue responses to implanted microelectrodes through mechanical and biochemical means

Implantable intracortical microelectrodes face an uphill struggle for widespread clinical use. Their potential for treating a wide range of traumatic and degenerative neural disease is hampered by their unreliability in chronic settings. A major factor in this decline in chronic performance is a reactive response of brain tissue, which aims to isolate the implanted device from the rest of the healthy tissue. In this review we present a discussion of materials approaches aimed at modulating the reactive tissue response through mechanical and biochemical means. Benefits and challenges associated with these approaches are analyzed, and the importance of multimodal solutions tested in emerging animal models are presented.

Neuromodulation of the lumbar spinal locomotor circuit

The lumbar spinal cord contains the necessary circuitry to independently drive locomotor behaviors. This function is retained following spinal cord injury (SCI) and is amenable to rehabilitation. Although the effectiveness of task-specific training and pharmacologic modulation has been repeatedly demonstrated in animal studies, results from human studies are less striking. Recently, lumbar epidural stimulation (EDS) along with locomotor training was shown to restore weight-bearing function and lower-extremity voluntary control in a chronic, motor-complete human SCI subject. Related animal studies incorporating EDS as part of the therapeutic regiment are also encouraging. EDS is emerging as a promising neuromodulatory tool for SCI.

Functional electrical stimulation for bladder, bowel, and sexual function

The principles of using electrical stimulation of peripheral nerves or nerve roots for restoring useful bladder, bowel, and sexual function after damage or disease of the central nervous system are described. Activation of somatic or parasympathetic efferent nerves can produce contraction of striated or smooth muscle in the bladder, rectum, and sphincters. Activation of afferent nerves can produce reflex activation of somatic muscle and reflex inhibition or activation of smooth muscle in these organs. In clinical practice these techniques have been used to produce effective emptying of the bladder and bowel in patients with spinal cord injury and to improve continence of urine and feces. Stimulation of parasympathetic efferents can produce sustained erection of the penis, and stimulation of the nerves to the seminal vesicles can produce seminal emission. Reflex erection and ejaculation can also be produced by stimulation of afferent nerves. Experimental techniques for controlling emptying and continence by a single device, and prospects for comprehensive control of bladder, bowel, and sexual function by electrical techniques are described. These may include more selective electrodes, inactivation of nerves by specific stimulus parameters, greater use of sensors, and networking of implanted components connected to the central and peripheral nervous system.

Control of urinary bladder function with devices: successes and failures

The management of urinary tract dysfunction is crucial for the health and well-being of people with spinal cord injury. Devices, specifically catheters, play an important role in the daily regime of bladder management for most people with spinal cord injury. However, the high incidence of complications associated with the use of catheters, and the fact that the spinal segments involved in lower urinary tract control remain intact in most cord-injured people, continue to motivate research into devices that could harness the nervous system to provide greater control over lower urinary tract function. Mechanical devices discussed in this review include catheters, artificial urethral sphincters, urethral stents and intraurethral pumps. Additionally, many attempts to restore control of the lower urinary tract with electrical stimulation have been made. Stimulation sites have included: inside the bladder, bladder wall, thigh, pelvic floor, dorsal penile nerve, pelvic nerve, tibial nerve, sacral roots, sacral nerves and spinal cord. Catheters and sacral root stimulators are two techniques whose efficacy is well established. Some approaches have proven less successful and others are still in the development stage. Modifications to sacral root stimulation including posterior root stimulation, anodal blockade and high-frequency blockade as well as new techniques including intraspinal microstimulation, urethral afferent stimulation and injectable microstimulators are also discussed. No single device has yet restored the control and function of the lower urinary tract to the pre-injury state, but new techniques are bringing this possibility closer to reality.

Movements generated by intraspinal microstimulation in the intermediate gray matter of the anesthetized, decerebrate, and spinal cat

The intermediate laminae of the lumbosacral spinal cord are suggested to contain a small number of specialized neuronal circuits that form the basic elements of movement construction ("movement primitives"). Our aim was to study the properties and state dependence of these hypothesized circuits in comparison with movements elicited by direct nerve or muscle stimulation. Microwires for intraspinal microstimulation (ISMS) were implanted in intermediate laminae throughout the lumbosacral enlargement. Movement vectors evoked by ISMS were compared with those evoked by stimulation through muscle and nerve electrodes in cats that were anesthetized, then decerebrated, and finally spinalized. Similar movements could be evoked under anesthesia by ISMS and nerve and muscle stimulation, and these covered the full work space of the limb. ISMS-evoked movements were associated with the actions of nearby motoneuron pools. However, after decerebration and spinalization, ISMS-evoked movements were dominated by flexion, with few extensor movements. This indicates that the outputs of neuronal networks in the intermediate laminae depend significantly on descending input and on the state of the spinal cord. Frequently, the outputs also depended on stimulus intensity. These experiments suggest that interneuronal circuits in the intermediate and ventral regions of the spinal cord overlap and their function may be to process reflex and descending activity in a flexible manner for the activation of nearby motoneuron pools.

Intraspinal microstimulation generates functional movements after spinal-cord injury

Restoring locomotion after spinal-cord injury has been a difficult problem to solve with traditional functional electrical stimulation (FES) systems. Intraspinal microstimulation (ISMS) is a novel approach to FES that takes advantage of spinal-cord locomotor circuits by stimulating in the spinal cord directly. Previous studies in spinal-cord intact cats showed near normal recruitment order, reduced fatigue, and functional, synergistic movements induced by stimulation through a few microwires implanted over a 3-cm region in the lumbosacral cord. The present study sought to test the feasibility of ISMS for restoring locomotion after complete spinal-cord transection. In four adult male cats, the spinal cord was severed at T10, T11, or T12. Two to four weeks later, 30 wires (30 microm, stainless steel) were implanted, under anesthesia, in both sides of the lumbosacral cord. The cats were then decerebrated. Stimulus pulses (40-50 Hz, 200 micros, biphasic) with amplitudes ranging from 1-4x threshold (threshold = 32 +/- 19 microA) were delivered through each unipolar electrode. Kinetics, kinematics, and electromyographic (EMG) measurements were obtained with the cats suspended over a stationary treadmill with embedded force platforms for the hindlimbs. Phasic, interleaved stimulation through electrodes generating flexor or extensor movements produced bilateral weight-bearing stepping of the hindlimbs with ample foot clearance during swing. Minimal changes in kinematics and little fatigue were seen during episodes of 40 consecutive steps. The results indicate that ISMS is a promising technique for restoring locomotion after injury.

Long-term stimulation and recording with a penetrating microelectrode array in cat sciatic nerve

We studied the consequences of long-term implantation of a penetrating microelectrode array in peripheral nerve over the time course of 4-6 mo. Electrode arrays without lead wires were implanted to test the ability of different containment systems to protect the array and nerve during contractions of surrounding muscles. Treadmill walking was monitored and the animals showed no functional deficits as a result of implantation. In a different set of experiments, electrodes with lead wires were implanted for up to 7 mo and the animals were tested at 2-4 week intervals at which time stimulation thresholds and recorded sensory activity were monitored for every electrode. It was shown that surgical technique highly affected the long-term stimulation results. Results between measurement sessions were compared, and in the best case, the stimulation properties stabilized in 80% of the electrodes over the course of the experiment (162 days). The recorded sensory signals, however, were not stable over time. A histological analysis performed on all implanted tissues indicated that the morphology and fiber density of the nerve around the electrodes were normal.

Modularity of motor output evoked by intraspinal microstimulation in cats

We studied the forces produced at the cat's hindpaw by microstimulation of the ipsi- and contralateral lumbar spinal cord in spinal intact alpha-chloralose anesthetized (n = 3) or decerebrate (n = 3) animals. Isometric force and EMG responses were measured at 9-12 limb configurations, with the paw attached to a force transducer and with the hip and femur fixed. The active forces elicited at different limb configurations were summarized as force fields representing the sagittal plane component of the forces produced at the paw throughout the workspace. The forces varied in amplitude over time but the orientations were stable, and the pattern of an active force field was invariant through time. The active force fields divided into four distinct types, and a few of the fields showed convergence to an equilibrium point. The fields were generally produced by coactivation of the hindlimb muscles. In addition, some of the fields were consistent with known spinal reflexes and the stimulation sites producing them were in laminae where the interneurons associated with those reflexes are known to be located. Muscle activation produced by intraspinal stimulation, as assessed by intramuscular EMG activity, was modified with limb configuration, suggesting that the responses were not fixed, but were modified by position-dependent sensory feedback. The force responses may represent basic outputs of the spinal circuitry and may be related to similar spinal primitives found in the frog and rat.

Electrical stimulation for the treatment of bladder dysfunction: current status and future possibilities

Electrical stimulation of peripheral nerves can be used to cause muscle contraction, to activate reflexes, and to modulate some functions of the central nervous system (neuromodulation). If applied to the spinal cord or nerves controlling the lower urinary tract, electrical stimulation can produce bladder or sphincter contraction, produce micturition, and can be applied as a medical treatment in cases of incontinence and urinary retention. This article first reviews the history of electrical stimulation applied for treatment of bladder dysfunction and then focuses on the implantable Finetech-Brindley stimulator to produce bladder emptying, and on external and implantable neuromodulation systems for treatment of incontinence. We conclude by summarizing some recent research efforts including: (a) combined sacral posterior and anterior sacral root stimulator implant (SPARSI), (b) selective stimulation of nerve fibers for selective detrusor activation by sacral ventral root stimulation, (c) microstimulation of the spinal cord, and (d) a newly proposed closed-loop bladder neuroprosthesis to treat incontinence caused by bladder overactivity.

Dermatome electrical stimulation as a therapeutic ambulatory aid for incomplete spinal cord injured patients

Electrical stimulation of the L-3,4 dermatome during treadmill walking is proposed as a gait training modality in incomplete spinal cord injured patients. The dermatome stimulation proved to be efficient in diminishing the extensor tone occurring after loading of the paralyzed limb during the stance phase of walking and resulting in improved flexion of the leg during the swing phase.

Neural prostheses

Assuming that neural regeneration after spinal cord injury (SCI) will eventually become a clinical reality, functional recovery will probably remain incomplete. Assistive devices will therefore continue to play an important role in rehabilitation. Neural prostheses (NPs) are assistive devices that restore functions lost as a result of neural damage. NPs electrically stimulate nerves and are either external or implanted devices. Surface stimulators for muscle exercise are now commonplace in rehabilitation clinics and many homes. Regarding implantable NPs, since 1963 over 40 000 have been implanted to restore hearing, bladder control and respiration. Epidural spinal cord stimulators and deep brain stimulators are routinely implanted to control pain, spasticity, tremor and rigidity. Implantable NPs have also been developed to restore limb movements using electrodes tunnelled under the skin to muscles and nerves. Spinal cord microstimulation (SC[mu]stim) is under study as an alternative way of restoring movement and bladder control. Improvement in bladder and bowel function is a high priority for many SCI people. Sacral root stimulation to elicit bladder contraction is the current NP approach, but this usually requires dorsal rhizotomies to reduce reflex contractions of the external urethral sphincter. It is possible that the spinal centres coordinating the bladder-sphincter synergy could be activated with SC[mu]stim. Given the large and growing number of NPs in use or development, it is surprising how little is known about their long-term interactions with the nervous system. Physiological research will play an important role in elucidating the mechanisms underlying these interactions.