Bladder and urethral pressures evoked by microstimulation of the sacral spinal cord in cats

The inhibitory effect of intraspinal microstimulation of the sacral spinal cord on nonlinear bladder reflex dynamics in cats

Objective

Electrical stimulation of the pudendal nerve, pelvic nerve, sacral dorsal root ganglion (DRG), and spinal cord has been explored to treat urinary incontinence and overactive bladder (OAB). This study introduces sacral intraspinal microstimulation (ISMS) as a novel method to inhibit spontaneous bladder reflexes in anesthetized cats. In addition, we investigated the effects of intermittent and switching stimulation patterns on bladder inhibition.

Methods

The electrode was implanted in the dorsal horn of the S2 spinal cord. Bladder pressure was recorded under isovolumetric conditions, and the stimulation parameters were adjusted to inhibit spontaneous bladder contractions. Nonlinear dynamic methods, including chaos theory, were employed to analyze the complexity of bladder reflexes.

Results

Results demonstrated that ISMS targeting the dorsal horn of the S2 spinal segment effectively suppressed high-amplitude spontaneous contractions. Furthermore, bladder reflexes exhibited complex dynamics, ranging from regular to chaotic patterns, with transitions between these states. Importantly, ISMS was able to stabilize these chaotic dynamics, leading to more controlled bladder behavior.

Conclusion

These findings suggest that sacral ISMS offers a promising, targeted alternative to traditional stimulation therapies, potentially providing a new therapeutic approach for managing OAB and urinary incontinence by regulating chaotic bladder activity.

Functional mapping of the lower urinary tract by epidural electrical stimulation of the spinal cord in decerebrated cat model

Several neurologic diseases including spinal cord injury, Parkinson's disease or multiple sclerosis are accompanied by disturbances of the lower urinary tract functions. Clinical data indicates that chronic spinal cord stimulation can improve not only motor function but also ability to store urine and control micturition. Decoding the spinal mechanisms that regulate the functioning of detrusor (Detr) and external urethral sphincter (EUS) muscles is essential for effective neuromodulation therapy in patients with disturbances of micturition. In the present work we performed a mapping of Detr and EUS activity by applying epidural electrical stimulation (EES) at different levels of the spinal cord in decerebrated cat model. The study was performed in 5 adult male cats, evoked potentials were generated by EES aiming to recruit various spinal pathways responsible for LUT and hindlimbs control. Recruitment of Detr occurred mainly with stimulation of the lower thoracic and upper lumbar spinal cord (T13-L1 spinal segments). Responses in the EUS, in general, occurred with stimulation of all the studied sites of the spinal cord, however, a pronounced specificity was noted for the lower lumbar/upper sacral sections (L7-S1 spinal segments). These features were confirmed by comparing the normalized values of the slope angles used to approximate the recruitment curve data by the linear regression method. Thus, these findings are in accordance with our previous data obtained in rats and could be used for development of novel site-specific neuromodulation therapeutic approaches.

Spinal stimulation for motor rehabilitation immediately modulates nociceptive transmission

Objective. Spinal cord injury (SCI) often results in debilitating movement impairments and neuropathic pain. Electrical stimulation of spinal neurons holds considerable promise both for enhancing neural transmission in weakened motor pathways and for reducing neural transmission in overactive nociceptive pathways. However, spinal stimulation paradigms currently under development for individuals living with SCI continue overwhelmingly to be developed in the context of motor rehabilitation alone. The objective of this study is to test the hypothesis that motor-targeted spinal stimulation simultaneously modulates spinal nociceptive transmission.Approach. We characterized the neuromodulatory actions of motor-targeted intraspinal microstimulation (ISMS) on the firing dynamics of large populations of discrete nociceptive specific and wide dynamic range (WDR) neurons. Neurons were accessed via dense microelectrode arrays implantedin vivointo lumbar enlargement of rats. Nociceptive and non-nociceptive cutaneous transmission was induced before, during, and after ISMS by mechanically probing the L5 dermatome.Main results. Our primary findings are that (a) sub-motor threshold ISMS delivered to spinal motor pools immediately modulates concurrent nociceptive transmission; (b) the magnitude of anti-nociceptive effects increases with longer durations of ISMS, including robust carryover effects; (c) the majority of all identified nociceptive-specific and WDR neurons exhibit firing rate reductions after only 10 min of ISMS; and (d) ISMS does not increase spinal responsiveness to non-nociceptive cutaneous transmission. These results lead to the conclusion that ISMS parameterized to enhance motor output results in an overall net decrease n spinal nociceptive transmission.Significance. These results suggest that ISMS may hold translational potential for neuropathic pain-related applications and that it may be uniquely suited to delivering multi-modal therapeutic benefits for individuals living with SCI.

A general framework for automatic closed-loop control of bladder voiding induced by intraspinal microstimulation in rats

Individuals with spinal cord injury or neurological disorders have problems in voiding function due to the dyssynergic contraction of the urethral sphincter. Here, we introduce a closed-loop control of intraspinal microstimulation (ISMS) for efficient bladder voiding. The strategy is based on asynchronous two-electrode ISMS with combined pulse-amplitude and pulse-frequency modulation without requiring rhizotomy, neurotomy, or high-frequency blocking. Intermittent stimulation is alternately applied to the two electrodes that are implanted in the S2 lateral ventral horn and S1 dorsal gray commissure, to excite the bladder motoneurons and to inhibit the urethral sphincter motoneurons. Asynchronous stimulation would lead to reduce the net electric field and to maximize the selective stimulation. The proposed closed-loop system attains a highly voiding efficiency of 77.2-100%, with an average of 91.28 ± 8.4%. This work represents a promising approach to the development of a natural and robust motor neuroprosthesis device for restoring bladder functions.

The Brain and the Bladder: Forebrain Control of Urinary (In)Continence

Neural circuits extending from the cerebral cortex to the bladder maintain urinary continence and allow voiding when it is socially appropriate. Injuries to certain brain regions produce a specific disruption known as urge incontinence. This neurologic symptom is distinguished by bladder spasticity, with sudden urges to void and frequent inability to maintain continence. The precise localization of neural circuit disruptions responsible for urge incontinence remains poorly defined, partly because the brain regions, cell types, and circuit connections that normally maintain continence are unknown. Here, we review what is known about the micturition reflex circuit and about forebrain control of continence from experimental animal studies and human lesion data. Based on this information, we hypothesize that urge incontinence results from damage to a descending pathway that normally maintains urinary continence. This pathway begins with excitatory neurons in the prefrontal cortex and relays subcortically, through inhibitory neurons that may help suppress reflex micturition during sleep and until it is safe and socially appropriate to void. Identifying the specific cell types and circuit connections that constitute the continence-promoting pathway, from the forebrain to the brainstem, will help us better understand why some brain lesions and neurodegenerative diseases disrupt continence. This information is needed to pave the way toward better treatments for neurologic patients suffering from urge incontinence.

Spinal cord stimulation for the restoration of bladder function after spinal cord injury

Spinal cord injury (SCI) results in the inability to empty the bladder voluntarily, and neurogenic detrusor overactivity (NDO) and detrusor sphincter dyssynergia (DSD) negatively impact both the health and quality of life of persons with SCI. Current approaches to treat bladder dysfunction in persons with SCI, including self-catheterisation and anticholinergic medications, are inadequate, and novel approaches are required to restore continence with increased bladder capacity, as well as to provide predictable and efficient on-demand voiding. Improvements in bladder function following SCI have been documented using a number of different modalities of spinal cord stimulation (SCS) in both persons with SCI and animal models, including SCS alone or SCS with concomitant activity-based training. Improvements include increased volitional voiding, voided volumes, bladder capacity, and quality of life, as well as decreases in NDO and DSD. Further, SCS is a well-developed therapy for chronic pain, and existing Food And Drug Administration (FDA)-approved devices provide a clear pathway to sustainable commercial availability and impact. However, the effective stimulation parameters and the appropriate timing and location of stimulation for SCS-mediated restoration of bladder function require further study, and studies are needed to determine underlying mechanisms of action.

Respiratory resetting elicited by single pulse spinal stimulation

Intraspinal microstimulation (ISMS) can effectively activate spinal motor circuits, but the impact on the endogenous respiratory pattern has not been systematically evaluated. Here we delivered ISMS in spontaneously breathing adult rats while simultaneously recording diaphragm and external intercostal electromyography activity. ISMS pulses were delivered from C2-T1 along two rostrocaudal tracts located 0.5 or 1 mm lateral to midline. A tungsten electrode was incrementally advanced from the dorsal spinal surface and 300μs biphasic pulses (10-90 μA) were delivered at depth increments of 600 μm. Dorsal ISMS often produced fractionated inspiratory bursting or caused early termination of the inspiratory effort. Conversely, ventral stimulation had no discernable impact on respiratory resetting. We conclude that ISMS targeting the ventral spinal cord is unlikely to directly alter the respiratory rhythm. Dorsal ISMS, however, may terminate the inspiratory burst through activation of spinobulbar pathways. We suggest that respiratory patterns should be included as an outcome variable in preclinical studies of ISMS.

Voluntary urination control by brainstem neurons that relax the urethral sphincter

Voluntary urination ensures that waste is eliminated when safe and socially appropriate, even without a pressing urge. Uncontrolled urination, or incontinence, is a common problem with few treatment options. Normal urine release requires a small region in the brainstem known as Barrington's nucleus (Bar), but specific neurons that relax the urethral sphincter and enable urine flow are unknown. Here we identify a small subset of Bar neurons that control the urethral sphincter in mice. These excitatory neurons express estrogen receptor 1 (Bar^ESR1), project to sphincter-relaxing interneurons in the spinal cord and are active during natural urination. Optogenetic stimulation of Bar^ESR1 neurons rapidly initiates sphincter bursting and efficient voiding in anesthetized and behaving animals. Conversely, optogenetic and chemogenetic inhibition reveals their necessity in motivated urination behavior. The identification of these cells provides an expanded model for the control of urination and its dysfunction.

Neurophysiology and neural engineering: a review

Neurophysiology is the branch of physiology concerned with understanding the function of neural systems. Neural engineering (also known as neuroengineering) is a discipline within biomedical engineering that uses engineering techniques to understand, repair, replace, enhance, or otherwise exploit the properties and functions of neural systems. In most cases neural engineering involves the development of an interface between electronic devices and living neural tissue. This review describes the origins of neural engineering, the explosive development of methods and devices commencing in the late 1950s, and the present-day devices that have resulted. The barriers to interfacing electronic devices with living neural tissues are many and varied, and consequently there have been numerous stops and starts along the way. Representative examples are discussed. None of this could have happened without a basic understanding of the relevant neurophysiology. I also consider examples of how neural engineering is repaying the debt to basic neurophysiology with new knowledge and insight.

Barrington's nucleus: Neuroanatomic landscape of the mouse "pontine micturition center"

Barrington's nucleus (Bar) is thought to contain neurons that trigger voiding and thereby function as the "pontine micturition center." Lacking detailed information on this region in mice, we examined gene and protein markers to characterize Bar and the neurons surrounding it. Like rats and cats, mice have an ovoid core of medium-sized Bar neurons located medial to the locus coeruleus (LC). Bar neurons express a GFP reporter for Vglut2, develop from a Math1/Atoh1 lineage, and exhibit immunoreactivity for NeuN. Many neurons in and around this core cluster express a reporter for corticotrophin-releasing hormone (BarCRH ). Axons from BarCRH neurons project to the lumbosacral spinal cord and ramify extensively in two regions: the dorsal gray commissural and intermediolateral nuclei. BarCRH neurons have unexpectedly long dendrites, which may receive synaptic input from the cerebral cortex and other brain regions beyond the core afferents identified previously. Finally, at least five populations of neurons surround Bar: rostral-dorsomedial cholinergic neurons in the laterodorsal tegmental nucleus; lateral noradrenergic neurons in the LC; medial GABAergic neurons in the pontine central gray; ventromedial, small GABAergic neurons that express FoxP2; and dorsolateral glutamatergic neurons that express FoxP2 in the pLC and form a wedge dividing Bar from the dorsal LC. We discuss the implications of this new information for interpreting existing data and future experiments targeting BarCRH neurons and their synaptic afferents to study micturition and other pelvic functions.

Modeling the spinal pudendo-vesical reflex for bladder control by pudendal afferent stimulation

Electrical stimulation of the pudendal nerve (PN) is a promising approach to restore continence and micturition following bladder dysfunction resulting from neurological disease or injury. Although the pudendo-vesical reflex and its physiological properties are well established, there is limited understanding of the specific neural mechanisms that mediate this reflex. We sought to develop a computational model of the spinal neural network that governs the reflex bladder response to PN stimulation. We implemented and validated a neural network architecture based on previous neuroanatomical and electrophysiological studies. Using synaptically-connected integrate and fire model neurons, we created a network model with realistic spiking behavior. The model produced expected sacral parasympathetic nucleus (SPN) neuron firing rates from prescribed neural inputs and predicted bladder activation and inhibition with different frequencies of pudendal afferent stimulation. In addition, the model matched experimental results from previous studies of temporal patterns of pudendal afferent stimulation and selective pharmacological blockade of inhibitory neurons. The frequency- and pattern-dependent effects of pudendal afferent stimulation were determined by changes in firing rate of spinal interneurons, suggesting that neural network interactions at the lumbosacral level can mediate the bladder response to different frequencies or temporal patterns of pudendal afferent stimulation. Further, the anatomical structure of excitatory and inhibitory interneurons in the network model was necessary and sufficient to reproduce the critical features of the pudendo-vesical reflex, and this model may prove useful to guide development of novel, more effective electrical stimulation techniques for bladder control.

Inhibitory effects of endomorphin-2 on excitatory synaptic transmission and the neuronal excitability of sacral parasympathetic preganglionic neurons in young rats

The function of the urinary bladder is partly controlled by parasympathetic preganglionic neurons (PPNs) of the sacral parasympathetic nucleus (SPN). Our recent work demonstrated that endomorphin-2 (EM-2)-immunoreactive (IR) terminals form synapses with μ-opioid receptor (MOR)-expressing PPNs in the rat SPN. Here, we examined the effects of EM-2 on excitatory synaptic transmission and the neuronal excitability of the PPNs in young rats (24–30 days old) using a whole-cell patch-clamp approach. PPNs were identified by retrograde labeling with the fluorescent tracer tetramethylrhodamine-dextran (TMR). EM-2 (3 μM) markedly decreased both the amplitude and the frequency of the spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) of PPNs. EM-2 not only decreased the resting membrane potentials (RMPs) in 61.1% of the examined PPNs with half-maximal response at the concentration of 0.282 μM, but also increased the rheobase current and reduced the repetitive action potential firing of PPNs. Analysis of the current–voltage relationship revealed that the EM-2-induced current was reversed at −95 ± 2.5 mV and was suppressed by perfusion of the potassium channel blockers 4-aminopyridine (4-AP) or BaCl₂ or by the addition of guanosine 5′-[β-thio]diphosphate trilithium salt (GDP-β-S) to the pipette solution, suggesting the involvement of the G-protein-coupled inwardly rectifying potassium (GIRK) channel. The above EM-2-invoked inhibitory effects were abolished by the MOR selective antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), indicating that the effects of EM-2 on PPNs were mediated by MOR via pre- and/or post-synaptic mechanisms. EM-2 activated pre- and post-synaptic MORs, inhibiting excitatory neurotransmitter release from the presynaptic terminals and decreasing the excitability of PPNs due to hyperpolarization of their membrane potentials, respectively. These inhibitory effects of EM-2 on PPNs at the spinal cord level may explain the mechanism of action of morphine treatment and morphine-induced bladder dysfunction in the clinic.

Spinal primitives and intra-spinal micro-stimulation (ISMS) based prostheses: a neurobiological perspective on the "known unknowns" in ISMS and future prospects

The current literature on Intra-Spinal Micro-Stimulation (ISMS) for motor prostheses is reviewed in light of neurobiological data on spinal organization, and a neurobiological perspective on output motor modularity, ISMS maps, stimulation combination effects, and stability. By comparing published data in these areas, the review identifies several gaps in current knowledge that are crucial to the development of effective intraspinal neuroprostheses. Gaps can be categorized into a lack of systematic and reproducible details of: (a) Topography and threshold for ISMS across the segmental motor system, the topography of autonomic recruitment by ISMS, and the coupling relations between these two types of outputs in practice. (b) Compositional rules for ISMS motor responses tested across the full range of the target spinal topographies. (c) Rules for ISMS effects' dependence on spinal cord state and neural dynamics during naturally elicited or ISMS triggered behaviors. (d) Plasticity of the compositional rules for ISMS motor responses, and understanding plasticity of ISMS topography in different spinal cord lesion states, disease states, and following rehabilitation. All these knowledge gaps to a greater or lesser extent require novel electrode technology in order to allow high density chronic recording and stimulation. The current lack of this technology may explain why these prominent gaps in the ISMS literature currently exist. It is also argued that given the "known unknowns" in the current ISMS literature, it may be prudent to adopt and develop control schemes that can manage the current results with simple superposition and winner-take-all interactions, but can also incorporate the possible plastic and stochastic dynamic interactions that may emerge in fuller analyses over longer terms, and which have already been noted in some simpler model systems.

Electrical stimulation for the treatment of lower urinary tract dysfunction after spinal cord injury

Electrical stimulation for bladder control is an alternative to traditional methods of treating neurogenic lower urinary tract dysfunction (NLUTD) resulting from spinal cord injury (SCI). In this review, we systematically discuss the neurophysiology of bladder dysfunction following SCI and the applications of electrical stimulation for bladder control following SCI, spanning from historic clinical approaches to recent pre-clinical studies that offer promising new strategies that may improve the feasibility and success of electrical stimulation therapy in patients with SCI. Electrical stimulation provides a unique opportunity to control bladder function by exploiting neural control mechanisms. Our understanding of the applications and limitations of electrical stimulation for bladder control has improved due to many pre-clinical studies performed in animals and translational clinical studies. Techniques that have emerged as possible opportunities to control bladder function include pudendal nerve stimulation and novel methods of stimulation, such as high frequency nerve block. Further development of novel applications of electrical stimulation will drive progress towards effective therapy for SCI. The optimal solution for restoration of bladder control may encompass a combination of efficient, targeted electrical stimulation, possibly at multiple locations, and pharmacological treatment to enhance symptom control.

Spinal stimulation of the upper lumbar spinal cord modulates urethral sphincter activity in rats after spinal cord injury

After spinal cord injury (SCI), the neurogenic bladder is observed to develop asynchronous bladder and external urethral sphincter (EUS) contractions in a condition known as detrusor-sphincter dyssnergia (DSD). Activation of the EUS spinal controlling center located at the upper lumbar spinal cord may contribute to reduce EUS dyssynergic contractions and decrease urethral resistance during voiding. However, this mechanism has not been well studied. This study aimed at evaluating the effects of epidural stimulation (EpS) over the spinal EUS controlling center (L3) in combination with a serotonergic receptor agonist on EUS relaxation in naive rats and chronic (6-8 wk) T8 SCI rats. Cystometrogram and EUS electromyography (EMG) were obtained before and after the intravenous administration of 5HT-1A receptor agonist and antagonist. The latency, duration, frequency, amplitude, and area under curve of EpS-evoked EUS EMG responses were analyzed. EpS on L3 evoked an inhibition of EUS tonic contraction and an excitation of EUS intermittent bursting/relaxation correlating with urine expulsion in intact rats. Combined with a 5HT-1A receptor agonist, EpS on L3 evoked a similar effect in chronic T8 SCI rats to reduce urethral contraction (resistance). This study examined the effect of facilitating the EUS spinal controlling center to switch between urine storage and voiding phases by using EpS and a serotonergic receptor agonist. This novel approach of applying EpS on the EUS controlling center modulates EUS contraction and relaxation as well as reduces urethral resistance during voiding in chronic SCI rats with DSD.

Optical stimulation for restoration of motor function after spinal cord injury

Spinal cord injury can be defined as a loss of communication between the brain and the body due to disrupted pathways within the spinal cord. Although many promising molecular strategies have emerged to reduce secondary injury and promote axonal regrowth, there is still no effective cure, and recovery of function remains limited. Functional electrical stimulation (FES) represents a strategy developed to restore motor function without the need for regenerating severed spinal pathways. Despite its technological success, however, FES has not been widely integrated into the lives of spinal cord injury survivors. In this review, we briefly discuss the limitations of existing FES technologies. Additionally, we discuss how optogenetics, a rapidly evolving technique used primarily to investigate select neuronal populations within the brain, may eventually be used to replace FES as a form of therapy for functional restoration after spinal cord injury.

Neuroprosthetic technology for individuals with spinal cord injury

CONTEXT

Spinal cord injury (SCI) results in a loss of function and sensation below the level of the lesion. Neuroprosthetic technology has been developed to help restore motor and autonomic functions as well as to provide sensory feedback.

FINDINGS

This paper provides an overview of neuroprosthetic technology that aims to address the priorities for functional restoration as defined by individuals with SCI. We describe neuroprostheses that are in various stages of preclinical development, clinical testing, and commercialization including functional electrical stimulators, epidural and intraspinal microstimulation, bladder neuroprosthesis, and cortical stimulation for restoring sensation. We also discuss neural recording technologies that may provide command or feedback signals for neuroprosthetic devices.

CONCLUSION/CLINICAL RELEVANCE

Neuroprostheses have begun to address the priorities of individuals with SCI, although there remains room for improvement. In addition to continued technological improvements, closing the loop between the technology and the user may help provide intuitive device control with high levels of performance.

Therapeutic intraspinal microstimulation improves forelimb function after cervical contusion injury

OBJECTIVE

Intraspinal microstimulation (ISMS) is a promising method for activating the spinal cord distal to an injury. The objectives of this study were to examine the ability of chronically implanted stimulating wires within the cervical spinal cord to (1) directly produce forelimb movements, and (2) assess whether ISMS stimulation could improve subsequent volitional control of paretic extremities following injury.

APPROACH

We developed a technique for implanting intraspinal stimulating electrodes within the cervical spinal cord segments C6-T1 of Long-Evans rats. Beginning 4 weeks after a severe cervical contusion injury at C4-C5, animals in the treatment condition received therapeutic ISMS 7 hours/day, 5 days/week for the following 12 weeks.

MAIN RESULTS

Over 12 weeks of therapeutic ISMS, stimulus-evoked forelimb movements were relatively stable. We also explored whether therapeutic ISMS promoted recovery of forelimb reaching movements. Animals receiving daily therapeutic ISMS performed significantly better than unstimulated animals during behavioural tests conducted without stimulation. Quantitative video analysis of forelimb movements showed that stimulated animals performed better in the movements reinforced by stimulation, including extending the elbow to advance the forelimb and opening the digits. While threshold current to elicit forelimb movement gradually increased over time, no differences were observed between chronically stimulated and unstimulated electrodes suggesting that no additional tissue damage was produced by the electrical stimulation.

SIGNIFICANCE

The results indicate that therapeutic intraspinal stimulation delivered via chronic microwire implants within the cervical spinal cord confers benefits extending beyond the period of stimulation, suggesting future strategies for neural devices to promote sustained recovery after injury.

Synaptic connections between endomorphin 2-immunoreactive terminals and μ-opioid receptor-expressing neurons in the sacral parasympathetic nucleus of the rat

The urinary bladder is innervated by parasympathetic preganglionic neurons (PPNs) that express μ-opioid receptors (MOR) in the sacral parasympathetic nucleus (SPN) at lumbosacral segments L6-S1. The SPN also contains endomorphin 2 (EM2)-immunoreactive (IR) fibers and terminals. EM2 is the endogenous ligand of MOR. In the present study, retrograde tract-tracing with cholera toxin subunit b (CTb) or wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) via the pelvic nerve combined with immunohistochemical staining for EM2 and MOR to identify PPNs within the SPN as well as synaptic connections between the EM2-IR terminals and MOR-expressing PPNs in the SPN of the rat. After CTb was injected into the pelvic nerve, CTb retrogradely labeled neurons were almost exclusively located in the lateral part of the intermediolateral gray matter at L6-S1 of the lumbosacral spinal cord. All of the them also expressed MOR. EM2-IR terminals formed symmetric synapses with MOR-IR, WGA-HRP-labeled and WGA-HRP/MOR double-labeled neuronal cell bodies and dendrites within the SPN. These results provided morphological evidence that EM2-containing axon terminals formed symmetric synapses with MOR-expressing PPNs in the SPN. The present results also show that EM2 and MOR might be involved in both the homeostatic control and information transmission of micturition.

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.

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.

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.

Neural stimulation and recording electrodes

Electrical stimulation of nerve tissue and recording of neural electrical activity are the basis of emerging prostheses and treatments for spinal cord injury, stroke, sensory deficits, and neurological disorders. An understanding of the electrochemical mechanisms underlying the behavior of neural stimulation and recording electrodes is important for the development of chronically implanted devices, particularly those employing large numbers of microelectrodes. For stimulation, materials that support charge injection by capacitive and faradaic mechanisms are available. These include titanium nitride, platinum, and iridium oxide, each with certain advantages and limitations. The use of charge-balanced waveforms and maximum electrochemical potential excursions as criteria for reversible charge injection with these electrode materials are described and critiqued. Techniques for characterizing electrochemical properties relevant to stimulation and recording are described with examples of differences in the in vitro and in vivo response of electrodes.

Blind source separation of neural recordings and control signals

Neural signals recorded in parts of the body where voluntary movement has been retained can be used to control prosthetic devices that assist patients to regain lost function. The number of signals recorded to control these devices can be increased by using a single multi-contact electrode placed over a multi-fasciculated peripheral nerve. Recordings made using these electrodes can then be separated using blind signal separation (BSS) methods to recover individual fascicular neural activity. In this study, we investigate the feasibility of separating peripheral neural recordings, obtained using a multi-contact electrode, to recover individual fascicular signals. We implement BSS through independent component analysis (ICA) and investigate the effects of the number of contacts used and electrode layout on separation. Peripheral neural signals were simulated using a finite element model of the hypoglossal nerve of adult beagle dogs with a multi-contact cuff electrode placed around it. FastICA was then used to separate simulated neural signals. The separated and post-ICA processed neural signals were then compared to the original signals in the fascicles that caused them through correlation coefficient (CC) calculations. For n=50 trials, the CC values obtained were all higher than 0.9 indicating that BSS can be used to recover linearly mixed independent fascicular neural signals recorded using a multi-contact cuff electrode.

Intraspinal stimulation for bladder voiding in cats before and after chronic spinal cord injury

The long-term objective of this study is to develop neural prostheses for people with spinal cord injuries who are unable to voluntarily control their bladder. This feasibility study was performed in 22 adult cats. We implanted an array of microelectrodes into locations in the sacral spinal cord that are involved in the control of micturition reflexes. The effect of microelectrode stimulation was studied under light Propofol anesthesia at monthly intervals for up to 14 months. We found that electrical stimulation in the sacral parasympathetic nucleus at S(2) level or in adjacent ventrolateral white matter produced bladder contractions insufficient for inducing voiding, while stimulation at or immediately dorsal to the dorsal gray commissure at S(1) level produced strong (at least 20 mmHg) bladder contractions as well as strong (at least 40 mm Hg) external urethral sphincter relaxation, resulting in bladder voiding in 14 animals. In a subset of three animals, spinal cord transection was performed. For several months after the transection, intraspinal stimulation continued to be similarly or even more effective in inducing the bladder voiding as before the transection. We speculate that in the absence of the supraspinal connections, the plasticity in the local spinal circuitry played a role in the improved responsiveness to intraspinal stimulation.

Intraspinal stimulation for bladder voiding in cats before and after chronic spinal cord injury

The long-term objective of this study is to develop neural prostheses for people with spinal cord injuries who are unable to voluntarily control their bladder. This feasibility study was performed in 22 adult cats. We implanted an array of microelectrodes into locations in the sacral spinal cord that are involved in the control of micturition reflexes. The effect of microelectrode stimulation was studied under light Propofol anesthesia at monthly intervals for up to 14 months. We found that electrical stimulation in the sacral parasympathetic nucleus at S(2) level or in adjacent ventrolateral white matter produced bladder contractions insufficient for inducing voiding, while stimulation at or immediately dorsal to the dorsal gray commissure at S(1) level produced strong (at least 20 mmHg) bladder contractions as well as strong (at least 40 mm Hg) external urethral sphincter relaxation, resulting in bladder voiding in 14 animals. In a subset of three animals, spinal cord transection was performed. For several months after the transection, intraspinal stimulation continued to be similarly or even more effective in inducing the bladder voiding as before the transection. We speculate that in the absence of the supraspinal connections, the plasticity in the local spinal circuitry played a role in the improved responsiveness to intraspinal stimulation.

"Safe" charge-injection waveforms for iridium oxide (AIROF) microelectrodes

Use of anodic bias improves the charge-injection limits of activated iridium oxide (AIROF) microelectrodes. Asymmetric waveforms, in which the charge balancing anodic phase is delivered at a lower current density and longer pulse width, has been found to allow for higher values of anodic bias voltages, thus maximizing the AIROF charge-injection capacity. Limiting the voltage excursion of the AIROF below the value at which electrolysis of water occurs is essential to maintaining the long-term viability of implanted electrodes. However, maintaining the electrodes at an anodic bias state while keeping the electrode voltage within these electrochemically "safe" limits complicates the topology of the electronic driver circuitry. We present two possible driver topologies that use compliance-voltage limitation in combination with cathodic current modification.

Blind source separation of peripheral nerve recordings

Prosthetic devices can be controlled using signals recorded in parts of the body where sensation and/or voluntary movement have been retained. Although neural prosthetic applications have used single-channel recordings, multiple-channel recordings could provide a significant increase in useable control signals. Multiple control signals can be acquired from recordings of a single implant by using a multi-contact electrode placed over a multi-fasciculated peripheral nerve. These recordings can be separated to recover the individual fascicular signals. Blind source separation (BSS) algorithms have been developed to extract independent source signals from recordings of their mixtures. The hypothesis that BSS algorithms can recover individual fascicular signals from nerve cuff recordings at physiological signal-to-noise ratio (SNR approximately 3-10 dB) was investigated in this study using a finite-element model (FEM) of a beagle hypoglossal nerve with a flattening interface nerve electrode (FINE). Known statistical properties of fascicular signals were used to generate a set of four sources from which the neural signals recorded at the surface of the nerve with a multi-contact FINE were simulated. Independent component analysis (ICA) was then implemented for BSS of the simulated recordings. A novel post-ICA processing algorithm was developed to solve ICA's inherent permutation ambiguities. The similarity between the estimated and original fascicular signals was quantified by calculating their correlation coefficients. The mean values of the correlation coefficients calculated were higher than 0.95 (n = 50). The effects of the geometric layout of the FINE electrode and noise on the separation algorithm were also investigated. The results show that four distinct overlapping fascicular source signals can be simultaneously recovered from neural recordings obtained using a FINE with five or more contacts at SNR levels higher than 8 dB making them available for use as control signals.

The influence of electrolyte composition on the in vitro charge-injection limits of activated iridium oxide (AIROF) stimulation electrodes

The effects of ionic conductivity and buffer concentration of electrolytes used for in vitro measurement of the charge-injection limits of activated iridium oxide (AIROF) neural stimulation electrodes have been investigated. Charge-injection limits of AIROF microelectrodes were measured in saline with a range of phosphate buffer concentrations from [PO(4)(3-)] = 0 to [PO(4)(3-)] = 103 mM and ionic conductivities from 2-28 mS cm(-1). The charge-injection limits were insensitive to the buffer concentration, but varied significantly with ionic conductivity. Using 0.4 ms cathodal current pulses at 50 Hz, the charge-injection limit increased from 0.5 mC cm(-2) to 2.1 mC cm(-2) as the conductivity was increased from 2 mS cm(-1) to 28 mS cm(-1). An explanation is proposed in which the observed dependence on ionic conductivity arises from non-uniform reduction and oxidation within the porous AIROF and from uncorrected iR-drops that result in an overestimation of the redox potential during pulsing. Conversely, slow-sweep-rate cyclic voltammograms (CVs) were sensitive to buffer concentration with the potentials of the primary Ir(3+)/Ir(4+) reduction and oxidation reactions shifting approximately 300 mV as the buffer concentration decreased from [PO(4)(3-)] = 103 mM to [PO(4)(3-)] = 0 mM. The CV response was insensitive to ionic conductivity. A comparison of in vitro AIROF charge-injection limits in commonly employed electrolyte models of extracellular fluid revealed a significant dependence on the electrolyte, with more than a factor of 4 difference under some pulsing conditions, emphasizing the need to select an electrolyte model that closely matches the conductivity and ionic composition of the in vivo environment.

The recording properties of a multi-contact nerve electrode as predicted by a finite element model of the canine hypoglossal nerve

Most functional electrical stimulation (FES) systems rely only on unidirectional (i.e., efferent) activation of the target organ to yield therapeutic outcomes. For applications involving multi-fasciculated nerves, however, artificial sensors have exhibited limited results. As such, the flat-interface-nerve-electrode (FINE) is presented as a means of obtaining an effective closed-loop control system. To investigate the ability of this electrode to achieve selective recordings at physiological signal-to-noise ratio (SNR), a finite element model (JFEM) of a beagle hypoglossal nerve with an implanted FINE was constructed. Action potentials (AP) were generated at various SNR levels and the performance of the electrode was assessed with a selectivity index (0 < or = SI < or = 1; ability of the electrode to distinguish two active sources). Computer simulations yielded a selective range (0.05 < or = SI < or = 0.76) that was (1) related to the inter-fiber distance and (2) used to predict the minimum inter-fiber distance (0.23 mm < or = d < or = 1.42 mm) required for selective recording. The results of this study suggest that the FINE can record neural activity from a multi-fasciculated nerve and, more importantly, distinguish neural activity from pairs of fascicles at physiologic SNR.

Potential-biased, asymmetric waveforms for charge-injection with activated iridium oxide (AIROF) neural stimulation electrodes

The use of potential biasing and biphasic, asymmetric current pulse waveforms to maximize the charge-injection capacity of activated iridium oxide (AIROF) microelectrodes used for neural stimulation is described. The waveforms retain overall zero net charge for the biphasic pulse, but employ an asymmetry in the current and pulse widths of each phase, with the second phase delivered at a lower current density for a longer period of time than the leading phase. This strategy minimizes polarization of the AIROF by the charge-balancing second phase and permits the use of a more positive anodic bias for cathodal-first pulsing or a more negative cathodic bias for anodal-first pulsing to maximize charge injection. Using 0.4-ms cathodal-first pulses, a maximum charge-injection capacity of 3.3 mC/cm2 was obtained with an 0.6-V bias (versus Ag/AgCl) and a pulse asymmetry of 1:8 in the cathodal and anodal pulse widths. For anodal-first pulsing, a maximum charge capacity of 9.6 mC/cm2 was obtained with an asymmetry of 1:3 at an 0.1-V bias. These measurements were made in vitro in carbonate-buffered saline using microelectrodes with a 2000 microm2 surface area.

Bladder emptying by intermittent electrical stimulation of the pudendal nerve

Persons with a suprasacral spinal cord injury cannot empty their bladder voluntarily. Bladder emptying can be restored by intermittent electrical stimulation of the sacral nerve roots (SR) to cause bladder contraction. However, this therapy requires sensory nerve transection to prevent dyssynergic contraction of the external urethral sphincter (EUS). Stimulation of the compound pudendal nerve trunk (PN) activates spinal micturition circuitry, leading to a reflex bladder contraction without a reflex EUS contraction. The present study determined if PN stimulation could produce bladder emptying without nerve transection in cats anesthetized with alpha-chloralose. With all nerves intact, intermittent PN stimulation emptied the bladder (64 +/- 14% of initial volume, n = 37 across six cats) more effectively than either distention-evoked micturition (40 +/- 19%, p < 0.001, n = 27 across six cats) or bilateral intermittent SR stimulation (25 +/- 23%, p < 0.005, n = 4 across two cats). After bilateral transection of the nerves innervating the urethral sphincter, intermittent SR stimulation voided 79 +/- 17% (n = 12 across three cats), comparable to clinical results obtained with SR stimulation. Voiding via intermittent PN stimulation did not increase after neurotomy (p > 0.10), indicating that PN stimulation was not limited by bladder-sphincter dyssynergia. Intermittent PN stimulation holds promise for restoring bladder emptying following spinal injury without requiring nerve transection.

Sacral neuromodulation in lower urinary tract dysfunction

Vesico-urethral dysfunction is a major problem in daily medical practice due to its psychological disturbances, its social costs and its high impact on quality of life. Recently, sacral neuromodulation, namely the electrical stimulation of the sacral nerves, appears to have become an alternative for radical bladder surgery particularly in cases of idiopathic bladder overactivity. The mechanism of action is only partially understood but it seems to involve a modulation in the spinal cord due to stimulation of inhibitory interneurons. Temporary sacral nerve stimulation is the first step. It comprises the temporary application of neuromodulation as a diagnostic test to determine the best location for the implant and to control the integrity of the sacral root. If test stimulation is successful, a permanent device is implanted. This procedure is safe in experienced hands. So-called idiopathic bladder overactivity still the major indication for this technique. Patients not likely to benefit from the procedure were those with complete or almost complete spinal lesions, but incomplete spinal lesions seemed to be a potential indication. This technique is now also indicated in the case of idiopathic chronic retention and chronic pelvic pain syndrome. When selection is performed, more than three-quarters of the patients showed a clinically significant response with 50% or more reduction in the frequency of incontinent episodes, but the results vary according to the author's mode of evaluation. From the economic point of view, the initial investment in the device is amortized in the mid-term by savings related to lower urinary tract dysfunction. Finally, this technique requires an attentive follow-up and adjustments to the electric parameters so as to optimize the equilibrium between the neurological systems.

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.

Selective recording of the canine hypoglossal nerve using a multicontact flat interface nerve electrode

A flat-interface nerve electrode (FINE) is presented as a potential solution for using multifascicle nerve recordings as part of a closed-loop control system. To investigate the ability of this electrode to achieve selective recordings at physiological signal-to-noise ratio (SNR), a finite-element model (FEM) of a beagle hypoglossal nerve with an implanted FINE was constructed. Action potentials (AP) were generated at various SNR levels and the performance of the electrode was assessed with a selectivity index (0 < or = SI < or = 1; ability of the electrode to distinguish two active sources). Computer simulations yielded a selective range (0.05 < or = SI < or = 0.76) that was 1) related to the interfiber distance and 2) used to predict the minimum interfiber distance (0.23 mm < or = d < or = 1.42 mm) for selective recording at each SNR. The SI was further evaluated using recorded compound APs elicited from electrically activating the branches of the beagle hypoglossal nerve. For all experiments (n = 7), the selectivity (SI = 0.45 +/- 0.16) was within the range predicted by the FEM. This study suggests that the FINE can record the activity from a multifasciculated nerve and, more importantly, distinguish neural signals from pairs of fascicles at physiologic SNR.

Polarization of a Spherical Cell in a Nonuniform Extracellular Electric Field

Polarization of cells by extracellular fields is relevant to neural stimulation, cardiac pacing, cardiac defibrillation, and electroporation. The electric field generated by an extracellular electrode may be nonuniform, and highly nonuniform fields are produced by microelectrodes and near the edges of larger electrodes. We solved analytically for the transmembrane voltage (phi(m)) generated in a spherical cell by a nonuniform extracellular field, as would arise from a point electrode. Phi(m) reached its steady state value with a time constant much shorter than the membrane time constant in both uniform and nonuniform fields. The magnitude of phi(m) generated in the hemisphere of the cell toward the electrode was larger than in the other hemisphere in the nonuniform field, while symmetric polarization occurred in the uniform field. The transmembrane potential in oocytes stained with the voltage sensitive dye Di-8-ANEPPS was measured in a nonuniform field at three different electrode-to-cell distances. Asymmetric biphasic polarization and distance-dependent patterns of membrane voltage were observed in the measurements, as predicted from the analytical solution. These results highlight the differences in cell polarization in uniform and nonuniform electric fields, and these differences may impact excitation and poration by extracellular fields.

Bladder and urethral sphincter responses evoked by microstimulation of S2 sacral spinal cord in spinal cord intact and chronic spinal cord injured cats

Urinary bladder and urethral sphincter responses evoked by bladder distention, ventral root stimulation, or microstimulation of S2 segment of the sacral spinal cord were investigated under alpha-chloralose anesthesia in cats with an intact spinal cord and in chronic spinal cord injured (SCI) cats 6-8 weeks after spinal cord transection at T9-T10 spinal segment. Both SCI and normal cats exhibited large amplitude reflex bladder contractions when the bladder was fully distended. SCI cats also exhibited hyperreflexic bladder contractions during filling and detrusor-sphincter dyssynergia during voiding, neither was observed in normal cats. Electrical stimulation of the ventral roots revealed that the S2 sacral spinal cord was the most effective segment for evoking large amplitude bladder contractions or voiding in both types of cats. Microstimulation with a stimulus intensity of 100 microA and duration of 30-60 s via a single microelectrode in the S2 lateral ventral horn or ventral funiculus evoked large amplitude bladder contractions with small urethral contractions in both normal and SCI cats. However, this stimulation evoked incomplete voiding due to either co-activation of the urethral sphincter or detrusor-sphincter dyssynergia. Stimulation in the S2 dorsal horn evoked large amplitude sphincter responses. The effectiveness of spinal cord microstimulation with a single electrode to induce prominent bladder and urethral sphincter responses in SCI animals demonstrates the potential for using microstimulation techniques to modulate lower urinary tract function in patients with neurogenic voiding dysfunctions.

Arrays for chronic functional microstimulation of the lumbosacral spinal cord

Our objective is to develop neural prostheses based on an array of microelectrodes implanted into the sacral spinal cord, that will allow persons with spinal cord injuries to regain control of their bladder and bowels. For our chronic cat model, we have developed two microelectrode arrays, one type containing nine discrete activated iridium microelectrodes and the second utilizing silicon substrate probes with multiple electrode sites on each probe. Both types can elicit an increase in the pressure within the urinary bladder of more than 40-mm Hg and/or relaxation of the urethral sphincter. A stimulus of 100 microA and 400 micros/ph at 20 Hz (charge-balanced pulses) was required to induce a large increase in bladder pressure or relaxation of the urethral sphincter. We found that 24 h of continuous stimulation with these parameters induced tissue injury (disrupted neuropil, infiltration of inflammatory cells, and loss of neurons close to the tip sites). However, a neural prosthesis that is intended to restore bladder control after spinal cord injury would not operate continuously. Thus, when this stimulus was applied for 24 h, at a 10% duty cycle (1 min of stimulation, then 9 min without stimulation) only minimal histologic changes were observed.

Spinal micturition reflex mediated by afferents in the deep perineal nerve

Reflexes mediated by urethral sensory pathways are integral to urinary function. This study investigated the changes in bladder pressure and urethral sphincter activity resulting from electrical stimulation of afferents in the deep perineal nerve (DP), which innervates the urethra and surrounding muscles, before and after acute spinal cord transection (SCT) in cats anesthetized with alpha-chloralose monitored by blood pressure and heart rate. DP stimulation elicited bladder contractions before and after SCT but only if the bladder contained a sufficient volume of fluid (78% of the volume needed to cause distention-evoked reflex contractions). The volume dependency was mediated by a neuronal mechanism in the lumbosacral spinal cord and was not attributable to length-tension properties of the detrusor muscle. Stimulation at 2-40 Hz initiated bladder contractions, but 20-40 Hz was more effective than lower frequencies in evoking and sustaining bladder contractions for the duration of the stimulus train. Decreases in urethral sphincter activity occurred during sustained bladder contractions evoked by 20- to 40-Hz stimulation before and within 16 h after SCT. After SCT, average bladder pressure increases evoked by DP stimulation were smaller than those evoked before SCT, but in some animals, bladder pressures elicited by DP stimulation continued to increase as time after SCT increased and reached pretransection amplitudes at 8-16 h posttransection. These data confirm the presence of a spinal circuit that can mediate coordinated bladder-sphincter responses and show that afferents from the DP can activate this circuit under appropriate conditions.

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.

Over-pulsing degrades activated iridium oxide films used for intracortical neural stimulation

Microelectrodes using activated iridium oxide (AIROF) charge-injection coatings have been pulsed in cat cortex at levels from near-threshold for neural excitation to the reported in vitro electrochemical charge-injection limits of AIROF. The microelectrodes were subjected to continuous biphasic current pulsing, using an 0.4V (versus Ag|AgCl) anodic bias with equal cathodal and anodal pulse widths, for periods up to 7h at a frequency of either 50Hz or 100Hz. At charge densities of 3mC/cm(2), histology revealed iridium-containing deposits in tissue adjacent to the charge-injection sites and scanning electron microscopy of explanted electrodes revealed a thickened and poorly adherent AIROF coating. Microelectrodes pulsed at 2mC/cm(2) or less remained intact, with no histologic evidence of non-biologic deposits in the tissue. AIROF microelectrodes challenged in vitro under the same pulsing conditions responded similarly, with electrodes pulsed at 3mC/cm(2) showing evidence of AIROF delamination after only 100s of pulsing at 100Hz (10,000 pulses total), while electrodes pulsed at 2mC/cm(2) for 7h at 50Hz (1.3 x 10(6) pulses total) showed no evidence of damage. In vitro electrochemical potential transient measurements in buffered physiologic saline indicate that polarizing the AIROF beyond the potential window for electrolysis of water (-0.6 to 0.8V versus Ag|AgCl) results in the observed degradation.

Artificial autonomic reflexes: using functional electrical stimulation to mimic bladder reflexes after injury or disease

Autonomic reflexes controlling bladder storage (continence) and emptying (micturition) involve spinal and supraspinal nerve pathways, with complex mechanisms coordinating smooth muscle activity of the lower urinary tract with voluntary muscle activity of the external urethral sphincter (EUS). These reflexes can be severely disrupted by various diseases and by neurotrauma, particularly spinal cord injury (SCI). Functional electrical stimulation (FES) refers to a group of techniques that involve application of low levels of electrical current to artificially induce or modify nerve activation or muscle contraction, in order to restore function, improve health or rectify physiological dysfunction. Various types of FES have been developed specifically for improving bladder function and while successful for many urological patients, still require substantial refinement for use after spinal cord injury. Improved knowledge of the neural circuitry and physiology of human bladder reflexes, and the mechanisms by which various types of FES alter spinal outflow, is urgently required. Following spinal cord injury, physical and chemical changes occur within peripheral, spinal and supraspinal components of bladder reflex circuitry. Better understanding of this plasticity may determine the most suitable methods of FES at particular times after injury, or may lead to new FES approaches that exploit this remodeling or perhaps even influence the plasticity. Advances in studies of the neuroanatomy, neurophysiology and plasticity of lumbosacral nerve circuits will provide many further opportunities to improve FES approaches, and will provide "artificial autonomic reflexes" that much more closely resemble the original, healthy neuronal regulatory mechanisms.

Mapping of spinal cord circuits controlling the bladder and external urethral sphincter functions in the rabbit

AIMS

A primary purpose of this study was to evaluate the rabbit as a model for studying the spinal circuitry controlling the bladder emptying. We aimed to map the locations of the neuronal circuitry controlling the external urethral sphincter (EUS) and the detrusor by stimulating at different spinal cord locations with a microelectrode, while recording the responses from these muscles.

METHODS

Spinal cord microstimulation was performed in the intermediate zone of the gray matter at the L7-S4 spinal cord levels in eight rabbits with empty and full bladders. Bladder activity was measured as intravesical pressure (IVP) changes and EUS activity was measured via electromyographic (EMG) electrodes positioned within the urethra.

RESULTS

Under both bladder conditions, EUS activation was produced from similar locations in the spinal cord comprising a continuous area in the intermediate zone of the S2-S3 spinal cord. This region extended 25 mm in the rostrocaudal dimension, at least 1 mm lateral to the midline, and 0.5-1 mm in the dorsoventral dimension at a depth of 2-3 mm beneath the dorsal surface. No locations in the intermediate zone produced EUS inhibition. The S2-S3 spinal region, stimulation of which produced the strongest EUS activation, also produced modest bladder contractions.

CONCLUSIONS

Overall, the results indicate that spinal cord networks controlling bladder and EUS activation in the rabbit are overlapping and clustered into columns extending rostrocaudally. The lack of spinal locations producing EUS inhibition and large bladder contractions make the rabbit an unattractive model for studies of neuroprosthetic spinal control of micturition.

Activation and coordination of spinal motoneuron pools after spinal cord injury

Goal-directed movements of the limbs involve the coordinated activation of dozens of muscles. The neural signals activating these muscles are organized at spinal and supraspinal levels, the end result being trains of action potentials delivered to muscles by ensembles of spinal motoneurons (MNs). A new modelling approach has allowed us to visualize the activity of MNs controlling cat hindlimb locomotion. This reveals a rostrocaudal oscillation of MN activity distributed over about 30 mm of the lumbosacral spinal cord. The coordination and topographical distribution of MN pools thus revealed put an interesting perspective on the restoration of motor function with regeneration and neuroprosthetic techniques. Recent progress in the area of intraspinal microstimulation (ISMS) is reviewed, including the synthesis of locomotor movements with a small number of implanted microwires, eliciting movements after a spinal transection and facilitating weak voluntary movements with subliminal ISMS. We suggest that neuroprostheses may be useful in maximizing the benefits of neural regeneration in the future.

Extracellular stimulation of central neurons: influence of stimulus waveform and frequency on neuronal output

The objective of this project was to examine the influence of stimulus waveform and frequency on extracellular stimulation of neurons with their cell bodies near the electrode (local cells) and fibers of passage in the CNS. Detailed computer-based models of CNS cells and axons were developed that accurately reproduced the dynamic firing properties of mammalian motoneurons including afterpotential shape, spike-frequency adaptation, and firing frequency as a function of stimulus amplitude. The neuron models were coupled to a three-dimensional finite element model of the spinal cord that solved for the potentials generated in the tissue medium by an extracellular electrode. Extracellular stimulation of the CNS with symmetrical charge balanced biphasic stimuli resulted in activation of fibers of passage, axon terminals, and local cells around the electrode at similar thresholds. While high stimulus frequencies enhanced activation of fibers of passage, a much more robust technique to achieve selective activation of targeted neuronal populations was via alterations in the stimulus waveform. Asymmetrical charge-balanced biphasic stimuli, consisting of a long-duration low-amplitude cathodic prepulse phase followed by a short-duration high-amplitude anodic stimulus phase, enabled selective activation of local cells. Conversely, an anodic prepulse phase followed by a cathodic stimulus phase enabled selective activation of fibers of passage. The threshold for activation of axon terminals in the vicinity of the electrode was lower than the threshold for direct activation of local cells, independent of the stimulus waveform. As a result, stimulation induced trans-synaptic influences (indirect depolarization/hyperpolarization) on local cells altered their neural output, and this indirect effect was dependent on stimulus frequency. If the indirect activation of local cells was inhibitory, there was little effect on the stimulation induced neural output of the local cells. However, if the indirect activation of the local cells was excitatory, attempts to activate selectively fibers of passage over local cells was limited. These outcomes provide a biophysical basis for understanding frequency-dependent outputs during CNS stimulation and provide useful tools for selective stimulation of the CNS.

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.

Intraspinal micro stimulation generates locomotor-like and feedback-controlled movements

Intraspinal microstimulation (ISMS) may provide a means for improving motor function in people suffering from spinal cord injuries, head trauma, or stroke. The goal of this study was to determine whether microstimulation of the mammalian spinal cord could generate locomotor-like stepping and feedback-controlled movements of the hindlimbs. Under pentobarbital anesthesia, 24 insulated microwires were implanted in the lumbosacral cord of three adult cats. The cats were placed in a sling leaving all limbs pendent. Bilateral alternating stepping of the hindlimbs was achieved by stimulating through as few as two electrodes in each side of the spinal cord. Typical stride lengths were 23.5 cm, and ample foot clearance was achieved during swing. Mean ground reaction force during stance was 36.4 N, sufficient for load-bearing. Feedback-controlled movements of the cat's foot were achieved by reciprocally modulating the amplitude of stimuli delivered through two intraspinal electrodes generating ankle flexion and extension such that the distance between a sensor on the cat's foot and a free sensor moved back and forth by the investigators was minimized. The foot tracked the displacements of the target sensor through its normal range of motion. Stimulation through electrodes with tips in or near lamina IX elicited movements most suitable for locomotion. In chronically implanted awake cats, stimulation through dorsally located electrodes generated paw shakes and flexion-withdrawals consistent with sensory perception but no weight-bearing extensor movements. These locations would not be suitable for ISMS in incomplete spinal cord injuries. Despite the complexity of the spinal neuronal networks, our results demonstrate that by stimulating through a few intraspinal microwires, near-normal bipedal locomotor-like stepping and feedback-controlled movements could be achieved.