A computational model for estimating recruitment of primary afferent fibers by intraneural stimulation in the dorsal root ganglia

Dorsal Root Ganglion Stimulation for Chronic Pain: Hypothesized Mechanisms of Action

Dorsal root ganglion stimulation (DRGS) is a neuromodulation therapy for chronic pain that is refractory to conventional medical management. Currently, the mechanisms of action of DRGS-induced pain relief are unknown, precluding both our understanding of why DRGS fails to provide pain relief to some patients and the design of neurostimulation technologies that directly target these mechanisms to maximize pain relief in all patients. Due to the heterogeneity of sensory neurons in the dorsal root ganglion (DRG), the analgesic mechanisms could be attributed to the modulation of one or many cell types within the DRG and the numerous brain regions that process sensory information. Here, we summarize the leading hypotheses of the mechanisms of DRGS-induced analgesia, and propose areas of future study that will be vital to improving the clinical implementation of DRGS. PERSPECTIVE: This article synthesizes the evidence supporting the current hypotheses of the mechanisms of action of DRGS for chronic pain and suggests avenues for future interdisciplinary research which will be critical to fully elucidate the analgesic mechanisms of the therapy.

Stimulation of the dorsal root ganglion using an Injectrode®

Objective. The goal of this work was to compare afferent fiber recruitment by dorsal root ganglion (DRG) stimulation using an injectable polymer electrode (Injectrode^®) and a more traditional cylindrical metal electrode.Approach. We exposed the L6 and L7 DRG in four cats via a partial laminectomy or burr hole. We stimulated the DRG using an Injectrode or a stainless steel (SS) electrode using biphasic pulses at three different pulse widths (80, 150, 300μs) and pulse amplitudes spanning the range used for clinical DRG stimulation. We recorded antidromic evoked compound action potentials (ECAPs) in the sciatic, tibial, and common peroneal nerves using nerve cuffs. We calculated the conduction velocity of the ECAPs and determined the charge-thresholds and recruitment rates for ECAPs from Aα, Aβ, and Aδfibers. We also performed electrochemical impedance spectroscopy measurements for both electrode types.Main results. The ECAP thresholds for the Injectrode did not differ from the SS electrode across all primary afferents (Aα, Aβ, Aδ) and pulse widths; charge-thresholds increased with wider pulse widths. Thresholds for generating ECAPs from Aβfibers were 100.0 ± 32.3 nC using the SS electrode, and 90.9 ± 42.9 nC using the Injectrode. The ECAP thresholds from the Injectrode were consistent over several hours of stimulation. The rate of recruitment was similar between the Injectrodes and SS electrode and decreased with wider pulse widths.Significance. The Injectrode can effectively excite primary afferents when used for DRG stimulation within the range of parameters used for clinical DRG stimulation. The Injectrode can be implanted through minimally invasive techniques while achieving similar neural activation to conventional electrodes, making it an excellent candidate for future DRG stimulation and neuroprosthetic applications.

Gate Mechanism and Parameter Analysis of Anodal-First Waveforms for Improving Selectivity of C-Fiber Nerves

PURPOSE

Few investigations have been conducted on the selective stimulation of small-radius unmyelinated C nerves (C), which are critical to both the recovery of damaged nerves and pain suppression. The purpose of this study is to understand how an anodal pulse in an anodal-first stimulation could improve C-selectivity over myelinated nociceptive Aδ nerves (Aδ) and to further clarify the landscape of the solution space.

MATERIALS AND METHODS

An adapted Hodgkin-Huxley (HH) model and the McIntyre-Richardson-Grill (MRG) model were used for modeling C and Aδ, respectively, to analyze the underlying ion dynamics and the influence of relevant stimulation waveforms, including monopolar, polarity-symmetric, and asymmetric pulses.

RESULTS

The results showed that polarity asymmetric waveforms with preceding anodal stimulations benefit C-selectivity the most, underlain by the decrease in the potassium ion current of C.

CONCLUSION

The optimal parameters for C-selectivity have been identified in the low-frequency band, remarkably benefiting the design of selective stimulation waveforms for the recovery of damaged nerves and pain management.

The Dorsal Root Ganglion as a Novel Neuromodulatory Target to Evoke Strong and Reproducible Motor Responses in Chronic Motor Complete Spinal Cord Injury: A Case Series of Five Patients

OBJECTIVES

Current strategies for motor recovery after spinal cord injury (SCI) aim to facilitate motor performance through modulation of afferent input to the spinal cord using epidural electrical stimulation (EES). The dorsal root ganglion (DRG) itself, the first relay station of these afferent inputs, has not yet been targeted for this purpose. The current study aimed to determine whether DRG stimulation can facilitate clinically relevant motor response in motor complete SCI.

MATERIALS AND METHODS

Five patients with chronic motor complete SCI were implanted with DRG leads placed bilaterally on level L4 during five days. Based on personalized stimulation protocols, we aimed to evoke dynamic (phase 1) and isotonic (phase 2) motor responses in the bilateral quadriceps muscles. On days 1 and 5, EMG-measurements (root mean square [RMS] values) and clinical muscle force measurements (MRC scoring) were used to measure motor responses and their reproducibility.

RESULTS

In all patients, DRG-stimulation evoked significant phase 1 and phase 2 motor responses with an MRC ≥4 for all upper leg muscles (rectus femoris, vastus lateralis, vastus medialis, and biceps femoris) (p < 0.05 and p < 0.01, respectively), leading to a knee extension movement strong enough to facilitate assisted weight bearing. No significant differences in RMS values were observed between days 1 and 5 of the study, indicating that motor responses were reproducible.

CONCLUSION

The current paper provides first evidence that bilateral L4 DRG stimulation can evoke reproducible motor responses in the upper leg, sufficient for assisted weight bearing in patients with chronic motor complete SCI. As such, a new target for SCI treatment has surfaced, using existing stimulation devices, making the technique directly clinically accessible.

DRG microstimulation evokes postural responses in awake, standing felines

Objective.

We have demonstrated previously that microstimulation in the dorsal root ganglia (DRG) can selectively evoke activity in primary afferent neurons in anesthetized cats. This study describes the results of experiments focused on characterizing the postural effects of DRG microstimulation in awake cats during quiet standing.

Approach.

To understand the parameters of stimulation that can affect these postural shifts, we measured changes in ground reaction forces (GRF) while varying stimulation location and amplitude. Four animals were chronically implanted at the L6 and L7 DRG with penetrating multichannel microelectrode arrays. During each week of testing, we identified electrode channels that recruited primary afferent neurons with fast (80–120 m s⁻¹) and medium (30–75 m s⁻¹) conduction velocities, and selected one channel to deliver current-controlled biphasic stimulation trains during quiet standing.

Main results.

Postural responses were identified by changes in GRFs and were characterized based on their magnitude and latency. During DRG microstimulation, animals did not exhibit obvious signs of distress or discomfort, which could be indicative of pain or aversion to a noxious sensation. Across 56 total weeks, 13 electrode channels evoked behavioral responses, as detected by a significant change in GRF. Stimulation amplitude modulated the magnitude of the GRF responses for these 13 channels (p < 0.001). It was not possible to predict whether or not an electrode would drive a behavioral response based on information including conduction velocity, recruitment threshold, or the DRG in which it resided.

Significance.

The distinct and repeatable effects on the postural response to low amplitude (<40 μA) DRG microstimulation support that this technique may be an effective way to restore somatosensory feedback after neurological injuries such as amputation.

Selectivity of afferent microstimulation at the DRG using epineural and penetrating electrode arrays

OBJECTIVE

We have shown previously that microstimulation of the lumbar dorsal root ganglia (L5-L7 DRG) using penetrating microelectrodes, selectively recruits distal branches of the sciatic and femoral nerves in an acute preparation. However, a variety of challenges limit the clinical translatability of DRG microstimulation via penetrating electrodes. For clinical translation of a DRG somatosensory neural interface, electrodes placed on the epineural surface of the DRG may be a viable path forward. The goal of this study was to evaluate the recruitment properties of epineural electrodes and compare their performance with that of penetrating electrodes. Here, we compare the number of selectively recruited distal nerve branches and the threshold stimulus intensities between penetrating and epineural electrode arrays.

APPROACH

Antidromically propagating action potentials were recorded from multiple distal branches of the femoral and sciatic nerves in response to epineural stimulation on 11 ganglia in four cats to quantify the selectivity of DRG stimulation. Compound action potentials (CAPs) were recorded using nerve cuff electrodes implanted around up to nine distal branches of the femoral and sciatic nerve trunks. We also tested stimulation selectivity with penetrating microelectrode arrays implanted into ten ganglia in four cats. A binary search was carried out to identify the minimum stimulus intensity that evoked a response at any of the distal cuffs, as well as whether the threshold response selectively occurred in only a single distal nerve branch.

MAIN RESULTS

Stimulation evoked activity in just a single peripheral nerve through 67% of epineural electrodes (35/52) and through 79% of the penetrating microelectrodes (240/308). The recruitment threshold (median  =  9.67 nC/phase) and dynamic range of epineural stimulation (median  =  1.01 nC/phase) were significantly higher than penetrating stimulation (0.90 nC/phase and 0.36 nC/phase, respectively). However, the pattern of peripheral nerves recruited for each DRG were similar for stimulation through epineural and penetrating electrodes.

SIGNIFICANCE

Despite higher recruitment thresholds, epineural stimulation provides comparable selectivity and superior dynamic range to penetrating electrodes. These results suggest that it may be possible to achieve a highly selective neural interface with the DRG without penetrating the epineurium.

Design and assessment of stimulation parameters for a novel peripheral nerve interface

Bi-directional interfaces for peripheral nerve stimulation and recording aim to improve control and acceptance of sensorized prosthetic limbs. The implantable multimodal peripheral recording and stimulation system (IMPRESS) is an intraneural interface technology supporting a high-density transverse intrafascicular multichannel electrode (hd-TIME). Herein we report on in vivo selectivity studies using a passive hd-TIME, and computational modeling towards optimal stimulation parameters for fiber recruitment.

Effects of Synchronous Electrode Pulses on Neural Recruitment During Multichannel Microstimulation

The recent proliferation of high-density microelectrode arrays has inspired several new applications of electrical microstimulation, including restoration of sensory functions in the visual, auditory, and somatosensory systems. In each case, the goal is to achieve precisely targeted activation of neurons, while patterning the location and timing of stimulation across the array to mimic naturalistic patterns of neural activity. However, when two or more electrodes deliver stimulation pulses at the same time, the electric fields created by each electrode interact. The effects of field interactions on neuronal recruitment depend on several factors, which have been studied extensively at the macro-scale but have been overlooked in the case of high density arrays. Here, we report that field interactions can significantly affect neural recruitment, even with low amplitude stimulation. We created a computational model of peripheral nerve axons to estimate stimulation parameters sufficient to generate neural recruitment during synchronous and asynchronous stimulation on two microelectrodes located within the peripheral nerve. Across a range of stimulus amplitudes, the model predicted that synchronous stimulation on adjacent electrodes (400 µm separation), would recruit 2–3 times more neurons than during asynchronous stimulation. Our results suggest that field interactions should not be ignored when designing multichannel microstimulation paradigms, even at threshold-level stimulus amplitudes.

Mechanisms of Dorsal Root Ganglion Stimulation in Pain Suppression: A Computational Modeling Analysis

OBJECTIVE

The mechanisms of dorsal root ganglion (DRG) stimulation for chronic pain remain unclear. The objective of this work was to explore the neurophysiological effects of DRG stimulation using computational modeling.

METHODS

Electrical fields produced during DRG stimulation were calculated with finite element models, and were coupled to a validated biophysical model of a C-type primary sensory neuron. Intrinsic neuronal activity was introduced as a 4 Hz afferent signal or somatic ectopic firing. The transmembrane potential was measured along the neuron to determine the effect of stimulation on intrinsic activity across stimulation parameters, cell location/orientation, and membrane properties.

RESULTS

The model was validated by showing close correspondence in action potential (AP) characteristics and firing patterns when compared to experimental measurements. Subsequently, the model output demonstrated that T-junction filtering was amplified with DRG stimulation, thereby blocking afferent signaling, with cathodic stimulation at amplitudes of 2.8-5.5 × stimulation threshold and frequencies above 2 Hz. This amplified filtering was dependent on the presence of calcium and calcium-dependent small-conductance potassium channels, which produced a hyperpolarization offset in the soma, stem, and T-junction with repeated somatic APs during stimulation. Additionally, DRG stimulation suppressed somatic ectopic activity by hyperpolarizing the soma with cathodic or anodic stimulation at amplitudes of 3-11 × threshold and frequencies above 2 Hz. These effects were dependent on the stem axon being relatively close to and oriented toward a stimulating contact.

CONCLUSIONS

These results align with the working hypotheses on the mechanisms of DRG stimulation, and indicate the importance of stimulation amplitude, polarity, and cell location/orientation on neuronal responses.

Bionic intrafascicular interfaces for recording and stimulating peripheral nerve fibers

The network of peripheral nerves presents extraordinary potential for modulating and/or monitoring the functioning of internal organs or the brain. The degree to which these pathways can be used to influence or observe neural activity patterns will depend greatly on the quality and specificity of the bionic interface. The anatomical organization, which consists of multiple nerve fibers clustered into fascicles within a nerve bundle, presents opportunities and challenges that may necessitate insertion of electrodes into individual fascicles to achieve the specificity that may be required for many clinical applications. This manuscript reviews the current state-of-the-art in bionic intrafascicular interfaces, presents specific concerns for stimulation and recording, describes key implementation considerations and discusses challenges for future designs of bionic intrafascicular interfaces.

A 3D Computational Model of Transcutaneous Electrical Nerve Stimulation for Estimating Aβ Tactile Nerve Fiber Excitability

Tactile sensory feedback plays an important role in our daily life. Transcutaneous electrical nerve stimulation (TENS) is widely accepted to produce artificial tactile sensation. To explore the underlying mechanism of tactile sensation under TENS, this paper presented a novel 3D TENS computational model including an active Aβ tactile nerve fiber (TNF) model and a forearm finite element model with the fine-layered skin structure. The conduction velocity vs. fiber diameter and strength-duration relationships in this combined TENS model matched well with experimental data. Based on this validated TENS model, threshold current variation were further investigated under different stimulating electrode sizes with varied fiber diameters. The computational results showed that the threshold current intensity increased with electrode size, and larger nerve fibers were recruited at lower current intensities. These results were comparable to our psychophysical experimental data from six healthy subjects. This novel 3D TENS model would further guide the floorplan of the surface electrodes, and the stimulating paradigms for tactile sensory feedback.

Microstimulation of the lumbar DRG recruits primary afferent neurons in localized regions of lower limb

Patterned microstimulation of the dorsal root ganglion (DRG) has been proposed as a method for delivering tactile and proprioceptive feedback to amputees. Previous studies demonstrated that large- and medium-diameter afferent neurons could be recruited separately, even several months after implantation. However, those studies did not examine the anatomical localization of sensory fibers recruited by microstimulation in the DRG. Achieving precise recruitment with respect to both modality and receptive field locations will likely be crucial to create a viable sensory neuroprosthesis. In this study, penetrating microelectrode arrays were implanted in the L5, L6, and L7 DRG of four isoflurane-anesthetized cats instrumented with nerve cuff electrodes around the proximal and distal branches of the sciatic and femoral nerves. A binary search was used to find the recruitment threshold for evoking a response in each nerve cuff. The selectivity of DRG stimulation was characterized by the ability to recruit individual distal branches to the exclusion of all others at threshold; 84.7% (n = 201) of the stimulation electrodes recruited a single nerve branch, with 9 of the 15 instrumented nerves recruited selectively. The median stimulation threshold was 0.68 nC/phase, and the median dynamic range (increase in charge while stimulation remained selective) was 0.36 nC/phase. These results demonstrate the ability of DRG microstimulation to achieve selective recruitment of the major nerve branches of the hindlimb, suggesting that this approach could be used to drive sensory input from localized regions of the limb. This sensory input might be useful for restoring tactile and proprioceptive feedback to a lower-limb amputee.

Current approaches to model extracellular electrical neural microstimulation

Nowadays, high-density microelectrode arrays provide unprecedented possibilities to precisely activate spatially well-controlled central nervous system (CNS) areas. However, this requires optimizing stimulating devices, which in turn requires a good understanding of the effects of microstimulation on cells and tissues. In this context, modeling approaches provide flexible ways to predict the outcome of electrical stimulation in terms of CNS activation. In this paper, we present state-of-the-art modeling methods with sufficient details to allow the reader to rapidly build numerical models of neuronal extracellular microstimulation. These include (1) the computation of the electrical potential field created by the stimulation in the tissue, and (2) the response of a target neuron to this field. Two main approaches are described: First we describe the classical hybrid approach that combines the finite element modeling of the potential field with the calculation of the neuron's response in a cable equation framework (compartmentalized neuron models). Then, we present a “whole finite element” approach allowing the simultaneous calculation of the extracellular and intracellular potentials, by representing the neuronal membrane with a thin-film approximation. This approach was previously introduced in the frame of neural recording, but has never been implemented to determine the effect of extracellular stimulation on the neural response at a sub-compartment level. Here, we show on an example that the latter modeling scheme can reveal important sub-compartment behavior of the neural membrane that cannot be resolved using the hybrid approach. The goal of this paper is also to describe in detail the practical implementation of these methods to allow the reader to easily build new models using standard software packages. These modeling paradigms, depending on the situation, should help build more efficient high-density neural prostheses for CNS rehabilitation.