Biology subjects

Ludwig, K. A.

Publications and source records attributed to Ludwig, K. A..

18 recordsLinked to original sources

bioRxiv2025

Systematic Errors in DC Offset Measurement and Mitigation at Neurostimulation Electrodes: Causes, Implications and Solutions

Objective: Neuromodulation therapies are becoming increasingly common as a therapeutic strategy to treat neurological disorders and deficits. Despite their widespread use, the frameworks used to evaluate device longevity and electrochemical safety are limited, producing variable results for platinum (Pt) electrodes across studies. In addition, next-generation neuromodulation electrodes like thin-film microelectrode arrays (TFMEAs) are more prone to failure by virtue of their smaller size, higher edge to bulk ratios, and greater sensitivity to electrochemical dissolution processes. Two potential contributors to the variability in reported electrochemical safety limits and device performance are particularly sensitive to scaling electrodes to smaller geometries, namely: 1) insufficient measurement device input impedance and 2) unmitigated stimulator leakage currents. Approach: To explore these effects, we first characterize the impact of electrode size (1.27 x 10-4 to 7.85 x 10-3 cm2) on apparent open circuit potential (VaOCP) and interpulse potential (VIPP) measurements at platinum electrodes across measurement instrumentation configurations. Next, we evaluated the effectiveness of commonly used DC mitigation strategies, including stimulation with capacitive coupling (CC), and with capacitive coupling combined with cathode/anode shorting after each pulse (CC+ES), as a function of electrode size. Main Results: Without proper DC mitigation, insufficient measurement input impedance leads to an underestimation of DC offsets ({Delta}VIPP) and, consequently, a misrepresentation of charge injection limits (Qinj) at Pt electrodes. In contrast, when DC offsets are properly mitigated and the input impedance of the measurement device is sufficient, the experimentally derived charge injection limits for Pt electrodes are larger than what has been reported historically. Significance: By identifying the instrumentation-related confounds biasing measurements of electrochemical safety limits, this work provides a more consistent basis for the assessment of Qinj for safety, as well as a framework for identifying and mitigating DC offset as a potential source of variability in electrophysiological findings across studies using TFMEAs.

neuroscience↗

bioRxiv2025

Towards Optimizing Target Engagement in Non-Invasive Trigeminal Nerve Stimulation: Anatomical Characterization and Computational Modeling of the Human Trigeminal Nerve

ObjectiveCranial nerve stimulation (CNS) uses electric current to modulate higher-order brain activity and organ function via nerves, including the vagus and trigeminal, with applications in migraine, epilepsy, and pediatric ADHD. The trigeminal nerve is an emerging target for non-invasive neuromodulation due to the superficial trajectory of its branches, the supraorbital (SON), infraorbital (ION), and mental nerves (MN), and the predominantly sensory composition of the SON and ION. However, the parameters and outcomes of trigeminal nerve stimulation (TNS) remain varied. ApproachThis study characterizes the anatomical course, tissue composition, and activation profiles of the SON, ION, and MN using five human donors. CT imaging was utilized to localize each nerves exit foramen and distance to midline. Microdissections quantified nerve circumference and depth relative to the skin surface. Histological analysis described the number of fascicles and fascicular tissue area. Nerve depths were incorporated into computational models to illustrate the activation function across tissue layers, comparing expected nociceptor and nerve trunk activation functions as a measure of neural engagement. Main ResultsThe SON was found to be significantly more superficial than the ION and MN and had a higher nerve-to-connective tissue ratio relative to the MN. Computational modeling demonstrated that the activation function at the depths of nociceptors was orders of magnitude greater than within the main nerve trunks, suggesting preferential recruitment of cutaneous nociceptors, dependent on nociceptor density. SignificanceThe SON presents the most accessible and anatomically favorable target for transcutaneous trigeminal nerve stimulation among the branches examined due to its superficial location. However, preferential activation of low-threshold nociceptors compared to nerve trunks may lead to treatment-limiting off-target side effects, favoring strategies that target fibers of interest within the skin. These findings offer an anatomically informed framework to guide further computational modeling and electrode design for targeted trigeminal neuromodulation.

neuroscience↗

bioRxiv2025

Field-Programmable Gate Array Based Ultra-Low Power Discrete Fourier Transforms for Closed-Loop Neural Sensing

Digital implementations of discrete Fourier transforms (DFT) are a mainstay in feature assessment of recorded biopotentials, particularly in the quantification of biomarkers of neurological disease state for adaptive deep brain stimulation. Fast Fourier transform (FFT) algorithms and architectures present a substantial power demand from onboard batteries in implantable medical devices, necessitating the development of ultra-low power Fourier transform methods in resource-constrained environments. Numerous FFT architectures aim to optimize power and resource demand through computational efficiency; however, prioritizing the reduction of logic complexity at the cost of additional computations can be equally or more effective. This paper introduces a minimal-architecture single-delay feedback discrete Fourier transform (mSDF-DFT) for use in ultra-low-power field programmable gate array applications and shows energy and power improvements over state-of-the-art low-power DFT and FFT methods. In a neural sensing application, we observe a 33% reduction in dynamic power and 4% reduction in resource utilization when compared to state-of-the-art FFT algorithms; 38% reduction in dynamic power and 4% reduction in resource utilization when compared to Goertzel Algorithm. While designed for use in closed-loop deep brain stimulation and medical device implementations, the mSDF-DFT is also easily extendable to any ultra-low-power embedded application.

neuroscience↗

bioRxiv2025

Combined Central and Peripheral Nerve Stimulation Improves Functional Recovery of Mixed Peripheral Nerve Injury in a Rat Forelimb Model

IntroductionPeripheral nerve reinnervation following nerve injury is often a slow and incomplete process, resulting in significant morbidity and permanent loss of function of the injured extremity in many patients. Prior studies have shown the efficacy of electrical stimulation to synchronize the axonal regeneration of both motor and sensory neurons in peripheral nerve injury models. Moreover, separate investigations have also shown the use of cranial nerve stimulation, principally the vagus nerve, to improve functional outcomes. However, no study has investigated the synergistic effects of both intraoperative electrical stimulation and cranial nerve stimulation for functional improvement within a peripheral nerve injury model. This investigation quantifies the efficacy of combined intraoperative electrical stimulation and trigeminal nerve stimulation on motor and sensory functional recovery in a rat peripheral nerve injury model. MethodsTwelve adult male Lewis rats were trained in a reach and pull task for a food reward using their right forelimb with baseline force thresholds and percent success of the pull task recorded. Baseline sensory data was retrieved using an automated von Frey monofilament test. All rats underwent surgical transection and 2mm gap repair of their median and ulnar nerve of their right forelimb followed by 1 hour of intraoperative electrical stimulation. Trigeminal nerve stimulation throughout the rehabilitation period was completed via supraorbital nerve headcap electrodes. Motor and sensory data were compared to historic cohorts comprised of sham surgery (no nerve injury), brief intraoperative electrical stimulation, trigeminal nerve stimulation, and a sham peripheral and trigeminal nerve stimulation group. Polarization sensitive optical computed tomography (PS-OCT) was used to assess nerve regeneration in fixed tissue samples. ResultsThe combined cohort of rodents were able to recover to their pre-injury motor function by the third week of rehabilitation, faster than either of the singular electrical stimulation cohorts assessed previously. Moreover, functional sensory data of the combined stim cohort demonstrated no change when compared to their pre-injury baseline. ConclusionsPeripheral nerve electrical stimulation and trigeminal nerve stimulation are two separately acting mechanisms of therapy that employ electric waveforms to improve the functional recovery of injured peripheral nerves. The former acts within the periphery to synchronize axonal growth and regeneration of the injured neurons, while the latter acts centrally to augment neuroplasticity. When used simultaneously in a rodent peripheral nerve injury model, these modalities have shown to build upon each other to deliver a faster functional motor recovery, while sensory recovery outcomes remain to be demonstrated.

neuroscience↗

bioRxiv2024

Assessing changes in whole-brain structural connectivity in the unilateral 6-hydroxydopamine rat model of Parkinson's Disease using diffusion imaging and tractography

I.Parkinsons disease (PD) is a multifactorial, progressive neurodegenerative disease that has a profound impact on those it afflicts. Its hallmark pathophysiology is characterized by degeneration of dopaminergic neurons in the midbrain which trigger a host of motor and non- motor symptoms. Many preclinical research efforts utilize unilateral lesion models to assess the neural mechanisms of PD and explore new therapeutic approaches because these models produce similar motor symptoms to those of PD patients. The goal of this work is to examine changes in brain structure resulting from a unilateral lesion both within the nigrostriatal system, where dopaminergic neurons are lost, and throughout the brain. Using multi-shell diffusion magnetic resonance imaging and correlational tractography, we assessed microstructural changes throughout the brain resulting from unilateral injection of 6-hydroxydopamine (6-OHDA) in the median forebrain bundle (MFB). Following lesioning, the PD phenotype was confirmed using behavioral and histological assessment. Correlational tractography found networks of fiber tracts that were either positively or negatively correlated with lesion status throughout the brain. Analyzing patterns of intra- and inter-hemispheric connectivity between the positively and negatively correlated fiber tracts revealed two separate neural networks. The first contained only negatively correlated fibers in the lesioned hemisphere consistent with the local effects of the lesion (i.e. dopaminergic depletion in the nigrostriatal system). The second contained systematically overlapping fiber tracts in the lesioned and non-lesioned hemispheres including the olfactory system and cerebellum, which we suggest are indicative of adaptive mechanisms to compensate for the lesion. Taken together, these results suggest that correlational tractography is a reasonable tool to examine whole brain structural changes in rodent models of neurodegenerative disease, and may have future translational value as a diagnostic tool for patients with PD.

neuroscience↗

bioRxiv2023

Computational modeling of dorsal root ganglion stimulation using an Injectrode

ObjectiveMinimally invasive neuromodulation therapies like the Injectrode, which is composed of a tightly wound polymer-coated platinum/iridium microcoil, offer a low-risk approach for administering electrical stimulation to the dorsal root ganglion (DRG). This flexible electrode is aimed to conform to the DRG. The stimulation occurs through a transcutaneous electrical stimulation (TES) patch, which subsequently transmits the stimulation to the Injectrode via a subcutaneous metal collector. However, effectiveness of stimulation relies on the specific geometrical configurations of the Injectrode-collector-patch system. Hence, there is a need to investigate which design parameters influence the activation of targeted neural structures. ApproachWe employed a hybrid computational modeling approach to analyze the impact of the Injectrode system design parameters on charge delivery and the neural response to stimulation. We constructed multiple finite element method models of DRG stimulation and multi-compartment models of DRG neurons. We simulated the neural responses using parameters based on prior acute preclinical experiments. Additionally, we developed multiple human-scale computational models of DRG stimulation to investigate how design parameters like Injectrode size and orientation influenced neural activation thresholds. Main resultsOur findings were in accordance with acute experimental measurements and indicated that the Injectrode system predominantly engages large-diameter afferents (A{beta}-fibers). These activation thresholds were contingent upon the surface area of the Injectrode. As the charge density decreased due to increasing surface area, there was a corresponding expansion in the stimulation amplitude range before triggering any pain-related mechanoreceptor (A{delta}-fibers) activity. SignificanceThe Injectrode demonstrates potential as a viable technology for minimally invasive stimulation of the DRG. Our findings indicate that utilizing a larger surface area Injectrode enhances the therapeutic margin, effectively distinguishing the desired A{beta} activation from the undesired A{delta}-fiber activation.

bioengineering↗

bioRxiv2023

Vagus nerve stimulation using an endovascular electrode array

Vagus nerve stimulation (VNS), which involves a surgical procedure to place electrodes directly on the vagus nerve (VN), is approved clinically for the treatment of epilepsy, depression, and to facilitate rehabilitation in stroke. VNS at surgically implanted electrodes is often limited by activation of motor nerve fibers near and within the VN that cause neck muscle contraction. In this study we investigated endovascular VNS that may allow activation of the VN at locations where the motor nerve fibers are not localized. We used endovascular electrodes within the nearby internal jugular vein (IJV) to electrically stimulate the VN while recording VN compound action potentials and neck muscle motor evoked potentials in an acute intraoperative swine experiment. We show that the stimulation electrode position within the IJV is critical for efficient activation of the VN. We also demonstrate use of fluoroscopy (cone beam CT mode) and ultrasound to determine the position of the endovascular stimulation electrode with respect to the VN and IJV. At the most effective endovascular stimulation locations tested, thresholds for VN activation were several times higher than direct stimulation of the nerve using a cuff electrode; however, this work demonstrates the feasibility of VNS with endovascular electrodes and provides tools to optimize endovascular electrode positions for VNS. The ability to stimulate the VN with endovascular electrodes creates the possibility to stimulate at VN locations that would be otherwise too invasive and at VN locations where structures such as motor nerve fibers do not exist.

neuroscience↗

bioRxiv2022

Spatially selective stimulation of the pig vagus nerve to modulate target effect versus side effect

Electrical stimulation of the cervical vagus nerve using implanted electrodes (VNS) is FDA-approved for the treatment of drug-resistant epilepsy, treatment-resistant depression, and most recently, chronic ischemic stroke rehabilitation. However, VNS is critically limited by the unwanted stimulation of nearby neck muscles - a result of non-specific stimulation activating motor nerve fibers within the vagus. Prior studies suggested that precise placement of small epineural electrodes can modify VNS therapeutic effects, such as cardiac responses. However, it remains unclear if placement can alter the balance between intended effect and limiting side effect. We used an FDA investigational device exemption approved six-contact epineural cuff to deliver VNS in pigs and quantified how epineural electrode location impacts on- and off-target VNS activation. Detailed post-mortem histology was conducted to understand how the underlying neuroanatomy impacts observed functional responses. Here we report the discovery and characterization of clear neuroanatomy-dependent differences in threshold and saturation for responses related to both effect (change in heart rate) and side effect (neck muscle contractions). The histological and electrophysiological data were used to develop and validate subject-specific computation models of VNS, creating a well-grounded quantitative framework to optimize electrode location-specific activation of nerve fibers governing intended effect versus unwanted side effect.

bioengineering↗

bioRxiv2021

Augmented Transcutaneous Stimulation Using an Injectable Electrode

Minimally invasive neuromodulation technologies seek to marry the neural selectivity of implantable devices with the low-cost and non-invasive nature of transcutaneous electrical stimulation (TES). The Injectrode(R) is a needle-delivered electrode that is injected onto neural structures under image guidance. Power is then transcutaneously delivered to the Injectrode using surface electrodes. The Injectrode serves as a low-impedance conduit to guide current to the deep on-target nerve, reducing activation thresholds by an order of magnitude compared to using only surface stimulation electrodes. To minimize off-target recruitment of cutaneous fibers, the energy transfer efficiency from the surface electrodes to the Injectrode must be optimized. TES energy is transferred to the Injectrode through both capacitive and resistive mechanisms. Electrostatic finite element models generally used in TES research consider only the resistive means of energy transfer by defining tissue conductivities. Here, we present an electroquasistatic model, taking into consideration both the conductivity and permittivity of tissue, to understand transcutaneous power delivery to the Injectrode. The model was validated with measurements taken from (n=4) swine cadavers. We used the validated model to investigate system and anatomic parameters that influence the coupling efficiency of the Injectrode energy delivery system. Our work suggests the relevance of electroquasistatic models to account for capacitive charge transfer mechanisms when studying TES, particularly when high-frequency voltage components are present, such as those used for voltage-controlled pulses and sinusoidal nerve blocks.

bioengineering↗

bioRxiv2021

Stimulation of the Dorsal Root Ganglion using an Injectrode

ObjectiveThe goal of this work was to compare afferent fiber recruitment by dorsal root ganglion (DRG) stimulation using an injectable polymer electrode (Injectrode(R)) and a more traditional cylindrical metal electrode. ApproachWe 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 electrode using biphasic pulses at three different pulse widths (80, 150, 300 s) 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 A, A{beta}, and A{delta} fibers. We also performed electrochemical impedance spectroscopy measurements for both electrode types. Main ResultsThe ECAP thresholds for the Injectrode did not differ from the stainless steel electrode across all primary afferents (A, A{beta}, A{delta}) and pulse widths; charge-thresholds increased with wider pulse widths. Thresholds for generating ECAPs from A{beta} fibers were 100.0 {+/-} 32.3 nC using the stainless steel 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 stainless steel electrode and decreased with wider pulse widths. SignificanceThe 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.

bioengineering↗

bioRxiv2020

In vivo visualization of pig vagus nerve 'vagotopy' using ultrasound

BackgroundPlacement of the clinical vagus nerve stimulating cuff is a standard surgical procedure based on anatomical landmarks, with limited patient specificity in terms of fascicular organization or vagal anatomy. As such, the therapeutic effects are generally limited by unwanted side effects of neck muscle contractions, demonstrated by previous studies to result from stimulation of 1) motor fibers near the cuff in the superior laryngeal and 2) motor fibers within the cuff projecting to the recurrent laryngeal. ObjectiveThe use of patient-specific visualization of vagus nerve fascicular organization could better inform clinical cuff placement and improve clinical outcomes. MethodsThe viability of ultrasound, with the transducer in the surgical pocket, to visualize vagus nerve fascicular organization (i.e. vagotopy) was characterized in a pig model. Ultrasound images were matched to post-mortem histology to confirm the utility of ultrasound in identifying fascicular organization. ResultsHigh-resolution ultrasound accurately depicted the vagotopy of the pig vagus nerve intra-operatively, as confirmed via histology. The stereotypical pseudo-unipolar cell body aggregation at the nodose ganglion was identifiable, and these sensory afferent fascicular bundles were traced down the length of the vagus nerve. Additionally, the superior and recurrent laryngeal nerves were identified via ultrasound. ConclusionsIntraoperative visualization of vagotopy and surrounding nerves using ultrasound is a novel approach to optimize stimulating cuff placement, avoid unwanted activation of motor nerve fibers implicated in off-target effects, and seed patient-specific models of vagal fiber activation to improve patient outcomes.

neuroscience↗

bioRxiv2020

Sources of Off-Target Effects of Vagus Nerve Stimulation Using the Helical Clinical Lead in Domestic Pigs

Clinical data suggest that efficacious vagus nerve stimulation (VNS) is limited by side effects such as cough and dyspnea that have stimulation thresholds lower than those for therapeutic outcomes. VNS side effects are putatively caused by activation of nearby muscles within the neck, via direct muscle activation or activation of nerve fibers innervating those muscles. Our goal was to determine the thresholds at which various VNS-evoked effects occur in the domestic pig--an animal model with vagus anatomy similar to human--using the bipolar helical lead deployed clinically. Intrafascicular electrodes were placed within the vagus nerve to record electroneurographic (ENG) responses, and needle electrodes were placed in the vagal-innervated neck muscles to record electromyographic (EMG) responses. Contraction of the cricoarytenoid muscle occurred at low amplitudes ([~]0.3 mA) and resulted from activation of motor nerve fibers in the cervical vagus trunk within the electrode cuff which bifurcate into the recurrent laryngeal branch of the vagus. At higher amplitudes ([~]1.4 mA), contraction of the cricoarytenoid and cricothyroid muscles was generated by current leakage outside the cuff to activate motor nerve fibers running within the nearby superior laryngeal branch of the vagus. Activation of these muscles generated artifacts in the ENG recordings that may be mistaken for compound action potentials representing slowly conducting A{delta}-, B-, and C-fibers. Our data resolve conflicting reports of the stimulation amplitudes required for C-fiber activation in large animal studies (>10 mA) and human studies (<250 {micro}A). After removing muscle-generated artifacts, ENG signals with post-stimulus latencies consistent with A{delta}- and B-fibers occurred in only a small subset of animals, and these signals had similar thresholds to those that caused bradycardia. By identifying specific neuroanatomical pathways that cause off-target effects and characterizing the stimulation dose-response curves for on- and off-target effects, we hope to guide interpretation and optimization of clinical VNS.

neuroscience↗

bioRxiv2019

In vivo microstimulation with cathodic and anodic asymmetric waveforms modulates spatiotemporal calcium dynamics in cortical neuropil and pyramidal neurons.

Electrical stimulation has been critical in the development of an understanding of brain function and disease. Despite its widespread use and obvious clinical potential, the mechanisms governing stimulation in the cortex remain largely unexplored in the context of pulse parameters. Modeling studies have suggested that modulation of stimulation pulse waveform may be able to control the probability of neuronal activation to selectively stimulate either cell bodies or passing fibers depending on the leading polarity. Thus, asymmetric waveforms with equal charge per phase (i.e. increasing the leading phase duration and proportionately decreasing the amplitude) may be able to activate a more spatially localized or distributed population of neurons if the leading phase is cathodic or anodic, respectively. Here, we use two-photon and mesoscale calcium imaging of GCaMP6s expressed in excitatory pyramidal neurons of male mice to investigate the role of pulse polarity and waveform asymmetry on the spatiotemporal properties of direct neuronal activation with 10 Hz electrical stimulation. We demonstrate that increasing cathodic asymmetry effectively reduces neuronal activation and results in a more spatially localized subpopulation of activated neurons without sacrificing the density of activated neurons around the electrode. Conversely, increasing anodic asymmetry increases the spatial spread of activation and highly resembles spatiotemporal calcium activity induced by conventional symmetric cathodic stimulation. These results suggest that stimulation polarity and asymmetry can be used to modulate the spatiotemporal dynamics of neuronal activity thus increasing the effective parameter space of electrical stimulation to restore sensation and study circuit dynamics. Significance StatementElectrical stimulation has promise to restore sensation and motor function, as well as treat the symptoms of several neurological disorders. However, the mechanisms responsible for the beneficial effects of stimulation are not fully understood. This work supports modeling predictions by demonstrating that modulation of the stimulation waveform dramatically affects the spatial recruitment and activity level of neurons in vivo. These findings suggest that stimulation waveform symmetry represents a parameter that may be able to increase the dynamic range of stimulation applications. Further characterization of these parameters with frequency, and amplitude could provide further insight into the mechanisms of electrical stimulation.

bioengineering↗

bioRxiv2019

Functional Vagotopy in the Cervical Vagus Nerve of the Domestic Pig: Implications for Vagus Nerve Stimulation

Given current clinical interest in vagus nerve stimulation, there are surprisingly few studies characterizing the anatomy of the vagus nerve in large animal models as it pertains to on-and off-target engagement of local fibers. We sought to address this gap by evaluating vagal anatomy in the domestic pig, whose vagus nerve organization and size approximates the human cervical vagus nerve. We provide data on key features across the cervical vagus nerve including diameter, number and diameter of fascicles, and distance of fascicles from the epineural surface where stimulating electrodes are placed. We also characterized the relative locations of the superior and recurrent laryngeal branches of the vagus nerve that have been implicated in therapy limiting side effects with common electrode placement. We identified key variants across the cohort that may be important for vagus nerve stimulation with respect to changing sympathetic/parasympathetic tone, such as cross-connections to the sympathetic trunk. We discovered that cell bodies of pseudo-unipolar cells aggregate together to form a very distinct grouping within the nodose ganglion. This distinct grouping gives rise to a larger number of smaller fascicles as one moves caudally down the cervical vagus nerve. This often leads to a distinct bimodal organization, or vagotopy that may be advantageous to exploit in design of electrodes/stimulation paradigms. Finally, we placed our data in context of historic and recent histology spanning mouse, rat, canine, pig, non-human primate and human models, thus providing a comprehensive resource to understand similarities and differences across species.

neuroscience↗

bioRxiv2019

Is Vagus Stimulation Brain Washing?

Vagal nerve stimulation (VNS) is an FDA approved treatment method for intractable epilepsy, treatment resistant depression, cluster headaches and migraine with over 100,000 patients having received vagal nerve implants to date. Moreover, evidence in the literature has led to a growing list of possible clinical indications, with several small clinical trials applying VNS to treat conditions ranging from neurodegenerative diseases to arthritis, anxiety disorders, and obesity. Despite the growing list of therapeutic applications, the fundamental mechanisms by which VNS achieves its beneficial effects are poorly understood and an area of active research. In parallel, the glymphatic and meningeal lymphatic systems have recently been proposed and experimentally validated to explain how the brain maintains a healthy homeostasis without a traditionally defined lymphatic system. In particular, the glymphatic system relates to the interchange of cerebrospinal fluid (CSF) and interstitial fluid (ISF) whose net effect is to wash through the brain parenchyma removing metabolic waste products and misfolded proteins from the interstitium. Of note, clearance is sensitive to adrenergic signaling, and a primary driver of CSF influx into the parenchyma appears to be cerebral arterial pulsations and respiration. As VNS has well-documented effects on cardiovascular and respiratory physiology as well as brain adrenergic signaling, we hypothesized that VNS delivered at clinically derived parameters would increase CSF influx in the brain. To test this hypothesis, we injected a low molecular weight (3 kD) lysine-fixable fluorescent tracer (TxRed) into the CSF system of mice with a cervical vagus nerve cuff implant and measured the amount of CSF penetrance following VNS. We found that the clinical VNS group showed a significant increase in CSF dye penetrance as compared to the naive control and sham groups. This study demonstrates that VNS therapeutic strategies already being applied in the clinic today may induce intended effects and/or unwanted side effects by altering CSF/ISF exchange in the brain. This may have broad ranging implications in the treatment of various CNS pathologies.\n\nOne Sentence SummaryCervical vagus nerve stimulation using clinically derived parameters enhances movement of cerebrospinal fluid into the brain parenchyma presenting a previously unreported effect of vagus nerve stimulation with potential clinical utility.

neuroscience↗

bioRxiv2019

A Truly Injectable Neural Stimulation Electrode Made From an In-Body Curing Polymer/Metal Composite

Implanted neural stimulation and recording devices hold vast potential to treat a variety of neurological conditions, but the invasiveness, complexity, and cost of the implantation procedure greatly reduce access to an otherwise promising therapeutic approach. To address this need, we have developed a novel electrode that begins as an uncured, flowable pre-polymer that can be injected around a neuroanatomical target to minimize surgical manipulation. Referred to as the Injectrode, the electrode conforms to target structures forming an electrically conductive interface which is orders of magnitude less stiff than conventional neuromodulation electrodes. To validate the Injectrode, we performed detailed electrochemical and microscopy characterization of its material properties and validated the feasibility of using it to electrically stimulate the nervous system in rats and swine. The silicone-metal-particle composite performed very similarly to pure wire of the same metal (silver) in all measures, including exhibiting a favorable cathodic charge storage capacity (CSCC) and charge injection limits compared to the clinical LivaNova stimulation electrode and silver wire electrodes. By virtue of being simpler than traditional electrode designs, less invasive, and more cost-effective, the Injectrode has the potential to increase the adoption of neuromodulation therapies for existing and new indications.

bioengineering↗

bioRxiv2018

Calcium imaging in freely-moving mice during electrical stimulation of deep brain structures

After decades of study in humans and animal models, there remains a lack of consensus regarding how the action of electrical stimulation on neuronal and non-neuronal elements - e.g. neuropil, cell bodies, glial cells, etc. - leads to the therapeutic effects of neuromodulation therapies. To further our understanding of neuromodulation therapies, there is a critical need for novel methodological approaches using state-of-the-art neuroscience tools to study neuromodulation therapy in preclinical models of disease. In this manuscript we outline one such approach combining chronic behaving single-photon microendoscope recordings in a pathological mouse model with electrical stimulation of a common deep brain stimulation (DBS) target. We describe in detail the steps necessary to realize this approach, as well as discuss key considerations for extending this experimental paradigm to other DBS targets for different therapeutic indications. Additionally, we make recommendations from our experience on implementing and validating the required combination of procedures that includes: the induction of a pathological model (6-OHDA model of Parkinsons disease) through an injection procedure, the injection of the viral vector to induce GCaMP expression, the implantation of the GRIN lens and stimulation electrode, and the installation of a baseplate for mounting the microendoscope. We proactively identify unique data analysis confounds occurring due to the combination of electrical stimulation and optical recordings and outline an approach to address these confounds. In order to validate the technical feasibility of this unique combination of experimental methods, we present data to demonstrate that 1) despite the complex multifaceted surgical procedures, chronic optical recordings of hundreds of cells combined with stimulation is achievable over week long periods 2) this approach enables measurement of differences in DBS evoked neural activity between anesthetized and awake conditions and 3) this combination of techniques can be used to measure electrical stimulation induced changes in neural activity during behavior in a pathological mouse model. These findings are presented to underscore the feasibility and potential utility of minimally constrained optical recordings to elucidate the mechanisms of DBS therapies in animal models of disease.

bioengineering↗

bioRxiv2018

Calcium activation of frequency dependent temporally phasic, localized, and dense population of cortical neurons by continuous electrical stimulation

Electrical stimulation of the brain has become a mainstay of fundamental neuroscience research and an increasingly prevalent clinical therapy. Despite decades of use in basic neuroscience research and the growing prevalence of neuromodulation therapies, gaps in knowledge regarding activation or inactivation of neural elements over time have limited its ability to adequately interpret evoked downstream responses or fine-tune stimulation parameters to focus on desired responses. In this work, in vivo two-photon microscopy was used to image neuronal calcium activity in layer 2/3 neurons of somatosensory cortex (S1) in male C57BL/6J-Tg(Thy1-GCaMP6s)GP4.3Dkim/J mice during 30 s of continuous electrical stimulation at varying frequencies. We show frequency-dependent differences in spatial and temporal somatic responses during continuous stimulation. Our results elucidate conflicting results from prior studies reporting either dense spherical activation of somas biased towards those near the electrode, or sparse activation of somas at a distance via axons near the electrode. These findings indicate that the neural element specific temporal response local to the stimulating electrode changes as a function of applied charge density and frequency. These temporal responses need to be considered to properly interpret downstream circuit responses or determining mechanisms of action in basic science experiments or clinical therapeutic applications.\n\nSignificance StatementMicrostimulation of small populations of neurons has the potential to ameliorate symptoms associated with several neurological disorders. However, the specific mechanisms by which microstimulation elicits therapeutic responses are unclear. This work examines the effects of continuous microstimulation on the local population of neurons surrounding the implanted electrode. Stimulation was found to elicit spatiotemporal neuronal responses in a frequency dependent manner. These findings suggest that stimulation frequency may be an important consideration for applications in research or therapy. Further research aimed at understanding these neuronal activation properties may provide insight into the mechanistic mode of action of continuous microstimulation.

neuroscience↗