A Neural Implant ASIC for the Restoration of Balance in Individuals with Vestibular Dysfunction

Published versio

An Energy-Efficient, Dynamic Voltage Scaling Neural Stimulator for a Proprioceptive Prosthesis

Accepted versio

Restoring Sensation of Gravitoinertial Acceleration through Prosthetic Stimulation of the Utricle and Saccule

Individuals with bilateral vestibular hypofunction suffer reduced quality of life due to loss of postural and ocular reflexes essential to maintaining balance and visual acuity during head movements. Vestibular stimulation has demonstrated success in restoring sensation of angular head rotations using electrical stimulation of the semi-circular canals (SCCs). Efforts toward utricle and saccule stimulation to restore sensation of gravitoinertial acceleration have been limited due to the complexity of the otolith end organs and otolith-ocular reflexes (OORs). Four key pieces of technology were developed to extend prosthetic stimulation to the utricle and saccule: a low-noise scleral coil system to record binocular 3D eye movements; a motion platform control system for automated presentation of rotational and translational stimuli; custom electrode arrays with fifty contacts targeting the SCCs, utricle and saccule; and a general-purpose neuroelectronic stimulator for vestibular and other neuromodulation applications. Using these new technologies, OORs were first characterized in six chinchillas to establish OOR norms during translations and static tilts. Results led to creation of a model that infers the axis of head tilt from measured binocular eye movements and thereby provides a context and means to assess the selectivity of prosthetic utricle and saccule stimulation. The model confirms the expectation that excitation of the left utricle and saccule primarily encodes tilts that bring the left ear down. Three of the chinchillas were implanted with electrode arrays in the left ear. Step changes in pulse rate were delivered to utricle and saccule electrodes near the maculae while measuring 3D binocular eye movements with the animal stationary in darkness. These stimuli elicited sustained ocular counter-roll responses that increased in magnitude as pulse rate or amplitude increased. Bipolar stimulation via neighboring electrodes elicited slow-rising or delayed onset of ocular counter-rolls (consistent with normal translational OOR low-pass filter behavior). Two chinchillas showed different direction of electrically-evoked ocular counter-roll between utricle versus saccule stimulation. Only near-neighbor bipolar electrode combinations elicited eye responses compensatory for tilts other than the ‘usual’ left ear down, suggesting the need for distributing multiple bipolar electrode pairs across the maculae to achieve selective stimulation and restore 3D sensation of gravitoinertial acceleration

Towards Integrating Vestibular Implant Stimulation of the Semicircular Canals and the Otolith End Organs to Drive Posture, Gait, and Eye-Stabilizing Reflexes

Individuals who suffer from bilateral vestibular hypofunction see an increased risk of falling and a decreased quality of life due to symptoms like imbalance, difficulty keeping their eyes on target during head movement, and difficulty walking. The current standard of care to address these symptoms involves rehabilitation exercises to train the central nervous system to rely on visual and proprioceptive signals to compensate for a loss in vestibular signal. However, no sensory substitution can compensate for the extremely fast ocular and spinal reflexes driven by the vestibular nerves. Decades of research led to the development of multichannel vestibular prostheses designed to electrically stimulate the vestibular nerve endings to provide head movement information from a three-axis motion processing unit. The Johns Hopkins Multichannel Vestibular Implant Early Feasibility Study implanted nine study participants with a unilateral vestibular implant targeting the three semicircular canals. The Johns Hopkins Multichannel Vestibular Implant for clinical use does not incorporate circuitry or hardware to stimulate the remaining two sensory organs of the vestibular system, the otolith end organs, responsible for encoding gravitoinertial accelerations that contribute to ocular and spinal reflexes. Stimulation to these end organs were not the priority because they typically elicit much smaller ocular reflexes and encode slower, low-frequency information that can more easily be compensated by the visual and proprioceptive systems. Additionally, their sensory epithelia encode a wider range of information, so mapping a three-axis accelerometer’s signal to stimulation parameters is not straightforward. The research described in this dissertation first assesses the need for otolith-targeted stimulation by measuring otolith-related vestibulo-spinal reflexes via clinical tests of posture and gait in the Multichannel Vestibular Implant Early Feasibility Study with individuals receiving only semicircular canal-targeted stimulation. In the following chapters, the research expands on previous work done in normally functioning chinchillas to explore otolith-targeted stimulation in a rodent model of bilateral vestibular hypofunction and to understand and optimize the stimulation parameters that will be physiologically relevant and useful to restore otolith-specific information to the vestibular nerve endings. The work described in this dissertation is a step toward a more complete vestibular prosthesis to help restore the otolith-driven reflexes to individuals with profound vestibular loss

Therapeutic approaches and development of genomic diagnostic tools for Usher syndrome

Tesis por compendio[ES] El síndrome de Usher (USH) es un trastorno raro autosómico recesivo definido principalmente por sordera neurosensorial (SNHL), y una distrofia retiniana conocida como retinosis pigmentaria (RP). La patología muestra heterogeneidad genética, puesto que se conocen al menos 10 genes responsables. No obstante, las mutaciones en USH2A son la causa más frecuente de la enfermedad, en gran medida por la recurrencia de la variante patogénica c.2299delG. En esta tesis se ha desarrollado un ensayo de edición génica para revertir dicha anomalía genética por medio del sistema CRISPR/Cas9. Se diseñaron y probaron varios complejos CRISPR específicos de locus, y el más eficiente fue usado para la corrección de la mutación c.2299delG en células derivadas de pacientes. La tasa de corrección de la mutación obtenida fue del 2.5%. Otro objetivo de esta tesis ha sido la caracterización genética de pacientes USH aún sin diagnóstico molecular. Una primera fase implicó la secuenciación masiva dirigida de las regiones codificantes de todos los genes asociados a la enfermedad. Este estudio, cuya cohorte incluyó 58 pacientes no escrutados previamente, permitió la identificación de 42 nuevas mutaciones presuntamente patológicas, y una tasa general de detección de alelos responsables de la enfermedad de prácticamente el 83%. Sorprendentemente, uno de los sujetos presentaba mutaciones en CEP250, uno de los últimos genes correlacionados con la enfermedad. Una exhaustiva revisión clínica reveló que la degeneración retiniana se trataba en realidad de una distrofia de conos y bastones en lugar de RP clásica, lo cual permitió consolidación del gen CEP250 como responsable de un fenotipo similar al USH. El resto de casos sin resolver induce a sospechar de la existencia de otros genes vinculados con USH. Así pues, se analizó el exoma íntegro de dichos casos negativos del panel por medio de secuenciación de exoma completo, lo cual proporcionó resultados relevantes en seis de las muestras estudiadas. Uno de tales sujetos resultó ser un claro caso de fenocopia de USH, al albergar mutaciones patogénicas en dos genes independientes, TECTA y REEP6, siendo el primero responsable de la SNHL y el segundo de la RP. De forma parecida, en otro paciente se detectaron variantes patológicas para RP en el gen EYS, pero no se identificó paralelamente ningún cambio genético que explicara la SNHL. Tres individuos adicionales resultaron haber sido erróneamente diagnosticados como USH, dada la conclusiva inexistencia o ambigüedad de la sordera. Uno de ellos fue definido como homocigoto de una mutación en CNGB1, ya reconocido como responsable de RP. En el segundo de dichos sujetos se identificó una mutación en homocigosis en el gen GRN, cuyos defectos en estado heterocigoto están asociados a demencia frontotemporal y más raramente combinada con RP si ambos alelos se encuentran alterados. Por otro lado, el tercer paciente fue resuelto como heterocigoto compuesto de variantes en WDR19, un gen asociado en mayor medida a una distrofia retiniana acompañada de trastornos renales y, más raramente, a la forma aislada del síntoma. En el último de los seis casos resaltados de este objetivo se detectó una mutación homocigota sin sentido en el gen ASIC5, cuyo papel en el organismo todavía se desconoce. Sin embargo, se han correlacionado funciones visuales y auditivas para miembros de la misma familia proteica. En conjunto, los hallazgos obtenidos en este trabajo avalan la importancia del uso de las más novedosas tecnologías en la búsqueda de soluciones para enfermedades raras, las cuales presentan por ahora un pronóstico terapéutico bastante desamparado. Asimismo, otras consecuencias positivas en cuanto a la caracterización genética de los pacientes son la corroboración (o rectificación) del diagnóstico inicial, así como la contribución a la estimación demográfica y correlaciones de genotipo-fenotipo, que en definit[CA] La síndrome d'Usher (USH) és una malaltia rara autosòmic recessiu definit principalment per sordera neurosensorial (SNHL) i una distròfia retiniana coneguda com a retinosi pigmentària (RP). La patologia mostra heterogeneïtat genètica, ja que es coneixen almenys 10 gens responsables. No obstant això, les mutacions en USH2A són la causa més freqüent de la malaltia, a causa de la recurrència de la variant patogènica c.2299delG. En aquesta tesi s'ha desenvolupat un assaig d'edició gènica per a revertir la dita anomalia genètica per mitjà del sistema CRISPR/Cas9. Es van dissenyar i van probar diversos complexos CRISPR específics de locus, i el més eficient va ser usat per a la correcció de la mutació en cèl·lules derivades de pacients. La taxa de correcció de la mutació obtinguda va ser del 2.5%. Un altre objectiu d'aquesta tesi ha sigut la caracterització genètica de pacients USH encara sense diagnòstic molecular. Una primera fase va implicar la seqüenciació massiva dirigida de les regions codificants de tots els gens associats a la malaltia. Aquest estudi, la cohort de la qual va incloure 58 pacients no escrutats prèviament, va permetre la identificació de 42 noves mutacions presumptament patològiques, i una taxa general de detecció d'al·lels responsables de la malaltia de pràcticament el 83%. Sorprenentment, un dels subjectes presentava mutacions en CEP250, un dels últims gens correlacionats amb la malaltia. Una exhaustiva revisió clínica va revelar que la degeneració retiniana es tractava en realitat d'una distròfia de cons i bastons en lloc de RP clàssica. Aquestes troballes han permés la consolidació del gen CEP250 com a responsable d'un fenotip similar al USH. La resta de casos sense resoldre induïx a sospitar de l'existència d'altres gens vinculats amb USH. Així, doncs, es va analitzar l'exoma íntegre dels casos negatius del panell a través de seqüenciació d'exoma complet, cosa que va proporcionar resultats rellevants en sis de les mostres estudiades. Un de tals subjectes va resultar ser un clar cas de fenocopia d'USH, a l'albergar mutacions patogèniques en dos gens independents, TECTA i REEP6, sent el primer responsable de la SNHL i el segon de la RP. De forma semblant, en un altre pacient es van detectar variants patològiques per a RP al gen EYS, però no es va identificar paral·lelament cap canvi genètic que explicara la SNHL. Tres individus addicionals van resultar haver sigut erròniament diagnosticats com USH, donada la final inexistència o ambigüitat de la sordera. Un d'ells va ser definit com a homozigot d'una mutació en CNGB1, ja reconegut com a responsable de RP. En el segon d'aquestes subjectes es va identificar una mutació en homozigosi en el gen GRN, els defectes del qual estan associats a demència frontotemporal en estat heterozigot, i més rarament en combinació amb RP si ambdós al·lels es troben alterats. D'altra banda, el tercer pacient va ser resolt com a heterozigot compost de variants en WDR19, un gen associat en major grau a una distròfia retiniana acompanyada de trastorns renals i, més rarament, a la forma aïllada del símptoma. En l'últim dels sis casos ressaltats d'aquest objectiu es va detectar una mutació homozigota sense sentit en el gen ASIC5, el paper en l'organisme del qual encara es desconeix. Amb tot, s'han correlacionat funcions visuals i auditives per a membres de la mateixa família proteica. En conjunt, les troballes obtingudes en aquest treball avalen la importància de l'ús de les més noves tecnologies en la recerca de solucions per a malalties rares, les quals presenten per ara un pronòstic terapèutic prou desemparat. Així mateix, altres conseqüències positives quant a la caracterització genètica dels pacients són la corroboració (o rectificació) del diagnòstic inicial, així com la contribució a l'estimació demogràfica i correlacions de genotip-fenotip, que en definitiva ajuden en la compressió d'US[EN] Usher syndrome (USH) is a rare autosomal recessive disorder defined essentially by sensorineural hearing loss (SNHL) and a retinal dystrophy known as retinitis pigmentosa (RP). The condition shows a genetic heterogeneity, since there are at least 10 genes known to be causative of the syndrome. However, mutations in USH2A are the most frequent cause of the disease, due in a large measure to the recurrence of the c.2299delG pathogenic variant. A gene editing assay to reverse this specific genetic anomaly was developed in this thesis by means of the groundbreaking CRISPR/Cas9 system. Several locus-specific CRISPR complexes were designed and tested, and the most efficient was used to proceed with the c.2299delG mutation correction on patient-derived cells. The trial resulted in a mutation correction rate of 2.5%. Another goal of this thesis was the genetic characterization of molecularly undiagnosed USH patients. Given the genetic diversity of the disease, the procedure required the implementation of high-throughput sequencing, a technology that enables in bulk sequencing of any number of selected loci (or the indiscriminate totality) of the genome. The first phase implied the targeted sequencing of the coding-relevant regions of all known causative or disease-associated genes at the moment. The study, comprising a cohort of 58 previously unscreened patients, enabled the identification of 42 novel putative pathogenic mutations, and an etiologic-allele detection ratio shy of 83%. Remarkably, one of the subjects harbored nonsense mutations in CEP250, which is one of the latest USH-associated genes. However, an exhaustive review of the clinical features unmasked the retinal degeneration as a cone-rod dystrophy rather than RP, which reinforced the linkage of the gene to an USH-like phenotype. The remaining portion of unresolved cases lead to suspicion of the existence of other genes accountable for USH. Hence, the complete exome of such panel-negative cases was screened through whole exome sequencing. This venture provided relevant findings in six of the surveyed samples. One subject was plainly exposed as an USH phenocopy by harboring pathogenic splice-site mutations in two independent genes, TECTA and REEP6, the former responsible for the SNHL and the latter for the RP. Similarly, RP-causative variants in EYS were detected in another patient, yet no pathogenic changes explaining the HL were discovered. Three additional individuals were ultimately unveiled as USH misdiagnosed cases, being the HL actually absent or ambiguous. One of the patients in this set was homozygous for a mutation in CNGB1, already known to be accountable for RP. The other two cases showed a more peculiar outcome being compound heterozygous for putatively pathogenic variants in genes generally associated to other disorders. One presented a homozygous mutation in GRN, a gene associated to frontotemporal dementia under heterozygous condition and less commonly to combined RP for homozygous alterations. The third subject was found to be a carrier of mutations in WDR19, a gene best associated with retinal disorders accompanied by renal signs and rarely with the isolated visual symptom. The last case presented a homozygous nonsense variant in the ASIC5 gene, whose role has yet to be learned. However, some correlations to visual and hearing functions have been reported for members of the same protein family. Altogether, the results obtained from this work attest to the importance of applying the most up-to-date technologies in the search of solutions for rare diseases that realistically pose a despairing therapeutic prognosis. In addition, the positive consequences of the genetic characterization of the patients are the corroboration (or else correction) of the initial diagnosis, and the contribution to the appraisal of demographic and genotype-phenotype correlations, which ultimately aid in the understanding USH and other related diseases.This work was financially supported by the Institute of Health Carlos III and FEDER funds (ISCIII; grants PI13/00638, PI16/00425, PI16/00539, and PIE13/00046), Fundación ONCE (grant 2015/0398), XVIII Fundaluce-FARPE, and “Telemaratón: Todos Somos Raros, Todos Somos Únicos” (grant IP58). C.F.-G. is a recipient of a fellowship (grant IFI14/00021) from the ISCIII. R.P.V.-M. is a Miguel Servet researcher (grant CP11/00090 funded by ISCIII, Madrid, Spain). The funds from the ISCIII are partially supported by the European Regional Development Fund. R.-P.V.M. is also a Marie Curie fellow (grant CIG322034 from the European Commission).Fuster García, C. (2020). Therapeutic approaches and development of genomic diagnostic tools for Usher syndrome [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/137034TESISCompendi

Argus® II Retinal Prosthesis System: Clinical & Functional Outcomes

Developing artificial visual systems to restore sight in blind patients has long been the dream of scientists, clinicians and the public at large. After decades of research, the greatest success in the field has been achieved with electronic retinal prostheses. To date, 3 retinal prosthetic systems have made the transition from laboratory / clinical research to entering the commercial market for clinical use, namely the Argus® II Retinal Prosthesis System (Second Sight), the alpha-IMS system (Retinal Implant AG), and the IRIS® II (Pixium Vision). The following body of work describes the Argus® II Retinal Prosthesis system, which obtained regulatory approval in the European Economic Area in 2011 (CE marking) and later on in the USA (FDA approval in February 2013), based on the results of an international multi-centre clinical feasibility trial (Clinical Trial identifier: NCT 00407602). This thesis aims to examine the long-term clinical and functional outcomes in an early cohort of subjects chronically implanted with the Argus® II system, from the original feasibility study. A further aim is to elucidate the characteristics of the artificial vision that is perceived and its long-term repeatability and reproducibility in individual subjects. These two broad aims will assist in understanding the nature of the visual performance provided by this device, as well as to add to the current data that is defining the feasibility of constructing predictable pixelated patterns to achieve useful artificial vision in the future. Finally, we explored the feasibility of real-time imaging of visual cortex activation in response to electrical retinal stimulation with the Argus® II system, using functional near infra-red spectroscopy (fNIRS). Development of this real-time imaging tool will enable future investigations into the differences in the cortical activities in response to different stimulations and in different subjects. This may in turn help us understand the variability in their visual performance, as well as to further explore the extent and effect of cross-modal plasticity at the cortical level, in this cohort of patients who have been deprived of visual inputs for decades. Visual function was assessed in terms of: a) form recognition and b) spatial localisation under both 2-dimensional (2D) screen-based laboratory settings and 3-dimensional (3D) paradigms simulating real-life settings. A prospective study of 11 Argus® II subjects showed that the subjects could identify distinct geometric shapes presented in high contrast better with the prosthetic system switched on (median % of correct identification = 20.0%, IQR = 18.8), versus off (median = 12.5%, IQR = 5.0). The accuracy of shapes identification could be further improved by enhancing the outlines of the geometric shape (median = 33.1%, IQR = 21.6). A further prospective study from a subset of 7 subjects showed that this 2D shape identification could be translated into improved identification of 3D objects. These subjects could identify 8 common daily-life objects presented in high contrast with the prosthetic system switched on (median = 31.3%, IQR = 20.3) versus off (median = 12.5%, IQR = 12.5). Scrambling of the transmission signals within the prosthetic system in order to separate light information from form information (i.e. “scrambled mode”) hindered the identification in some but not all subjects (median = 25.0%, IQR = 12.5). The accuracy of object identification could also be improved by enhancing the edges of objects (median = 43.8%, IQR = 15.6). Previously published data showed that Argus® II subjects were able to locate and point to white squares presented on touch screens against a black background more accurately with the prosthetic system switched on versus off. We demonstrated with a prospective study of 5 subjects that they could localise an object on the table, reach out and grasp the object (prehension) with great accuracy (66.7 – 100%) when the prosthetic system was switched on, versus no object prehension (0%) with the system switched off. A prospective study of 6 Argus® II subjects illustrated that while there was a wide variation in the shape and size of the phosphenes perceived by individual subjects, the elicited phosphenes were consistently reproducible in each subject using fixed stimulating parameters, with inter-stimuli intervals ranging from 20 minutes apart, down to 1 second. The perceived location of the phosphenes grossly matched retinotopic agreement, with 4 subjects drawing phosphenes in the same visual field quadrant as predicted by the relative stimulus-fovea position, and 2 subjects depicting phosphenes in the same hemi-field as the expected locations. A retrospective study of 3 Argus® II subjects who underwent MRI brain scan (for unrelated medical reasons) showed that MRI brain scans of up to 1.5 Tesla field strength appeared to have no detrimental effect on the subjects and their implant function. The Argus® II implant produced an artefact of around 50mm x 50mm in size which would prevent visualisation of structures within the orbit, but visualisation of surrounding tissues outside this areas are unaffected. The use of functional MRI as a tool of exploring visual cortex activation in Argus® II subjects was discounted, due to concerns of signal interference from the radiofrequency telemetry of Argus® II system with that of MRI. Subsequently, we have demonstrated in a prospective study that an alternative neuro-imaging technique, functional near infra-red spectroscopy (fNIRS), was capable of capturing real-time cortical activation in 5 out of 6 Argus® II subjects, and maybe a feasible tool for future investigation into cortical function and interactions. The work in this thesis has shown that the Argus® II retinal prosthesis system could improve visual function both in terms of form recognition, as well as object localisation in 3D in situations simulating real-life settings, in a cohort of patients with end-stage retinitis pigmentosa or other outer retinal diseases such as choroideremia. The wide variation in the visual performance level observed could in part be attributable to the diversity in the phosphene features perceived by these subjects. Nevertheless, the consistency and reproducibility with which these phosphenes could be elicited, with fixed stimulating parameters within each subject, provides an encouraging basis for the construction of more complicated pixelated images. Future work to determine the underlying factors influencing the perceived phosphene characteristics, may allow for better prediction of functional outcome, which could in turn be useful for patient selection and tailored preoperative counselling. For those subjects already implanted with the Argus® II system, future work into determining the suitable stimulating parameters for each electrode / quad stimulation may be required for individual subjects, to achieve the construction of optimised and useful, pixelated prosthetic vision

Afferent information modulates spinal network activity in vitro and in preclinical animal models

Primary afferents are responsible for the transmission of peripheral sensory information to the spinal cord. Spinal circuits involved in sensory processing and in motor activity are directly modulated by incoming input conveyed by afferent fibres. Current neurorehabilitation exploits primary afferent information to induce plastic changes within lesioned spinal circuitries. Plasticity and neuromodulation promoted by activity-based interventions are suggested to support both the functional recovery of locomotion and pain relief in subjects with sensorimotor disorders. The present study was aimed at assessing spinal modifications mediated by afferent information. At the beginning of my PhD project, I adopted a simplified in vitro model of isolated spinal cord from the newborn rat. In this preparation, dorsal root (DR) fibres were repetitively activated by delivering trains of electrical stimuli. Responses of dorsal sensory-related and ventral motor-related circuits were assessed by extracellular recordings. I demonstrated that electrostimulation protocols able to activate the spinal CPG for locomotion, induced primary afferent hyperexcitability, as well. Thus, evidence of incoming signals in modulating spinal circuits was provided. Furthermore, a robust sensorimotor interplay was reported to take place within the spinal cord. I further investigated hyperexcitability conditions in a new in vivo model of peripheral neuropathic pain. Adult rats underwent a surgical procedure where the common peroneal nerve was crushed using a calibrated nerve clamp (modified spared nerve injury, mSNI). Thus, primary afferents of the common peroneal nerve were activated through the application of a noxious compression, which presumably elicited ectopic activity constitutively generated in the periphery. One week after surgery, animals were classified into two groups, with (mSNI+) and without (mSNI-) tactile hypersensitivity, based on behavioral tests assessing paw withdrawal threshold. Interestingly, the efficiency of the mSNI in inducing tactile hypersensitivity was halved with respect to the classical SNI model. Moreover, mSNI animals with tactile hypersensitivity (mSNI+) showed an extensive neuroinflammation within the dorsal horn, with activated microglia and astrocytes being significantly increased with respect to mSNI animals without tactile hypersensitivity (mSNI-) and to sham-operated animals. Lastly, RGS4 (regulator of G protein signaling 4) was reported to be enhanced in lumbar dorsal root ganglia (DRGs) and dorsal horn ipsilaterally to the lesion in mSNI+ animals. Thus, a new molecular marker was demonstrated to be involved in tactile hypersensitivity in our preclinical model of mSNI. Lastly, we developed a novel in vitro model of newborn rat, where hindlimbs were functionally connected to a partially dissected spinal cord and passively-driven by a robotic device (Bipedal Induced Kinetic Exercise, BIKE). I aimed at studying whether spinal activity was influenced by afferent signals evoked during passive cycling. I first demonstrated that BIKE could actually evoke an afferent feedback from the periphery. Then, I determined that spinal circuitries were differentially affected by training sessions of different duration. On one side, a short exercise session could not directly activate the locomotor CPG, but was able to transiently facilitate an electrically-induced locomotor-like activity. Moreover, no changes in reflex or spontaneous activity of dorsal and ventral networks were promoted by a short training. On the other side, a long BIKE session caused a loss in facilitation of spinal locomotor networks and a depression in the area of motor reflexes. Furthermore, activity in dorsal circuits was long-term enhanced, with a significant increase in both electrically-evoked and spontaneous antidromic discharges. Thus, the persistence of training-mediated effects was different, with spinal locomotor circuits being only transiently modulated, whereas dorsal activity being strongly and stably enhanced. Motoneurons were also affected by a prolonged training, showing a reduction in membrane resistance and an increase in the frequency of post-synaptic currents (PSCs), with both fast- and slow-decaying synaptic inputs being augmented. Changes in synaptic transmission onto the motoneuron were suggested to be responsible for network effects mediated by passive training. In conclusion, I demonstrated that afferent information might induce changes within the spinal cord, involving both neuronal and glial cells. In particular, spinal networks are affected by incoming peripheral signals, which mediate synaptic, cellular and molecular modifications. Moreover, a strong interplay between dorsal and ventral spinal circuits was also reported. A full comprehension of basic mechanisms underlying sensory-mediated spinal plasticity and bidirectional interactions between functionally different spinal networks might lead to the development of neurorehabilitation strategies which simultaneously promote locomotor recovery and pain relief

2009 - 2010 University Catalog

Volume 99, Number 1, October 30, 2009 Published once a year, October 30, 2009https://scholarsrepository.llu.edu/univcatalog/1011/thumbnail.jp

An Electrophysiological Study Of Voluntary Movement and Spinal Cord Injury

Voluntary movement is generated from the interaction between neurons in our brain and the neurons in our spinal cord that engage our muscles. A spinal cord injury destroys the connection between these two regions, but parts of their underlying neural circuits survive. A new class of treatment (the brain-machine interface) takes advantage of this fact by either a) recording neural activity from the brain and predicting the intended movement (neural prosthetics) or b) stimulating neural activity in the spinal cord to facilitate muscle activity (spinal stimulation). This thesis covers new research studying the brain-machine interface and its application for spinal injury. First, the electrical properties of the microelectrode (the main tool of the brain-machine interface) are studied during deep brain recording and stimulation. This work shows that the insulation coating the electrode forms a capacitor with the surrounding neural tissue. This capacitance causes large spikes of voltage in the surrounding tissue during deep brain stimulation, which will cause electrical artifacts in neural recordings and may damage the surrounding neurons. This work also shows that a coaxially shielded electrode will block this effect. Second, the activity of neurons in the parietal cortex is studied during hand movements, which has applications for neural prosthetics. Prior work suggests that the parietal cortex encodes a state-estimator [1], which combines sensory feedback with the internal efference copy to predict the state of the hand. To test this idea, we used a visual lag to misalign sensory feedback from the efference copy. The expectation was that a state-estimator would unknowingly combine the delayed visual feedback with the current efference information, resulting in incorrect predictions of the hand. Our results show a drop in correlation between neural activity in the parietal cortex and hand movement during a visual lag, supporting the idea that the parietal cortex encodes a state-estimator. This correlation gradually recovers over time, showing that parietal cortex is adaptive to sensory delays. Third, while the intention of spinal stimulation was to interact locally with neural circuits in the spinal cord, results from the clinic show that electrical stimulation of the lumbosacral enlargement enables paraplegic patients to regain voluntary movement of their legs [2]. This means that spinal stimulation facilitates communication across an injury site. To further study this effect, we developed a new behavioral task in the rodent. Rats were trained to kick their right hindlimb in response to an auditory cue. The animals then received a spinal injury that caused paraplegia. After injury, the animals recovered the behavior (they could kick in response to the cue), but only during spinal stimulation. Their recovered behavior was slower and more stereotyped than their pre-injury response. Administering quipazine to these rodents disrupted their ability to respond to the cue, suggesting that serotonin plays an important role in the recovered pathway. This work proves that the new behavioral task is a successful tool for studying the recovery of voluntary movement. Future work will combine cortical recordings with this behavioral task in the rodent to study plasticity in the nervous system and improve treatment of spinal cord injuries. [1] Mulliken, Grant H., Sam Musallam, and Richard A. Andersen. "Forward estimation of movement state in posterior parietal cortex." Proceedings of the National Academy of Sciences105.24 (2008): 8170-8177. [2] Harkema, Susan, et al. "Effect of epidural stimulation of the lumbosacral spinal cord on voluntary movement, standing, and assisted stepping after motor complete paraplegia: a case study." The Lancet 377.9781 (2011): 1938-1947.</p