Title: Effect of surface electrode type on quality of decomposition of surface EMG signal Objectives: The aim of this thesis is to evaluate the effect of surface electrode type, which was used for EMG record, on the quality of decomposition of surface EMG signal using decomposition software EMGlab. Methods: The form of the thesis is an experimental essay. It was detected an EMG signal from one healthy volunteer during mild cyclic contraction. It was used three different types of electrodes (standard surface electrode, tetrode, tetrode with saw-off spikes) for EMG signal detection. These signals were decomposed in program EMGlab. The results were processed in program MS Excel, compared with each other and graphically displayed. Results: The measurement verified a hypothesis which claimed that the used type of electrode affects the quality of automatic decomposition. The best results of automatic decomposition provide the signal gained from the standard surface electrode. Key words: action potential, decomposition, surface electrodes, EMG signal, tetrode, electromyography

Le neurone est une cellule hautement spécialisée qui permet, par des impulsions électriques appelées potentiel d’action (PA) d’assurer la communication neuronale de manière rapide et efficace vers les autres neurones du cerveau. L’axone occupe une place privilégiée dans la genèse du PA. En effet, une région spécialisée de l’axone appelé segment initial de l’axone (SIA) concentre les protéines canaux qui sont à l’origine du PA, les canaux sodium.Le sujet de cette thèse a pour objet d’identifier les facteurs géométriques et électriques contrôlant le seuil du PA. Par une approche essentiellement électrophysiologique couplée à la modélisation, nous identifions ici pour la première fois l’importance de la résistance axiale de l’axone, des canaux sodium et de certains canaux potassium dans le seuil du PA mesuré au corps cellulaire. Cette étude devrait permettre d’affiner et de valider les modèles de seuil du PA en apportant une meilleure compréhension de l’excitabilité neuronale. / The neuron is a highly specialized cell which permits, thanks to electrical impulsion called action potential (AP), to ensure the neuronal communication in a quick and efficient manner towards the other neurons of the brain. The axon takes a privileged place in AP genesis. Indeed, a specified region of the axon, called the axon initial segment (AIS) concentrates channel proteins that are at the origin of the AP, the sodium channels.The subject of this thesis aims to identify the geometrical and electrical factors controlling the threshold of AP. Essentially using an electrophysiological approach coupled with modeling, we identify for the first time here the importance of the axial resistance of the axon, the sodium channels, and some of the potassium channels in the threshold of AP measured in the cell body. This study should permit to refine and validate models of AP threshold by bringing a better understanding of neuronal excitability.

Les cellules ciliées internes (CCI) sont les cellules sensorielles de l'organe de l'audition, elles transforment les ondes sonores en messages nerveux. Avant l’entrée en fonction de la cochlée, les CCI émettent spontanément des potentiels d’action (PA) calciques, ce qui active la voie auditive ascendante et assure le développement de l’axe tonotopique, à savoir la représentation du codage en fréquence, dans chaque relais de la voie auditive. Le profil et les mécanismes à l’origine des PA des CCI sont fortement débattus. Nous nous sommes donc attachés à étudier l’empreinte développementale des cellules sensorielles, c'est à dire déterminer le profil et les mécanismes à l’origine de leur activité.Après avoir incubé l’épithélium neuro-sensoriel avec la sonde calcique Fura2-AM, nous avons observé des vagues calciques se propageant le long des cellules de soutien et des cellules sensorielles. Plus précisément, l’activité des cellules ciliées se caractérisait par des élévations transitoires de calcium (pics calciques) à intervalles de temps réguliers. Nous avons ensuite démontré que les pics calciques des CCI correspondaient bien à des bouffées de PA en mesurant simultanément les oscillations calciques et l’émission de PA en patch-clamp. La fréquence, la durée et la distribution temporelle des pics calciques des CCI étaient en grande partie invariantes le long de l’axe base-apex de la cochlée. Enfin, les cellules voisines montraient une activité fortement synchrone à l’inverse des cellules spatialement éloignées. Ces résultats indiquent donc que l’activité des CCI est majoritairement identique le long de l’axe tonotopique de la cochlée.Nous nous sommes ensuite intéressés au mécanisme responsable de l’activité spontanée, la dépendance à l’ATP. L’incubation d’apyrase, une ecto-nucléotidase, entraine une diminution de l’activité des cellules de soutien, à savoir une réduction de l’aire et de la vitesse de propagation des vagues calciques. En revanche, l'activité des CCI n'est pas altérée par la déplétion d’ATP. Ces résultats suggèrent 2 mécanismes distincts, le premier ATP-dépendant et le second ATP-indépendant dans les cellules de soutien et sensorielles, respectivement.L’ensemble de ces résultats indique que la maturation des centres supérieurs serait déterminée par l’activation synchrone d’un nombre limité de cellules sensorielles. / During development, the sensory cells of the cochlea, the inner hair cells (IHCs), fire spontaneous calcium action potentials. This spontaneous spiking activity at the pre-hearing stage allows the IHCs to automatically stimulate the auditory nerve fibers and hence, ensures the proper shaping of the tonotopic organization along the ascending auditory pathway. Spontaneous spiking patterns may depend on the IHCs position on the cochlea (the tonotopic axis). Those patterns may also rely on ATP secretion from neighboring supporting cells. In this study, we used calcium imaging in the immature neuro-sensory epithelium of the cochlea, the Kölliker´s organ, to gain insights in the IHCs spiking activity. After loading the Kölliker´s organ with the calcium dye fura-2 AM, propagation of spontaneous calcium waves was readily observed across supporting and sensory cells. Both basal and apical IHCs were characterized by similar spontaneous calcium transients interspaced with silent periods, reminiscent of bursts of action potential recorded in patch-clamp. In addition, neighboring cells show a strong degree of synchronous activity. Incubation with apyrase, which hydrolyzes ATP, prevents the spontaneous calcium increase that propagates across the supporting cells within the Kölliker's organ. However, it leaves the spontaneous calcium transients in IHCs mostly unaffected. All these results show that the tonotopic map refinement in higher auditory centers comes from a coordinated activity of neighboring sensory cells, whose activity seems to be independent of ATP

The ability to perceive speech is related to the listener's ability to differentiate among frequencies (i.e. spectral resolution). Cochlear implant users exhibit variable speech perception and spectral resolution abilities, which can be attributed at least in part to electrode interactions at the periphery (i.e. spatial resolution). However, electrophysiological measures of peripheral spatial resolution have not been found to correlate with speech perception. The purpose of this study was to systematically evaluate auditory processing from the periphery to the cortex using both simple and spectrally complex stimuli in order to better understanding the underlying processes affecting spatial and spectral resolution and speech perception. Eleven adult cochlear implant users participated in this study. Peripheral spatial resolution was assessed using the electrically evoked compound action potential (ECAP) to measure channel interaction functions for thirteen probe electrodes. We evaluated central processing using the auditory change complex (ACC), a cortical response, elicited with both spatial (electrode pairs) and spectral (rippled noise) stimulus changes. Speech perception included a vowel-discrimination task and the BKB-SIN test of keyword recognition in noise. We varied the likelihood of electrode interactions within each participant by creating three experimental programs, or MAPs, using a subset of seven electrodes and varying the spacing between activated electrodes. Linear mixed model analysis was used to account for repeated measures within an individual, allowing for a within-subject interpretation. We also performed regression analysis to evaluate the relationships across participants. Both peripheral and central processing abilities contributed to the variability in performance observed across CI users. The spectral ACC was the strongest predictor of speech perception abilities across participants. When spatial resolution was varied within a person, all electrophysiological measures were significantly correlated with each other and with speech perception. However, the ECAP measures were the best single predictor of speech perception for the within-subject analysis, followed by the spectral ACC. Our results indicate that electrophysiological measures of spatial and spectral resolution can provide valuable information about perception. All three of the electrophysiological measures used in this study, including the ECAP channel interaction functions, demonstrated potential for clinical utility.

Current cochlear implant systems use fast pulsatile stimulation to deliver the temporal modulations of speech and to, potentially, improve the neural representation of such modulations by restoring the independence of neural firing. The realization of these benefits may vary with other pulse rate-dependent temporal interactions that occur at the neural membrane, e.g., per(i)stimulatory adaptation and its post-stimulatory or forward masking effects. This study attempted to characterize adaptation and recovery of the electrically evoked compound action potential (ECAP) using probe pulses delivered within and following brief (100 ms) high-rate masker (1800 pps) pulse trains at various current levels in adults and children. With this stimulus paradigm, the ECAP amplitude typically achieved a steady state during the course of pulse train stimulation. The ECAP amplitude at steady state was, on average, a similar proportion (50-70%) of the amplitude at onset for various stimulus levels and in both age groups. However, long-term adaptation effects, evidenced by the decrease in onset ECAP amplitude, were greater in adults particularly at lower levels in the ECAP dynamic range. Instances of alternation in ECAP amplitude were seen at stimulus levels that were higher in the ECAP dynamic range. The forward masking effects of pulse train stimulation were quantified by the ECAP amplitude in response to a subsequent probe pulse normalized by the response to the same pulse presented alone. Pulse train forward masking increased with the level of the masker pulse train and decreased with the level of the probe stimulus. The recovery of the ECAP for probes that were lower in level than the masker pulse train was incomplete at 600 ms after masker offset, consistent with long-term cumulative effects observed in the response to the probe alone. Masker pulse trains that are lower in level than the probe pulse produced proportionally small decrements in the ECAP amplitude with complete recovery within 250 ms of pulse train offset particularly in adults. ECAP recovery of a probe preceded by a masker pulse train of equal level followed a monotonic or non-monotonic pattern consistent with a hypothesis of both adaptation and facilitation occurring with pulse train stimulation. The various patterns of recovery may attest to the occurrence of more than a single process in the same subset of nerve fibers or in different fibers. We hypothesize that the variations in the recovery patterns may be attributable to individual differences in the status of the auditory nerve and possibly, the variations in temporal interactions across the spatial domain at different stimulus levels. Finally, the probe-evoked ECAP amplitude at steady state in children and briefly, e.g., 20 ms, after pulse train offset in both age groups could be predicted by the ECAP amplitude in response to the same probe pulse when preceded at a brief interval (1.2 or 2 ms) by a single masker pulse of the same level as the masker pulse train. Further investigation may reveal if the observed differences in neural responsiveness to pulsatile stimulation, among individuals account for differences in psychophysical measures, including speech perception and whether there may be an "optimal" neural output that could be evoked by an individually "optimized" signal.

The purpose of this study was to investigate how design changes in Cochlear Nucleus cochlear implants (CIs) (CI24M, CI24R, CI24RE and CI422) affected electrode impedance and ECAP measures, and to determine if these design changes affected post-lingually deafened adults and children with congenital hearing loss in a similar way. Results of this study showed that electrode impedance was inversely related to the area of the electrode contacts in the array: lowest for the full-banded CI24M CI and highest for adults who used the CI422 device which has the smallest electrode contacts of all four devices. The noise floor of the NRT system likely plays a significant role in the finding that CI users with older devices (the CI24M, and CI24R CIs) had higher ECAP thresholds than individuals with the CI24RE electrode array. The position of the electrode array in the cochlea was also found to have a significant effect on ECAP measures. CI users with modiolar hugging (the CI24R and CI24RE CIs) electrode arrays were found to have lower ECAP thresholds than CI users whose electrode arrays were seated more laterally in the cochlear duct (e.g. the CI24M and CI422 implants). The position of the electrode contacts relative to the modiolus of the cochlea was found to be related to slope of the ECAP growth functions. The lowest slopes were found in CI24RE users. It also had a significant impact on the width of the channel interaction function. Electrode arrays seated further from the modiolus have significantly more channel interaction than electrode arrays that hug the modiolus of the cochlea. Differences between results recorded from post-lingually deafened adults and children with congenital hearing loss were minimal. The difference only reflected on the ECAP slopes. Slopes in children with congenital hearing loss were significantly steeper than those recorded from adults. This may indicate that children with congenital hearing loss may have better neural survival than adults with acquired hearing loss. In conclusion, the results of the current study show evidence of the effects of variations in design and function of the implanted components of the Nucleus CI. Perhaps the most significant finding from the current data set is that electrode arrays located closer to the modiolus of the cochlea have lower thresholds and exhibit less channel interaction than electrode arrays that are positioned more laterally. An argument could be made that lower stimulation levels and less channel interaction may result in better outcomes and/or longer battery life. For CI candidates who do not have significant residual acoustic hearing, the CI24RE implant might be a better choice than the more recently introduced CI422 electrode array.

[Truncated abstract] The present study investigated whether any of the characteristics of the compound action potential (CAP) waveform or the spectrum of the neural noise (SNN) recorded from the cochlea, could be used to examine abnormal spike generation in the type I primary afferent neurones, possibly due to pathologies leading to abnormal hearing such as tinnitus or tone decay. It was initially hypothesised that the CAP waveform and SNN contained components produced by the local action currents generated at the peripheral ends of the type I primary afferent neurones, and that changes in these local action currents occurred due to changes in the membrane potential of these neurones. It was further hypothesised that the lateral olivo-cochlear system (LOCS) efferent neurones regulate the membrane potential of the primary afferent dendrites to maintain normal action potential generation, where instability in the membrane potential might lead to abnormal primary afferent firing, and possibly one form of tinnitus. We had hoped that the activity of the LOCS efferent neurones could be observed through secondary changes in the CAP waveform and SNN, resulting from changes in the membrane potential of the primary afferent neurones. The origins of the neural activity generating the CAP waveform and SNN peaks, and the effects of the LOCS on the CAP and SNN were experimentally investigated in guinea pigs using lesions in the auditory system, transient ischemia and asphyxia, focal and systemic temperature changes, and pharmacological manipulations of different regions along the auditory pathway. ... Therefore, the CAP and SNN are altered by changes in the propagation of the action potential along the primary afferent neurones, by changes in the morphology of the tissues surrounding the cochlear nerve, and by changes in the time course of the action currents. If the CAP waveform is not altered, the amplitude of the 1kHz speak in the spontaneous SNN can be used as an objective measure of the spontaneous firing rate of the cochlear neurones. However, because the SNN contains a complex mixture of neural activity from all cochlear neurones, and the amplitude of the spontaneous SNN is variable, it would be difficult to use the spontaneous SNN alone as a differential diagnostic test of cochlear nerve pathologies. To record extratympanic electrocochleography (ET ECochG) from humans, a custom-designed, inexpensive, low-noise, optically isolated biological amplifier was built. Furthermore, a custom-designed extratympanic active electrode and ear canal indifferent electrode were designed, which increased the signal-to-noise ratio of the ECochG recording by a factor of 2, decreasing the overall recording time by 75%. The human and guinea pig CAP waveforms recorded in the present study appeared similar, suggesting that the origins of the human and guinea pig CAP waveforms were the same, and that experimental manipulations of the guinea pig CAP waveform can be used to diagnose the cause of abnormal human ECochG waveforms in cases of cochlear nerve pathologies.

<p>Disruption of coordinated impulse propagation in the heart as a result of fibrosis or myocardial infarction can create an asynchronous substrate with poor conduction and impaired contractility. This can ultimately lead to cardiac failure and make the heart more vulnerable to life-threatening arrhythmias and sudden cardiac death. The transplantation of exogenous cells into the diseased myocardium, "cardiac cell therapy," has been proposed as a treatment option to improve compromised cardiac function. Clinical trials of stem cell-based cardiac therapy have shown promising results, but also raised concerns about our inability to predict or control the fate of implanted cells and the electrical consequences of their interactions with host cardiomyocytes. Alternatively, genetically engineered somatic cells could be implanted to selectively and safely modify the cardiac electrical substrate, but their unexcitable nature makes them incapable of electrically repairing large conduction defects. The objective of this thesis was thus to develop a methodology to generate actively conducting excitable cells from an unexcitable somatic cell source and to demonstrate their utility for studies of basic electrophysiology and cardiac cell therapy.</p><p>First, based on the principles of cardiac action potential propagation, we applied genetic engineering techniques to convert human unexcitable cells (HEK-293) into an autonomous source of excitable and conducting cells by the stable forced expression of only three genes encoding an inward rectifier potassium (Kir2.1), a fast sodium (Na_v1.5), and a gap junction (Cx43) channel. Systematic pharmacological and electrical pacing studies in these cells revealed the individual contributions of each expressed channel to action potential shape and propagation speed. Conduction slowing and instability of induced arrhythmic activity was shown to be governed by specific mechanisms of I_Na inhibition by TTX, lidocaine, or flecainide. Furthermore, expression of the Na_v1.5 A1924T mutant sodium channel or Ca_v3.3 T-type calcium channel was utilized to study the specific roles of these channels in action potential conduction and demonstrate that genetic modifications of the engineered excitable cells in this platform allow quantitative correlations between single-cell patch clamp data and tissue-level function.</p><p>We further performed proof-of-concept experiments to show that networks of biosynthetic excitable cells can successfully repair large conduction defects within primary excitable tissue cultures. Specifically, genetically engineered excitable cells supported active action potential propagation between neonatal rat ventricular myocytes (NRVMs) separated by at least 2.5 cm in 2-dimensional and 1.3 cm in 3-dimensional cocultures. Using elastic films with micropatterned zig-zag NRVM networks that mimicked the tortuous conduction patterns observed in cardiac fibrosis, we showed that electrical resynchronization of cardiomyocyte activation by application of engineered excitable cells improved transverse conduction by 370% and increased cardiac twitch force amplitude by 64%. This demonstrated that despite being noncontractile, engineered excitable cells could potentially improve both the electrical and mechanical function of diseased myocardial tissue. </p><p>Lastly, we investigated how activation and repolarization gradients at the interface between cardiomyocytes and other excitable cells influence the vulnerability to conduction block. Microscopic optical mapping of action potential propagation was used to quantify dispersion of repolarization (DOR) in micropatterned heterocellular strands in which either well-coupled or poorly-coupled engineered excitable cells with a short action potential duration (APD), seamlessly interfaced with NRVMs that had a significantly longer APD. The resulting electrical gradients originating from the underlying heterogeneity in intercellular coupling and APD dispersion were further manipulated by the application of barium chloride (BaCl2) to selectively prolong APD in the engineered cells. We measured how the parameters of DOR affected the vulnerable time window (VW) of conduction block and found a strong linear correlation between the size of the repolarization gradient and VW. Reduction of DOR by BaCl2 significantly reduced VW and showed that VW correlated directly with dispersion height but not width. Conversely, at larger DOR, VW was inversely correlated with the dispersion width but independent of the dispersion height. In addition, despite their similar APDs, poorly-coupled excitable cells were found to significantly increase the maximum repolarization gradient and VW compared to well-coupled excitable cells, but only at larger DOR.</p><p>In summary, this thesis presents the novel concept of genetically engineering membrane excitability and impulse conduction in previously unexcitable somatic cells. This biosynthetic excitable cell platform is expected to enable studies of ion channel function in a reproducible tissue-level setting, promote the integration of theoretical and experimental studies of action potential propagation, and stimulate the development of novel gene and cell-based therapies for myocardial infarction and cardiac arrhythmias.</p> / Dissertation

<p>Cardiac arrhythmias triggered by both reentrant and focal sources are closely correlated with regions of tissue characterized by significant structural heterogeneity. Experimental and modeling studies of electrical activity in the heart have shown that local microscopic heterogeneities which average out at the macroscale in healthy tissue play a much more important role in diseased and aging cardiac tissue which have low levels of coupling and abnormal or reduced membrane excitability. However, it is still largely unknown how various combinations of microheterogeneity in the intracellular and interstitial spaces affect wavefront propagation in these critical regimes. </p> <p>This thesis uses biophysically realistic 1-D and 2-D computer models to investigate how heterogeneity in the interstitial and intracellular spaces influence both the initiation of ectopic beats and the escape of multiple ectopic beats from a poorly coupled region of tissue into surrounding well-coupled tissue. An approximate discrete monodomain model that incorporates local heterogeneity in both the interstitial and intracellular spaces was developed to represent the tissue domain. </p> <p>The results showed that increasing the effective interstitial resistivity in poorly coupled fibers alters the distribution of electrical load at the microscale and causes propagation to become more like that observed in continuous fibers. In poorly coupled domains, this nearly continuous state is modulated by cell length and is characterized by decreased gap junction delay, sustained conduction velocity, increased sodium current, reduced maximum upstroke velocity, and increased safety factor. In inhomogeneous fibers with adjacent well-coupled and poorly coupled regions, locally increasing the effective interstitial resistivity in the poorly coupled region reduces the size of the focal source needed to generate an ectopic beat, reduces dispersion of repolarization, and delays the onset of conduction block that is caused by source-load mismatch at the boundary between well-coupled and poorly-coupled regions. In 2-D tissue models, local increases in effective interstitial resistivity as well as microstructural variations in cell arrangement at the boundary between poorly coupled and well-coupled regions of tissue modulate the distribution of maximum sodium current which facilitates the unidirectional escape of focal beats. Variations in the distribution of sodium current as a function of cell length and width lead to directional differences in the response to increased effective interstitial resistivity. Propagation in critical regimes such as the ectopic substrate is very sensitive to source-load interactions and local increases in maximum sodium current caused by microheterogeneity in both intracellular and interstitial structure.</p> / Dissertation

The purpose of this thesis work is to model the gate control theory, which explains the modulation of pain signals, with a motivation of finding new possible targets for pain treatment and to find novel control algorithms that can be used in engineering practice. The difference of the current study from the previous modeling trials is that morphologies of neurons that constitute gate control system are also included in the model by which structure-function relationship can be observed. Model of an excitable neuron is constructed and the response of the model for different perturbations are investigated. The simulation results of the excitable cell model is obtained and when compared with the experimental findings obtained by using crayfish, it is found that they are in good agreement. Model encodes stimulation intensity information as firing frequency and also it can add sub-threshold inputs and fire action potentials as real neurons. Moreover, model is able to predict depolarization block. Absolute refractory period of the single cell model is found as 3.7 ms. The developed model, produces no action potentials when the sodium channels are blocked by tetrodotoxin. Also, frequency and amplitudes of generated action potentials increase when the reversal potential of Na is increased. In addition, propagation of signals along myelinated and unmyelinated fibers is simulated and input current intensity-frequency relationships for both type of fibers are constructed. Myelinated fiber starts to conduct when current input is about 400 pA whereas this minimum threshold value for unmyelinated fiber is around 1100 pA. Propagation velocity in the 1 cm long unmyelinated fiber is found as 0.43 m/s whereas velocity along myelinated fiber with the same length is found to be 64.35 m/s. Developed synapse model exhibits the summation and tetanization properties of real synapses while simulating the time dependency of neurotransmitter concentration in the synaptic cleft. Morphometric analysis of neurons that constitute gate control system are done in order to find electrophysiological properties according to dimensions of the neurons. All of the individual parts of the gate control system are connected and the whole system is simulated. For different connection configurations, results of the simulations predict the observed phenomena for the suppression of pain. If the myelinated fiber is dissected, the projection neuron generates action potentials that would convey to brain and elicit pain. However, if the unmyelinated fiber is dissected, projection neuron remains silent. In this study all of the simulations are preformed using Simulink.