Using primary afferent neural activity for predicting limb kinematics in cat

Kinematic state feedback is important for neuroprostheses to generate stable and adaptive movements of an extremity. State information, represented in the firing rates of populations of primary afferent neurons, can be recorded at the level of the dorsal root ganglia (DRG). Previous work in cats showed the feasibility of using DRG recordings to predict the kinematic state of the hind limb using reverse regression. Although accurate decoding results were attained, these methods did not make efficient use of the information embedded in the firing rates of the neural population. This dissertation proposes new methods for decoding limb kinematics from primary afferent firing rates. We present decoding results based on state-space modeling, and show that it is a more principled and more efficient method for decoding the firing rates in an ensemble of primary afferent neurons. In particular, we show that we can extract confounded information from neurons that respond to multiple kinematic parameters, and that including velocity components in the firing rate models significantly increases the accuracy of the decoded trajectory. This thesis further explores the feasibility of decoding primary afferent firing rates in the presence of stimulation artifact generated during functional electrical stimulation. We show that kinematic information extracted from the firing rates of primary afferent neurons can be used in a 'real-time' application as a feedback for control of FES in a neuroprostheses. It provides methods for decoding primary afferent neurons and sets a foundation for further development of closed loop FES control of paralyzed extremities. Although a complete closed loop neuroprosthesis for natural behavior seems far away, the premise of this work argues that an interface at the dorsal root ganglia should be considered as a viable option

Neuroprosthetic rehabilitation and translational mechanism after severe spinal cord injury

Traumatic SCIs have long-term health, economic and social consequences, stressing the urgency to develop interventions to improve recovery after such injuries. Today, the only proven effective interventions to enhance recovery after SCI are activity-based rehabilitation therapies, such as locomotor training. However, locomotor training shows no or very limited efficacy to improve function after a severe SCI that induces paralysis of the limbs. To mimic the outcome of severe but incomplete SCI in rodents, we developed a model of double opposite-side lateral hemisections termed staggered hemisection in adult rats. This model induced permanent paralysis below the level of injury but leaves an intervening gap of intact neural tissue that provides a substrate for recovery. We showed that this SCI leads to degradation of motor functions, which correlates with the formation of aberrant neuronal connections below the lesion. Robotic devices with a rehabilitative purpose should act as propulsive or postural neuroprosthesis allowing training under natural conditions. Our versatile robotic interface provides multidirectional bodyweight support during overground locomotion in rats. We next evaluated the effects of robot-assisted gait training enabled by electrochemical stimulation of spinal circuits to restore locomotion after staggered hemisection SCI. We found that after two months of daily training, paralyzed rats recovered the ability to initiate, sustain and adjust bipedal locomotion while supported in the robot under electrochemical stimulation. This recovery correlated with ubiquitous reorganization of corticospinal, brainstem, and intraspinal fibers. We next evaluated whether this treatment was capable of restoring supraspinal control of locomotion after a clinically relevant SCI. Rats received a severe contusion of the spinal cord that spared less than 10% of intact tissue. Robot-assisted rehabilitation restored weight-bearing locomotion in all the trained rats when stimulated electrochemicallay and in a subset of rats in the absence of any enabling factors which paralelled with the reorganization of axonal projections of reticulospinal fibers below the contusion. Virus-mediated silencing of reticulospinal neurons projecting to lumbar segments demonstrated that these inputs were necessary to initiate and sustain walking after training. When delaying the onset of training by two months, in the chronic stage, all the rats regained voluntary locomotor movements but the extent of the recovery was reduced compared to rats trained early after SCI. The results provide a strong rationale to evaluate the impact of neuroprosthetic training to improve motor functions in human patients with incomplete SCI. Translation of treatment paradigms developed in rodent models into effective clinical applications remains a major challenge in biomedical research. Here, we studied recovery of motor functions in more than 400 quadriplegic patients who presented various degree of spinal cord damage laterality. We found that recovery increases with the asymmetry of early motor deficits. We conclude that emergence of spinal cord decussating corticospinal fibers and bilateral motor cortex projections during mammalian evolution supports greater recovery after lateralized SCI primates compared to rodents. Novel experimental models and dedicated therapeutic strategies are necessary to take advantage of this powerful neuronal substrate for recovery after SCI

Identification of the optimal parameters for electrical stimulation to generate locomotor patterns in the rat isolated spinal cord

Recently, an innovative protocol of electrical stimulation, named \u201cfictive locomotion induced stimulation\u201d (FListim), which consists of an intrinsically variable noisy waveform, has been obtained from a segment of chemically-induced fictive locomotion (FL) sampled from the ventral root (VR) of an in vitro preparation of neonatal rat spinal cord. FListim delivered at sub-threshold intensities to a dorsal root (DR) has been shown to optimally activate the central pattern generators (CPGs) for locomotion (Taccola, 2011). In an attempt to introduce novel and improved protocols of stimulation in combination with neurochemicals, the current PhD project aims to identify the features that make sub-threshold noisy waveforms effective in activating locomotor patterns. In an attempt to introduce novel and improved protocols of stimulation in combination with neurochemicals, the current PhD project aims to identify the features that make sub-threshold noisy waveforms effective in activating locomotor patterns. To reach this aim, locomotor-like patterns in response to different noisy waveforms were compared. In order to obtain a wide palette of noisy protocols electromyographic (EMG) recordings were performed from leg muscles of adult volunteers during walking. These recordings were then delivered as stimulating patterns called real locomotion-induced stimulation (ReaListim). To reach this aim, locomotor-like patterns in response to different noisy waveforms were compared. In order to obtain a wide palette of noisy protocols electromyographic (EMG) recordings were performed from leg muscles of adult volunteers during walking. These recordings were then delivered as stimulating patterns called real locomotion-induced stimulation (ReaListim). ReaListim protocols, sampled during different motor behaviours, are equally able to induce an epoch of locomotor-like oscillations. Conversely, smooth kinematic profiles and non-phasic noisy patterns such as standing and isometric contraction, are unable to activate the locomotor CPGs. The complexity of noisy waveforms was then reduced at motoneuronal level, by recording electrical activity of a single motoneuron during FL. Long-lasting episode of FL, were evoked in response to intracellular patterns delivered at sub-threshold intensities. The analysis of motoneuronal firing during FL was used to identify four recurrent frequency values that optimally activated the locomotor CPGs when applied simultaneously in a multifrequency protocol. Different permutations were tried to further simplify the multifrequency protocol while isolating the most effective components of the four identified frequencies. The simplest asynchronous paradigm that can induce locomotor-like episodes consists of a train of rectangular pulses that contain two frequencies: 35 and 172 Hz. This protocol resulted already effective at subthreshold intensity even when delivered for a very short time (500 ms). The role of oxytocin in the modulation of neuronal networks is explored here on spinal networks. Intracellular recordings demonstrate that oxytocin dosedependently depolarizes single motoneurons with the appearance of sporadic bursts with superimposed firing. By applying the selective blocker of sodium channels, tetrodotoxin (TTX), the effects of oxytocin can be completely abolished, which suggest a premotoneuronal-level origin. The neuropeptide is capable to induce VRs depolarization with superimposed synchronous bursts of activity, while reflex responses induced by single pulses are depressed depending on the stimulus strength and peptide-concentration. The disinhibited bursting evoked by the pharmacological blockade of glycine and GABAA receptors blockers, strychnine and bicuculline, respectively, is accelerated by oxytocin, an effect that is suppressed by the selective oxytocin receptor antagonist atosiban. On spinal locomotor networks oxytocin facilitates the emergence of FL episodes in response either to weak noisy waveforms protocols or to the conjoint application of NMDA and 5HT at sub-threshold concentrations, even if the periodicity of a stable FL is not significantly affected by the neuropeptide. Interestingly, the facilitation of the locomotor CPGs by oxytocin is dependent on the endogenous release of 5HT, as is demonstrated by incubation with the inhibitor of 5HT synthesis, pchlorophenilalanine (PCPA). Low-frequency trains of stereotyped pulses (0.33 and 0.67Hz) delivered with a controlled time interval (delays 0.5 to 2 s) to multiple DRs converged on spinal locomotor circuits to generate locomotor rhythm. The same finding is confirmed by the phase resetting that is induced by single afferent stimuli during a simultaneous train of pulses delivered to another DR. Staggered protocols fail to elicit FL when simultaneously applied to multiple DRs, while a multi-site randomized pulse train is still effective in eliciting locomotor-like patterns. This thesis outlines new strategies for optimizing the reactivation of spinal locomotor networks after spinal damage. Though the technology that is currently available in clinics does not allow for the delivery of highly-variable stimulating patterns, experiments reported here indicate a way to overcome these limitations. Indeed, protocols that contain few distinct frequencies that are isolated from the spectrum of noisy waves can activate the CPGs even when delivered with a multisite approach. This suggests that it may be possible to separately supply multiple trains of pulses to several cord sites using different electrostimulators. The yield of stimulation in activating locomotor circuits will be further improved by the association with the neuropeptide oxytocin