An open question in contemporary neuroscience is how neuromodulators coregulate multiple conductances to maintain functional neuronal activity. Neuromodulators enact changes to properties of biophysical characteristics, such as the maximal conductance or voltage of half-activation of an ionic current, which determine the type and properties of neuronal activity. We apply dynamical systems theory to study the changes to neuronal activity that arise from neuromodulation. Neuromulators can act on multiple targets within a cell. The coregulation of mulitple ionic currents extends the scope of dynamic control on neuronal activity. Different aspects of neuronal activity can be independently controlled by different currents. The coregulation of multiple ionic currents provides precise control over the temporal characteristics of neuronal activity. Compensatory changes in multiple ionic currents could be used to avoid dangerous dynamics or maintain some aspect of neuronal activity. The coregulation of multiple ionic currents can be used as bifurcation control to ensure robust dynamics or expand the range of coexisting regimes. Multiple ionic currents could be involved in increasing the range of dynamic control over neuronal activity. The coregulation of multiple ionic currents in neuromodulation expands the range over which biophysical parameters support functional activity.

For those who have suffered a stroke, damage to the brain can result in a decreased ability to walk. The traditional therapy used for the recovery of walking, body weight supported treadmill training, has significant labour requirements that limit the availability of training to the larger stroke population. Thus, the conception and application of new, effective, and efficient rehabilitation therapies is required. To approach this, an understating of the intricate neural control behind walking is needed to form the principled foundation upon which locomotor therapies are based. Due to observations that the arms and legs are connected in the nervous system during walking, and that nervous system control is the same across rhythmic tasks, arm and leg (A&L) cycling training could provide an effective means of locomotor rehabilitation. Thus, the goal of this dissertation is focused upon exploring central nervous system control and interlimb coordination during rhythmic arm and leg movement and testing the extent to which A&L cycling training improves walking after stroke. The first objective of this dissertation was to provide further evidence of central nervous system control of walking. Through a literature review in Chapter 1 and experimental evidence in Chapter 2 of common subcortical control across rhythmic locomotor tasks, evidence for the existence of central pattern generating networks in humans is given. The second objective was to explore interlimb coordination during rhythmic movement. Results presented in Chapters 3 and 4 further our understanding of specific interlimb interactions during rhythmic arm and leg tasks. The third objective was to evaluate the effects of an A&L cycling training intervention in a post-stroke population. To support this objective, it was shown in Chapter 5 that a multiple baseline design is appropriate for use in intervention studies. In Chapter 6, it was determined that A&L cycling training can be used to improve walking ability. And in Chapter 7, it was shown that training induced plasticity in interlimb reflex pathways. Overall, results in this dissertation provide further knowledge on nervous system control and arm and leg interlimb interactions during rhythmic movements and their effect on locomotor recovery following a stroke. / Graduate / 2017-10-31

In this thesis, we study the neurocomputation by implementing two different neuron models. One is a semi magnetic micro p-n wire that emulates nerve fibres and supports the electrical propagation and regeneration. The other is a silicon neuron based on Hodgkin-Huxley conductance model that can generate spatiotemporal spiking patterns. The former model focuses on the spatial propagation of electrical pulses along a transmission line and presents the thesis that action potentials may be represented by solitary waves. The later model focuses on the dynamical properties such as how the output patterns of the active networks adapt to external stimulus. To demonstrate the dynamical properties of spiking networks, we present a central pattern generator (CPG) network with winnerless competition architecture. The CPG consists of three silicon neurons which are connected via reciprocally inhibitory synapses. The network of three neurons was stimulated with current steps possessing different time delays and that the voltage oscillations of the three neurons were recorded as a function of the strengths of inhibitory synaptic interconnections and internal parameters of neurons, such as voltage thresholds, time delays, etc. The architecture of the network is robust and sensitively depends on the stimulus. Stimulus dependent rhythms can be generated by the CPG network. The stimulus-dependent sequential switching between collective modes of oscillations in the network can explain the fundamental contradiction between sensitivity and robustness to external stimulus and the mechanism of pattern memorization. We successfully apply the CPG in modulating the heart rate of animal models (rats). The CPG was stimulated with respiratory signals and generated tri-phasic patterns corresponding to the respiratory cycles. The tri-phasic stimulus from the CPG was used to synchronize the heart rate with respiration. In this way, we artificially induce the respiratory sinus arrhythmia (RSA), which refers to the heart rate fluctuation in synchrony with respiration. RSA is lost in heart failure. Our CPG paves to way to novel medical devices that can provide a therapy for heart failure.

Movement is controlled by neuronal central pattern generator (CPG) networks that are segmentally organised in organisms across the animal kingdom. The precise role of neuromodulators in the function, development and, particularly, the maintenance of these circuits is currently unresolved. This study investigates the effects of chronically altered signalling of tyramine and/or octopamine, two well established neuromodulators, in Drosophila larval locomotion. It shows that tyramine reduces crawling speed in larvae, whereas octopamine increases speed up to a physiological maximum. Changes in crawling speed are mediated by modulating stride duration, whilst stride length remains constant. These two neuromodulators also affect segmental muscle contraction and relaxation rates, indicative that the effects on crawling speed are likely to be at least partially due to modulatory effects on muscle physiology. Muscle recordings from muscle M6 in two adjacent segments, during fictive forward locomotion show that stride duration is influenced by a variable time delay between segmental CPG outputs. Frequency and duration of individual segmental outputs, by contrast, remains constant. The behavioural and electrophysiological data suggest, therefore, that the segmental locomotor CPG outputs remain constant in response to chronically altered neuromodulatory signalling. This study also identified a close spatial proximity of motor neuronal dendritic branches and putatively octopaminergic and/or tyraminergic synaptic terminal varicosities in the ventral nerve cord (VNC) neuropil. Moreover, manipulation of a putatively octopaminergic and/or tyraminergic subpopulation of interneurons, located in anterior brain regions, is sufficient to induce a similar, albeit smaller, larval crawling deficit. This indicates that the effects of locomotion may be induced in the central nervous system. This is confirmed in identified motor neurons as chronic changes in octopaminergic and/or tyraminergic signalling increase the frequency of bursting of action potential firing. In addition, the synaptic current amplitudes are substantially reduced in both ventral and dorsal muscle- innervating motor neurons, indicative of an effect to presynaptic excitation. In contrast, the function of neuromuscular junction remains largely unchanged. Taken together, this data shows that neuromodulation is sufficient to alter the output of a relatively small group of neurons, that comprise the locomotor CPG. The site of action of these modulators is, however, likely to be diverse.

We constructed a model of the coordination of segmental heart motor neurons driving blood circulation in leeches. The heart motor neuron models were conductance-based; conductances of voltage-gated and synaptic currents were adjusted to match the firing pattern of heart motor neurons from the living system. Each motor neuron receives a specific pattern of inhibitory input from rhythmic premotor heart interneurons and translates this spatiotemporal pattern into the fictive heartbeat motor pattern. The temporal pattern of synaptic input to the model was derived from extracellularly recorded spikes of the premotor heart interneurons. We focused on determining the components necessary to produce side-to-side asymmetry in the motor pattern: motor neurons on one side fire nearly in synchrony (synchronous coordination), while on the other they fire in a rear-to-front progression (peristaltic coordination). The model reproduces the general trends in phasing and was used to investigate the effective contribution of several synaptic and cellular properties of the motor neurons. The spatial and temporal pattern of premotor synaptic input, the electrical coupling between the segmental motor neurons, intra-burst, short-term synaptic plasticity of the synaptic inputs, and the axonal conduction delays all were integrated with the intrinsic membrane properties to influence intersegmental phasing.

Spreading depression (SD) is an interesting and important phenomenon due to its role in mammalian pathologies such as migraine, seizures, and stroke. Until recently investigations of the mechanisms involved in SD have mostly utilized mammalian cortical tissue, however in my thesis I demonstrated that SD-like events occur in the CNS of an invertebrate model, Locusta migratoria. Locusts enter comas in response to stress during which neural and muscular systems shut down until the stress is removed, and this is believed to be an adaptive strategy to survive extreme environmental conditions. Using the ventilatory central pattern generator (vCPG) as a model circuit I was able to show that stress-induced arrest of vCPG function is associated with SD-like events in the locust metathoracic ganglion (MTG) that closely resemble cortical SD (CSD) in many respects, including mechanism of induction, extracellular potassium ion ([K+]o) changes, and propagation in areas equivalent to mammalian grey matter. SD-like events in the locust were characterized as abrupt [K+]o increases associated with electrical activity silence in the locust CNS that propagate to other areas within the MTG. In this thesis I described the generation of comas by several cellular stressors (hyperthermia, metabolic stressors, Na+/K+-ATPase inhibition, and KCl) and the associated SD-like events in the locust, provide a description of the similarities to CSD, and show how they can be manipulated both by stress preconditioning and pharmacologically. I showed that hyperthermic vCPG arrest can be preconditioned by prior heat shock (HS) treatment and induced-thermotolerance was associated with an increased rate of [K+]o clearance associated with vCPG recovery that was not linked to changes in ATP levels or total Na+/K+-ATPase activity. I also provided evidence for the involvement of the stress-sensor AMP-activated protein kinase (AMPK) in stress-induced comas in the locust. AMPK activation was linked to a switch in motor pattern behavior following recovery from anoxia-induced vCPG arrest and exacerbated repetitive SD-like events induced by ouabain (Na+/K+-ATPase inhibitor). I suggested that locust SD-like events are adaptive by conserving energy and preventing cellular damage, and I provided a model for the mechanism of SD onset and recovery in the locust nervous system. / Thesis (Ph.D, Biology) -- Queen's University, 2010-08-05 16:08:19.905

In this thesis we model two very different movement-related neuronal circuits, both of which produce oscillatory patterns of activity. In one case we study oscillatory activity in the basal ganglia under both normal and Parkinsonian conditions. First, we used a detailed Hodgkin-Huxley type spiking model to investigate the activity patterns that arise when oscillatory cortical input is transmitted to the globus pallidus via the subthalamic nucleus. Our model reproduced a result from rodent studies which shows that two anti-phase oscillatory groups of pallidal neurons appear under Parkinsonian conditions. Secondly, we used a population model of the basal ganglia to study whether oscillations could be locally generated. The basal ganglia are thought to be organised into multiple parallel channels. In our model, isolated channels could not generate oscillations, but if the lateral inhibition between channels is sufficiently strong then the network can act as a rhythm-generating ``pacemaker'' circuit. This was particularly true when we used a set of connection strength parameters that represent the basal ganglia under Parkinsonian conditions. Since many things are not known about the anatomy and electrophysiology of the basal ganglia, we also studied oscillatory activity in another, much simpler, movement-related neuronal system: the spinal cord of the Xenopus tadpole. We built a computational model of the spinal cord containing approximately 1,500 biologically realistic Hodgkin-Huxley neurons, with synaptic connectivity derived from a computational model of axon growth. The model produced physiological swimming behaviour and was used to investigate which aspects of axon growth and neuron dynamics are behaviourally important. We found that the oscillatory attractor associated with swimming was remarkably stable, which suggests that, surprisingly, many features of axonal growth and synapse formation are not necessary for swimming to emerge. We also studied how the same spinal cord network can generate a different oscillatory pattern in which neurons on both sides of the body fire synchronously. Our results here suggest that under normal conditions the synchronous state is unstable or weakly stable, but that even small increases in spike transmission delays act to stabilise it. Finally, we found that although the basal ganglia and the tadpole spinal cord are very different systems, the underlying mechanism by which they can produce oscillations may be remarkably similar. Insights from the tadpole model allow us to predict how the basal ganglia model may be capable of producing multiple patterns of oscillatory activity.

In this dissertation, we describe the development of a humanoid bipedal robot that fully physically models the human walking system, including the biomechanics of the leg, the sensory feedback pathways available in the body, and the neural structure of the central pattern generator (CPG). Using two different models of the CPG, we explore several issues in the neurobiology and robotics literature, including the role of reflexes in locomotion, the role of load reception and positive force feedback in generating the gait, and the degree to which central or peripheral control plays in human walking. We show that the walking pattern can be generated by a combination of a half-center CPG and reflex interactions phase modulated by the CPG, and that load receptors in the muscles can play a substantial role in generating the gait, using positive force feedback. We compare the gait of the robot to human subjects and show that this architecture produces human-like stepping. Varying the degree of direct central control of lower limb muscles by the CPG, we show that the most human-like gait is generated with a relatively weak central control signal, which modulates reflex responses that generate most of the muscle activation. These results allow us to conceive of locomotion as a series of nested loops, with a central CPG or rhythm generator modulating lower level reflex interactions, while higher centers modulate the CPG. Since locomotion is a primary mechanism by which animals interact with the world, this research is relevant to artificial intelligence researchers. Recent understanding of cognition holds that minds are embodied, situated relative to a set of goals, and exist in a feedback loop of interaction with the environment. In our robot, we model the dynamics of the body, the neural architecture and the sensory feedback channels in a complete dynamical feedback loop, and show that the robot entrains to the the natural dynamics of the world. We propose the concept of nested loops with descending phase modulation as a conceptual paradigm for a more general understanding of nervous system organization.

In the spinal cord, an intricate neural network generates and coordinates the patterning of limb movements during locomotion. This network, known as the locomotor central pattern generator (CPG), comprises of various cell populations that together orchestrate the output of motor neurons. Identification of CPG neurons through their specific gene expression is a valuable tool that can provide considerable insight to the character, intrinsic properties and role of a population, which represents a step toward understanding locomotor circuit function and correlating neural activity to behaviour. We selectively targeted two inhibitory CPG populations to investigate their molecular characteristics, circuitry and functional role; Renshaw cells (RCs) marked by their specific expression of the cholinergic nicotinic receptor α2 (Chrna2) and a subset of the dI6 population derived by their selective expression of the Doublesex and mab-3 related transcription factor 3 (Dmrt3). We found that RCs have hyperpolarisation-activated cation (Ih) and small calcium-activated potassium (ISK) modulatory currents that differentially regulate their excitation and firing properties, which influence the instantaneous feedback to motor neurons through the recurrent inhibition circuit. Due to previous difficulties isolating RCs from the surrounding locomotor circuits, their functional role remains poorly defined. For the first time, we selectively silenced RC inhibition and found that all aspects of motor behaviour, including coordination and gait were normal. The deletion of RC signalling instead altered the electrical and synaptic properties of the recurrent inhibitory circuit, suggesting that developmental plasticity compensates for the loss of RC inhibition. We reveal Dmrt3 neurons comprise a population of glycinergic inhibitory, spike-frequency adapting commissural interneurons active during locomotion. Conditional silencing of the Dmrt3 population resulted in considerable gait abnormalities in the neonatal and adult mouse. This manifested as an uncoordinated CPG output in vitro, impaired limb coordination in pups and increased fore- and hindlimb synchrony in adults that was exacerbated at faster locomotor speeds. Dmrt3 mediated inhibition subsequently impacts locomotion and suggests the Dmrt3 population contribute to coordinating speed dependent left-right limb alternation. This thesis provides cellular, circuit and behavioural insights into the Renshaw cell and Dmrt3 populations and enhances our knowledge regarding their probable function within the locomotor CPG.

Locomotion is an innate behaviour that is controlled by different areas of the central nervous system, which allow for effectiveness of movement. The spinal cord is an important centre involved in the generation and maintenance of rhythmic patterns of locomotor activity such as walking and running. Interneurons throughout the ventral horn of the spinal cord form the locomotor central pattern generator (CPG) circuit, which produces rhythmic activity responsible for hindlimb movement. Motoneurons within the lumbar region of the spinal cord innervate the leg muscles to convey rhythmic CPG output to drive appropriate muscle contractions. Intrinsic modulators, such as acetylcholine acting via M2 and M3 muscarinic receptors, regulate CPG circuitry to allow for flexibility of motor output. Using electrophysiology and genetic techniques, this work characterized the receptors involved in cholinergic modulation of locomotor networks and the role and mechanism of action of a subpopulation of genetically identified cholinergic interneurons in the lumbar region of the neonatal mouse spinal cord. Firstly, the effects of M2 and M3 muscarinic receptors on the output of the lumbar locomotor network were characterised. Experiments in which fictive locomotor output was recorded from the ventral roots of isolated spinal cord preparations revealed that M3 muscarinic receptors are important in stabilizing the locomotor rhythm while M2 muscarinic receptor activation seems to increase the irregularity of the locomotor frequency whilst increasing the strength of the motor output. This work then explored the cellular mechanisms through which M2 and M3 muscarinic receptors modulate motoneuron output. M2 and M3 receptor activation exhibited contrasting effects on motoneuron function suggesting that there is a fine balance between the activation of these two receptor subtypes. M2 receptor activation induces an outward current and decreases synaptic drive to motoneurons while M3 receptors are responsible for an inward current and increase in synaptic inputs to motoneurons. Despite the different effects of M2 and M3 receptor activation on synaptic drive and subthreshold properties of MNs, both M2 and M3 receptors are required for muscarine-induced increase in motoneuron output. CPG networks therefore appear to be subject to balanced cholinergic modulation mediated by M2 and M3 receptors, with the M2 subtype also being important for regulating the intensity of motor output. Next, using Designer Receptor Exclusively Activated by Designer Drug (DREADD) technology, the impact of the activation or inhibition of a genetically identified group of cholinergic spinal interneurons that express the Paired-like homeodomain 2 (Pitx2) transcription factor was explored. Stimulation of these interneurons increased motoneuron output through the activation of M2 muscarinic receptors and subsequent modulation of Kv2.1 channels. Inhibition of Pitx2+ interneurons during fictive locomotion decreased the amplitude of locomotor bursting. Genetic ablation of these cells confirmed that Pitx2+ interneurons increase the strength of locomotor output by activating M2 muscarinic receptors. Overall, this work provides new insights into the receptors and mechanisms involved in intraspinal cholinergic modulation. Furthermore, this study provides direct evidence of the mechanism through which Pitx2+ interneurons regulate motor output. This work is not only important for advancing understanding of locomotor networks that control hindlimb locomotion, but also for dysfunction and diseases where the cholinergic system is impaired such as Spinal Cord Injury and Amyotrophic Lateral Sclerosis.