Title of diploma thesis: Tactile discrimination and excitability of alpha motoneurons Objectives: The aim of this thesis is to detect whether tactile discrimination tasks affect the excitability of the alpha motoneurons. Methods: Seven volunteers aged between 20 and 26 years participated in this study. The H reflex, (M wave) were recorded during three control and three experimental conditions. The control conditions preceded each experimental condition. By stimulating the tibialis nerve in the popliteal fossa the H reflex was elicited and its amplitude and latency measured at rest (control) and during tactile discrimination tasks (experimental). As tactile discrimination tasks, three separate tasks were chosen-tactile stimulation, escape reaction to tactile stimulation, and two-point discrimination. We used an EMG stimulator with a constant voltage output and monophasic squared pulses, with a 0,5 ms interval. The stimulation was switched on manually every 3-5 seconds. To detect the electrical potential of the soleus muscle, we used a surface EMG device, a GrassTelefactor, with galvanic isolation complying with EU standards. The parameters measured were the latency and amplitude of the H reflex and M wave during the tactile discrimination tasks and these were then compared to the values at rest. The...

La sclérose latérale amyotrophique (SLA) est une maladie neurodégénérative fatale de l'âge adulte, caractérisée par une perte de motoneurones, conduisant à une atrophie et une faiblesse musculaires. Des mutations de la superoxyde dismutase-1 (SOD1) provoquent une forme génétique de SLA. Comme chez les patients atteints de SLA, le modèle animal de SLA, SOD1 mutant, révèle que tous les motoneurones sont inégalement sensibles à l'évolution de la maladie. Les mitochondries, centrales énergétiques des cellules, sont des organelles précocement touchées dans la pathologie de la SLA. Un mécanisme attrayant qui sous-tend la susceptibilité différentielle est la nécessité bioénergétique variable de sous-ensembles distincts de motoneurones. Cela implique que dans le système nerveux central, la demande bioénergétique pourrait moduler le seuil pathologique. Même en l'absence de perte bioénergétique, on peut imaginer une situation dans laquelle une contrainte pathologique modifie le niveau à partir duquel la production ou la livraison de l'ATP devient insuffisant, précipitant la chute des neurones les plus vulnérables. Dans les neurones, la majorité de l'ATP est produite par les mitochondries et l'homéostasie des gradients d'ions est le procédé le plus énergivore. La fonction mitochondriale est moindre pour modifier les propriétés électriques des motoneurones si la disponibilité en ATP devient insuffisante pour permettre aux pompes ioniques de maintenir des gradients appropriés. Nous avons démontré que la concentration intracellulaire basale d’ATP dans des cultures de neurones moteurs est diminuée dans les cellules mutées SOD1 par rapport au type sauvage. Paradoxalement à ce résultat, le taux de consommation d'oxygène des mitochondries est augmenté dans les motoneurones SOD1m et il n'existe aucune preuve d'une augmentation de la consommation. Nos résultats appuient l'hypothèse intéressante qu'il y a un découplage entre la chaîne respiratoire et la production d'ATP. Ce découplage peut être utilisé comme une stratégie pour minimiser les propriétés toxiques des mitochondries hyper stimulées. / Amyotrophic lateral sclerosis (ALS) is a fatal adult-onset neurodegenerative disorder characterized by a loss of motor neurons, leading to muscle wasting and weakness. Mutations in superoxide dismutase-1 (SOD1) cause a form of ALS. As in ALS patients, the mutant SOD1 animal model of ALS reveals that not all motor neurons are equally susceptible to the disease process. An attractive mechanism underlying differential susceptibility is the variable bioenergetics need of distinct subsets of motor neurons. This implies that within the CNS, bioenergetics can modulate the pathological threshold. Even in the absence of loss in bioenergetics, one can envision a situation in which a pathological stress alters the level at which either the production or delivery of ATP becomes insufficient, precipitating the demise of the most vulnerable neuron types. In neurons, majority of ATP is produced by mitochondria and the homeostasis of ion gradients is the most energy-consuming process. Reduced mitochondrial function will modify the electrical properties of motor neurons if ATP availability becomes insufficient to allow ion pumps to maintain appropriate gradients. We demonstrated that the basal ATP intra-cellular concentration in motor neuron cultures lower in SOD1 mutated cells compared to wild type. Paradoxically to this result, the oxygen consumption rate of mitochondria is increase in mSOD1 cells and there is no evidence for an increase of consumption. Our results support the interesting hypothesis that there is an uncoupling between the respiratory chain and the ATP production. This uncoupling might be used as a strategy to minor the toxic properties of hyper stimulated mitochondrion.

A precisão de uma ação motora depende de vários fatores, como: 1) grau de variabilidade da força gerada por cada músculo envolvido, 2) velocidade de geração da força, 3) coordenação das ativações dos músculos. A geração e o controle da força muscular possuem mecanismos que ainda precisam ser mais bem estabelecidos, tanto para o aprimoramento das teorias de controle motor, quanto para o desenvolvimento de técnicas que permitam a prevenção ou a compensação de certas deficiências. A perda de desempenho motor pode ser decorrente de doenças que afetam o sistema neuromuscular ou de alterações associadas ao envelhecimento. Sabe-se, por exemplo, que idosos podem possuir maior variabilidade e menor velocidade de desenvolvimento da força, quando comparados com jovens. Uma das formas de se entender os mecanismos responsáveis pelos fenômenos observados em experimentos neurofisiológicos, em indivíduos saudáveis, em pacientes ou em idosos, é por meio de uma representação adequada de tais mecanismos em modelos matemáticos. Tais modelos podem, pela escolha de um conjunto de parâmetros e de sinais de entrada, ser simulados, explorando-se toda gama de cenários plausíveis para a geração de um determinado fenômeno, tendo como referência os dados obtidos experimentalmente. Resumidamente, o presente trabalho trata do estudo do sistema neuromuscular por modelagem matemática e simulação computacional, com particular interesse nos músculos do tríceps sural e no primeiro interósseo dorsal (um músculo intrínseco da mão), sendo estes músculos amplamente utilizados em estudos experimentais e de modelagem. Maior enfoque é dado em contrações isométricas (i.e., ângulo articular mantido fixo), avaliando-se a organização do núcleo motor, em termos anatômicos e fisiológicos, recebendo como entrada a atividade sináptica das vias pré-motoneuronais, e estudando como diferentes arranjos das propriedades neurais podem resultar em características encontradas experimentalmente para a força muscular. Inicialmente foi feita uma ampla expansão de um simulador existente (ReMoto), tanto em aspectos de modelagem quanto de interface. Em seguida, este modelo expandido foi empregado para um estudo da influência do grau de rigidez muscular nas respostas reflexas do tornozelo. Posteriormente, um novo modelo de pool de motoneurônios, com ampla representação de características biofísicas, foi desenvolvido e, por fim, este modelo foi utilizado, em conjunto com modelos de tratos descendentes e da geração de força muscular, para representar a geração de forças isométricas em jovens e idosos. / The precision of a motor action depends on several factors such as: 1) the level of force variability of each involved muscle, 2) the rate of force development, 3) the coordination of the activations of the muscles involved. Several mechanisms underlying the force generation in a muscle and its control by the nervous system remain to be fully comprehended. An appropriate description of these mechanisms would allow an improvement in motor control theories and could contribute to the development of techniques for the prevention or compensation of some disabilities. Losses in motor performance may be caused by diseases affecting the neuromuscular system or due to aging processes. For instance, old adults may exhibit higher force variability and lower velocity of force development than young adults. Proper representations of such mechanisms in mathematical models constitute a promising way to test hypotheses raised by neurophysiological experiments to explain an observed phenomenon. These models can be used to investigate aspects of health/disease or young/old subjects and, by varying their parameter sets, it is possible to explore a broad range of plausible scenarios under which the experimentally observed phenomena are replicated. This project deals with the study of the neuromuscular system by mathematical modeling and computer simulations, applied to the triceps surae and the first dorsal interosseus (two of the most experimentally and theoretically studied muscles). The principal focus is on isometric contractions (i.e., fixed joint angle) and the study of the organization of the motor nucleus (anatomical and physiological aspects) receiving inputs from premotoneuronal pathways. The study analyzes how different patterns of organization result in experimentally observed aspects of muscle force. Initially, an existing simulator of the neuromuscular system (ReMoto) was broadly extended to include new models and a friendly interface. The extended model was used to investigate the influence of muscle stiffness on the reflex responses in the ankle joint. Next, a new motoneuron pool mathematical model was developed based on known biophysics. Finally, this model was integrated with models of pre-motoneuronal neurons estabilishing synapses with motoneurons and of muscle force generation in order to represent isometric force generation in young and old adults.

A precisão de uma ação motora depende de vários fatores, como: 1) grau de variabilidade da força gerada por cada músculo envolvido, 2) velocidade de geração da força, 3) coordenação das ativações dos músculos. A geração e o controle da força muscular possuem mecanismos que ainda precisam ser mais bem estabelecidos, tanto para o aprimoramento das teorias de controle motor, quanto para o desenvolvimento de técnicas que permitam a prevenção ou a compensação de certas deficiências. A perda de desempenho motor pode ser decorrente de doenças que afetam o sistema neuromuscular ou de alterações associadas ao envelhecimento. Sabe-se, por exemplo, que idosos podem possuir maior variabilidade e menor velocidade de desenvolvimento da força, quando comparados com jovens. Uma das formas de se entender os mecanismos responsáveis pelos fenômenos observados em experimentos neurofisiológicos, em indivíduos saudáveis, em pacientes ou em idosos, é por meio de uma representação adequada de tais mecanismos em modelos matemáticos. Tais modelos podem, pela escolha de um conjunto de parâmetros e de sinais de entrada, ser simulados, explorando-se toda gama de cenários plausíveis para a geração de um determinado fenômeno, tendo como referência os dados obtidos experimentalmente. Resumidamente, o presente trabalho trata do estudo do sistema neuromuscular por modelagem matemática e simulação computacional, com particular interesse nos músculos do tríceps sural e no primeiro interósseo dorsal (um músculo intrínseco da mão), sendo estes músculos amplamente utilizados em estudos experimentais e de modelagem. Maior enfoque é dado em contrações isométricas (i.e., ângulo articular mantido fixo), avaliando-se a organização do núcleo motor, em termos anatômicos e fisiológicos, recebendo como entrada a atividade sináptica das vias pré-motoneuronais, e estudando como diferentes arranjos das propriedades neurais podem resultar em características encontradas experimentalmente para a força muscular. Inicialmente foi feita uma ampla expansão de um simulador existente (ReMoto), tanto em aspectos de modelagem quanto de interface. Em seguida, este modelo expandido foi empregado para um estudo da influência do grau de rigidez muscular nas respostas reflexas do tornozelo. Posteriormente, um novo modelo de pool de motoneurônios, com ampla representação de características biofísicas, foi desenvolvido e, por fim, este modelo foi utilizado, em conjunto com modelos de tratos descendentes e da geração de força muscular, para representar a geração de forças isométricas em jovens e idosos. / The precision of a motor action depends on several factors such as: 1) the level of force variability of each involved muscle, 2) the rate of force development, 3) the coordination of the activations of the muscles involved. Several mechanisms underlying the force generation in a muscle and its control by the nervous system remain to be fully comprehended. An appropriate description of these mechanisms would allow an improvement in motor control theories and could contribute to the development of techniques for the prevention or compensation of some disabilities. Losses in motor performance may be caused by diseases affecting the neuromuscular system or due to aging processes. For instance, old adults may exhibit higher force variability and lower velocity of force development than young adults. Proper representations of such mechanisms in mathematical models constitute a promising way to test hypotheses raised by neurophysiological experiments to explain an observed phenomenon. These models can be used to investigate aspects of health/disease or young/old subjects and, by varying their parameter sets, it is possible to explore a broad range of plausible scenarios under which the experimentally observed phenomena are replicated. This project deals with the study of the neuromuscular system by mathematical modeling and computer simulations, applied to the triceps surae and the first dorsal interosseus (two of the most experimentally and theoretically studied muscles). The principal focus is on isometric contractions (i.e., fixed joint angle) and the study of the organization of the motor nucleus (anatomical and physiological aspects) receiving inputs from premotoneuronal pathways. The study analyzes how different patterns of organization result in experimentally observed aspects of muscle force. Initially, an existing simulator of the neuromuscular system (ReMoto) was broadly extended to include new models and a friendly interface. The extended model was used to investigate the influence of muscle stiffness on the reflex responses in the ankle joint. Next, a new motoneuron pool mathematical model was developed based on known biophysics. Finally, this model was integrated with models of pre-motoneuronal neurons estabilishing synapses with motoneurons and of muscle force generation in order to represent isometric force generation in young and old adults.

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.

Motoneuronai – tai nervinės ląstelės tiesiogiai valdančios raumenis. Motoneuronuose, kaip ir kitose neuronuose, įėjimo transformacija į išėjimą charakterizuojama perdavimo funkcija, kuri dažniausiai aprašoma tam tikro statumo tiesine priklausomybe. Didelis perdavimo funkcijos statumas leidžia išvystyti didelę raumens susitraukimo jėgą, o mažas – įgalina tikslų raumenų valdymą. Perdavimo funkcijos charakteristikas apsprendžia neurono membranoje esantys joniniai kanalai. Pavyzdžiui, veikimo potencialų adaptaciją sukeliantys joniniai kanalai perdavimo funkcijos statumą mažina. Be to, neuroninio tinklo veikimo metu išskirti neurotransmiteriai gali veikti joninius kanalus ir pritaikyti perdavimo funkciją konkretaus judesio vykdymui. Šio darbo tikslas buvo įvertinti nugaros smegenų motoneuronų perdavimo funkcijos ypatybes ir ištirti jos galimus modifikavimo mechanizmus. Tyrimams naudoti vėžlio nugaros smegenų motoneuronai. Disertacijoje parodėme, kad perdavimo funkcijos statumas įvertintas trikampiais srovės impulsais sutampa su stacionariu perdavimo funkcijos statumu, įvertintu stimuliuojant motoneuronus stačiakampiais srovės impulsais. Nustatėme, kad farmakologiškai padidintas motoneuronų membranos laidumas neįtakoja perdavimo funkcijos statumo. Taip pat parodėme, kad nuolatinė Na+ srovė sumažina pradinį ir ankstyvąjį perdavimo funkcijų statumus. / Motoneurons are the spinal neurons that directly control the muscle contraction. The gain characterizes how the synaptic input to motoneuron is converted in to action potential firing and subsequent muscle contraction. The high gain allows a high force and fast contraction, while the low gain is essential for a fine control of movements. The gain of motoneurons is mainly determined by a set of ion channels in membrane and therefore is a subject for modification. It is known, that the gain decreases during adaptation of action potential firing. Moreover, the neurotransmitters released during spinal network activity may modify the ion channel activity and therefore adjust the gain to the functional needs. The aim of this study was to evaluate the gain of spinal cord motoneurons and investigate mechanisms of its modification. Spinal motoneurons from adult turtle were used. We found that the gain of motoneurons estimated from triangular current ramps is the same as steady one obtained from square current steps. Pharmacologically increased conductance of motoneuron membrane does not change the gain. Finally, we demonstrated that persistent inward Na+ current increases excitability and reduces the transient and early gain of spinal motoneurons.

Motoneurons are the spinal neurons that directly control the muscle contraction. The gain characterizes how the synaptic input to motoneuron is converted in to action potential firing and subsequent muscle contraction. The high gain allows a high force and fast contraction, while the low gain is essential for a fine control of movements. The gain of motoneurons is mainly determined by a set of ion channels in membrane and therefore is a subject for modification. It is known, that the gain decreases during adaptation of action potential firing. Moreover, the neurotransmitters released during spinal network activity may modify the ion channel activity and therefore adjust the gain to the functional needs. The aim of this study was to evaluate the gain of spinal cord motoneurons and investigate mechanisms of its modification. Spinal motoneurons from adult turtle were used. We found that the gain of motoneurons estimated from triangular current ramps is the same as steady one obtained from square current steps. Pharmacologically increased conductance of motoneuron membrane does not change the gain. Finally, we demonstrated that persistent inward Na+ current increases excitability and reduces the transient and early gain of spinal motoneurons. / Motoneuronai – tai nervinės ląstelės tiesiogiai valdančios raumenis. Motoneuronuose, kaip ir kitose neuronuose, įėjimo transformacija į išėjimą charakterizuojama perdavimo funkcija, kuri dažniausiai aprašoma tam tikro statumo tiesine priklausomybe. Didelis perdavimo funkcijos statumas leidžia išvystyti didelę raumens susitraukimo jėgą, o mažas – įgalina tikslų raumenų valdymą. Perdavimo funkcijos charakteristikas apsprendžia neurono membranoje esantys joniniai kanalai. Pavyzdžiui, veikimo potencialų adaptaciją sukeliantys joniniai kanalai perdavimo funkcijos statumą mažina. Be to, neuroninio tinklo veikimo metu išskirti neurotransmiteriai gali veikti joninius kanalus ir pritaikyti perdavimo funkciją konkretaus judesio vykdymui. Šio darbo tikslas buvo įvertinti nugaros smegenų motoneuronų perdavimo funkcijos ypatybes ir ištirti jos galimus modifikavimo mechanizmus. Tyrimams naudoti vėžlio nugaros smegenų motoneuronai. Disertacijoje parodėme, kad perdavimo funkcijos statumas įvertintas trikampiais srovės impulsais sutampa su stacionariu perdavimo funkcijos statumu, įvertintu stimuliuojant motoneuronus stačiakampiais srovės impulsais. Nustatėme, kad farmakologiškai padidintas motoneuronų membranos laidumas neįtakoja perdavimo funkcijos statumo. Taip pat parodėme, kad nuolatinė Na+ srovė sumažina pradinį ir ankstyvąjį perdavimo funkcijų statumus.

No description available.

Brainstem derived noradrenaline (NA) in the spinal cord functions both to increase motoneuron excitability, by facilitating calcium-mediated persistent inward currents (Ca PICs), and to inhibit sensory afferent transmission to motoneurons (excitatory postsynaptic potentials; EPSPs). Spinal cord injury (SCI) results in a reduction of NA, causing a loss of Ca PICs in motoneurons below the lesion and exaggerated EPSPs to emerge. With time motoneuron Ca PICs gradually recover and are readily triggered by the exaggerated EPSPs, resulting in the development of muscle spasms. The role of the NA in the recovery of Ca PICs and muscle spasms after chronic SCI is examined in this thesis using a rat model of spasticity incorporating both the awake rat (in vivo) and the isolated rat spinal cord (in vitro). Specific activation of the adrenergic 1 receptor with agonists facilitated Ca PIC and spasms, whereas activation of the adrenergic 2 receptor with agonists decreased the EPSPs that trigger spasms. Both receptors were endogenously activated by a ligand in vivo, though the 1 receptor additionally exhibits constitutive activity (activity in the absence of NA), predominantly in vitro. The adrenergic 2 receptor was not found to be endogenously active in vitro. Use of amphetamine in rats, which causes a forced efflux of endogenous NA, confirmed the identity of the endogenous ligand as NA and demonstrated that a residual source of NA capable of facilitating the Ca PIC and spasms persists below a chronic transection. Immunohistochemical labelling for an enzyme involved in the synthesis of NA (dopamine--hydroxylase) revealed that NA is not synthesized in the spinal cord below a chronic transection, indicating that the endogenous NA is not intrinsic to the spinal cord. Peripheral injections of NA were used to demonstrate that the residual NA instead originates in the periphery (blood) and is both passively and actively transported across a compromised blood-brain barrier (BBB) after chronic injury. The peripherally derived NA activates central adrenergic receptors to modulate motoneuron excitability, sensory synaptic transmission and muscle spasms after chronic SCI. This novel finding highlights the importance of understanding the adaptations of neurotransmitter systems after injury when developing effective treatment strategies for spasticity.

x, 83 p. : ill. (some col.) A print copy of this thesis is available through the UO Libraries. Search the library catalog for the location and call number. / Programmed cell death (PCD) is a critical process during development and maturity of vertebrates and invertebrates. Aberrations in PCD are responsible for numerous developmental abnormalities and diseases in humans. Cell death pathways are surprisingly similar across species, so the study of PCD in simpler organisms such as insects provides important insight into the roles of cell death in higher animals including humans. Metamorphosis of the fruit fly, Drosophila melanogaster , provides an excellent model system in which to study PCD. During metamorphosis, many obsolete larval structures undergo PCD, largely in response to changes in circulating levels of steroid hormones known as ecdysteroids. These effects of ecdysteroids are particularly striking in the nervous system, where many larval neurons undergo PCD or functional remodeling during metamorphosis. One wave of neuronal PCD takes place during the first 24 hours of metamorphosis while a second follows adult emergence. Studies in another insect, Manduca sexta , suggested that the rise in ecdysteroids that initiates metamorphosis, the prepupal pulse, may trigger the first wave of neuronal PCD in Drosophila . This dissertation investigated steroid-regulated neuronal PCD in Drosophila by studying an individually-identified larval motoneuron, RP2. Using molecular genetics, ïmmunocytochemistry and primary cell culture, I showed that abdominal RP2s undergo PCD within the first 24 hours of Drosophila metamorphosis; identified a role for previously-identified PCD genes and ecdysteroid receptors in RP2's demise; and demonstrated that the prepupal pulse of ecdysteroids acts directly and cell-autonomously on RP2s to activate PCD. These experiments advance our understanding of hormonally-induced cell death and its regulation within the developing nervous system. This dissertation includes unpublished co-authored material. / Adviser: Janis C. Weeks