3 research outputs found
Emergent Central Pattern Generator Behavior in Gap-Junction-Coupled Hodgkin-Huxley Style Neuron Model
Most models of central pattern generators (CPGs) involve two distinct nuclei mutually inhibiting one another via synapses. Here, we present a single-nucleus model of biologically realistic Hodgkin-Huxley neurons with random gap junction coupling. Despite no explicit division of neurons into two groups, we observe a spontaneous division of neurons into two distinct firing groups. In addition, we also demonstrate this phenomenon in a simplified version of the model, highlighting the importance of afterhyperpolarization currents (I AHP ) to CPGs utilizing gap junction coupling. The properties of these CPGs also appear sensitive to gap junction conductance, probability of gap junction coupling between cells, topology of gap junction coupling, and, to a lesser extent, input current into our simulated nucleus
Emergent Central Pattern Generator Behavior in Gap-Junction-Coupled Hodgkin-Huxley Style Neuron Model
Most models of central pattern generators (CPGs) involve two distinct nuclei mutually inhibiting one another via synapses. Here, we present a single-nucleus model of biologically realistic Hodgkin-Huxley neurons with random gap junction coupling. Despite no explicit division of neurons into two groups, we observe a spontaneous division of neurons into two distinct firing groups. In addition, we also demonstrate this phenomenon in a simplified version of the model, highlighting the importance of afterhyperpolarization currents () to CPGs utilizing gap junction coupling. The properties of these CPGs also appear sensitive to gap junction conductance, probability of gap junction coupling between cells, topology of gap junction coupling, and, to a lesser extent, input current into our simulated nucleus
Development and plasticity of locomotor circuits in the zebrafish spinal cord
A fundamental goal in neurobiology is to understand the development and organization of neural circuits that drive behavior. In the embryonic spinal cord, the first motor activity is a slow coiling of the trunk that is sensory-independent and therefore appears to be centrally driven. Embryos later become responsive to sensory stimuli and eventually locomote, behaviors that are shaped by the integration of central patterns and sensory feedback. In this thesis I used a simple vertebrate model, the zebrafish, to investigate in three manners how developing spinal networks control these earliest locomotor behaviors.
For the first part of this thesis, I characterized the rapid transition of the spinal cord from a purely electrical circuit to a hybrid network that relies on both chemical and electrical synapses. Using genetics, lesions and pharmacology we identified a transient embryonic behavior preceding swimming, termed double coiling. I used electrophysiology to reveal that spinal motoneurons had glutamate-dependent activity patterns that correlated with double coiling as did a population of descending ipsilateral glutamatergic interneurons that also innervated motoneurons at this time. This work (Knogler et al., Journal of Neuroscience, 2014) suggests that double coiling is a discrete step in the transition of the motor network from an electrically coupled circuit that can only produce simple coils to a spinal network driven by descending chemical neurotransmission that can generate more complex behaviors.
In the second part of my thesis, I studied how spinal networks filter sensory information during self-generated movement. In the zebrafish embryo, mechanosensitive sensory neurons fire in response to light touch and excite downstream commissural glutamatergic interneurons to produce a flexion response, but spontaneous coiling does not trigger this reflex. I performed electrophysiological recordings to show that these interneurons received glycinergic inputs during spontaneous fictive coiling that prevented them from firing action potentials. Glycinergic inhibition specifically of these interneurons and not other spinal neurons was due to the expression of a unique glycine receptor subtype that enhanced the inhibitory current. This work (Knogler & Drapeau, Frontiers in Neural Circuits, 2014) suggests that glycinergic signaling onto sensory interneurons acts as a corollary discharge signal for reflex inhibition during movement.
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In the final part of my thesis I describe work begun during my masters and completed during my doctoral degree studying how homeostatic plasticity is expressed in vivo at central synapses following chronic changes in network activity. I performed whole-cell recordings from spinal motoneurons to show that excitatory synaptic strength scaled up in response to decreased network activity, in accordance with previous in vitro studies. At the network level, I showed that homeostatic plasticity mechanisms were not necessary to maintain the timing of spinal circuits driving behavior, which appeared to be hardwired in the developing zebrafish. This study (Knogler et al., Journal of Neuroscience, 2010) provided for the first time important in vivo results showing that synaptic patterning is less plastic than synaptic strength during development in the intact animal.
In conclusion, the findings presented in this thesis contribute widely to our understanding of the neural circuits underlying simple motor behaviors in the vertebrate spinal cord.Un objectif important en neurobiologie est de comprendre le deĢveloppement et l'organisation des circuits neuronaux qui entrainent les comportements. Chez l'embryon, la premieĢre activiteĢ motrice est une lente contraction spontaneĢe qui est entraineĢe par l'activiteĢ intrinseĢque des circuits spinaux. Ensuite, les embryons deviennent sensibles aux stimulations sensorielles et ils peuvent eĢventuellement nager, comportements qui sont facĢ§onneĢes par l'inteĢgration de l'activiteĢ intrinseĢque et le reĢtrocontroĢle sensoriel. Pour cette theĢse, j'ai utiliseĢ un modeĢle verteĢbreĢ simple, le poisson zeĢbre, afin d'eĢtudier en trois temps comment les reĢseaux spinaux se deĢveloppent et controĢlent les comportements locomoteurs embryonnaires.
Pour la premieĢre partie de cette theĢse j'ai caracteĢriseĢ la transition rapide de la moelle eĢpinieĢre d'un circuit entieĢrement eĢlectrique aĢ un reĢseau hybride qui utilise aĢ la fois des synapses chimiques et eĢlectriques. Nos expeĢriences ont reĢveĢleĢ un comportement embryonnaire transitoire qui preĢceĢde la natation et qu'on appelle Ā« double coiling Ā». J'ai deĢmontreĢ que les motoneurones spinaux preĢsentaient une activiteĢ deĢpendante du glutamate correĢleĢe avec le Ā« double coiling Ā» comme l'a fait une population d'interneurones glutamatergiques ipsilateĢraux qui innervent les motoneurones aĢ cet aĢge. Ce travail (Knogler et al., Journal of Neuroscience, 2014) suggeĢre que le Ā« double coiling Ā» est une eĢtape distincte dans la transition du reĢseau moteur aĢ partir d'un circuit eĢlectrique treĢs simple aĢ un reĢseau spinal entraineĢ par la neurotransmission chimique pour geĢneĢrer des comportements plus complexes.
Pour la seconde partie de ma theĢse, j'ai eĢtudieĢ comment les reĢseaux spinaux filtrent l'information sensorielle de mouvements auto-geĢneĢreĢs. Chez l'embryon, les neurones sensoriels meĢcanosensibles sont activeĢs par un leĢger toucher et ils excitent en aval des interneurones sensoriels pour produire une reĢponse de flexion. Par contre, les contractions spontaneĢes ne deĢclenchent pas ce reĢflexe meĢme si les neurones sensoriels sont toujours activeĢs. J'ai deĢmontreĢ que les interneurones sensoriels recĢ§oivent des entreĢes glycinergiques pendant les contractions spontaneĢes fictives qui les empeĢchaient de geĢneĢrer des potentiels d'action. L'inhibition glycinergique de ces interneurones, mais pas des autres neurones spinaux, est due aĢ l'expression d'un sous-type de reĢcepteur glycinergique unique qui augmente
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le courant inhibiteur. Ce travail (Knogler & Drapeau, Frontiers in Neural Circuits, 2014) suggeĢre que la signalisation glycinergique chez les interneurones sensoriels agit comme un signal de deĢcharge corolaire pour l'inhibition des reĢflexes pendant les mouvements auto- geĢneĢreĢs.
Dans la dernieĢre partie de ma theĢse, je deĢcris le travail commenceĢ aĢ la maiĢtrise et termineĢ au doctorat qui montre comment la plasticiteĢ homeĢostatique est exprimeĢe in vivo aux synapses centrales aĢ la suite des changements chroniques de l'activiteĢ du reĢseau. J'ai deĢmontreĢ que l'efficaciteĢ synaptique excitatrice de neurones moteurs spinaux est augmenteĢe aĢ la suite dāune diminution de l'activiteĢ du reĢseau, en accord avec des eĢtudes in vitro preĢceĢdentes. Par contre, au niveau du reĢseau j'ai deĢmontreĢ que la plasticiteĢ homeĢostatique n'eĢtait pas neĢcessaire pour maintenir la rythmiciteĢ des circuits spinaux qui entrainent les comportements embryonnaires. Cette eĢtude (Knogler et al., Journal of Neuroscience, 2010) a reĢveĢleĢ pour la premieĢre fois que l'organisation du circuit est moins plastique que l'efficaciteĢ synaptique au cours du deĢveloppement chez l'embryon.
En conclusion, les reĢsultats preĢsenteĢs dans cette theĢse contribuent aĢ notre compreĢhension des circuits neuronaux de la moelle eĢpinieĢre qui sous-tendent les comportements moteurs simples de l'embryon