摘要：

Effective movement is central to survival and it is essential for all animals to react inresponse to changes around them. In many animals the rhythmic signals that drivelocomotion are generated intrinsically by small networks of neurons in the nervoussystem which can be switched on and off. In this thesis I use a very simple animal,in which the behaviours and neuronal networks have been well characterised experimentally,to explore the salient features of such networks. Two days after hatching,tadpoles of the frog Xenopus laevis respond to a brief touch to the head by startingto swim. The swimming rhythm is driven by a small population of electricallycoupled brainstem neurons (called dINs) on each side of the tadpole. These neuronsalso receive synaptic input following head skin stimulation. I build biophysical computationalmodels of these neurons based on experimental data in order to addressquestions about the effects of electrical coupling, synaptic feedback excitation andinitiation pathways. My aim is better understanding of how swimming activity isinitiated and sustained in the tadpole.I find that the electrical coupling between the dINs causes their firing propertiesto be modulated. This allows two experimental observations to be reconciled: thata dIN only fires a single action potential in response to step current injections butthe population fires like pacemakers during swimming. I build on this hypothesisand show that long-lasting, excitatory feedback within the population of dINs allowsrhythmic pacemaker activity to be sustained in one side of the nervous system. Thisactivity can be switched on and off at short latency in response to biologically realisticsynaptic input. I further investigate models of synaptic input from a definedswim initiation pathway and show that electrical coupling causes a population ofdINs to be recruited to fire either as a group or not at all. This allows the animalto convert continuously varying sensory stimuli into a discrete decision. Finally Ifind that it is difficult to reliably start swimming-like activity in the tadpole modelusing simple, short-latency, symmetrical initiation pathways but that by using morecomplex, asymmetrical, neuronal-pathways to each side of the body, consistent withexperimental observations, the initiation of swimming is more robust. Throughoutthis work, I make testable predictions about the population of brainstem neuronsand also describe where more experimental data is needed. In order to manage theparameters and simulations, I present prototype libraries to build and manage thesebiophysical model networks.

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