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A developmental analysis of the reversal response in the nematode Caenorhabditis elegans

University of British Columbia

The nematode Caenorhabditis eleqans exhibits a number of locomotory behaviours, among them a family of reversal responses which can occur either spontaneously, in the absence of obvious stimuli, or reflexively, in response to mechanical stimuli. These behaviours are thought to be mediated by a common neural circuit, the touch withdrawal circuit, whose genetics, neuranatomy, and development have been exceptionally well-characterized. Despite known changes in the functional neuroanatomy of this circuit over development, at least one behaviour mediated by this circuit, the touch withdrawal reflex, shows no apparent changes over development. This thesis examined the possibility that these changes may be reflected in two other reversal responses, spontaneous reversals and the tap reversal reflex. In the experiments reported here, both spontaneous reversals and the tap reversal reflex were examined over six developmental stages: four larval stages (L1, L2, L3, and L4), and two adult stages (young adult and 4 day olds). In all experiments, animals were observed individually under a light microscope and their behaviour videotaped for later analysis. Results indicated that both spontaneous reversals and the tap reversal reflex show developmental changes in C. elegans. Young adult animals showed an increase in the frequency of spontaneous reversals relative to the other developmental stages; this increase was neither a function of increased activity or change in sensory threshold. Larval animals of all stages also showed significantly different patterns of responding to taps relative to adult stages, with L2 animals showing the most pronounced differences. Although animals of all stages reversed in response to touch, taps elicited not only reversals, but a previously undescribed response, accelerations. While adults consistently showed reversals in response to single taps, larval animals showed accelerations on approximately half the responses. The tap reversal reflex was also found to be graded, showing a significant trend to increase in magnitude with increasing stimulus magnitude. Increasing stimulus magnitude also increased the probability of accelerations relative to single taps for all developmental stages, though larval animals still showed fewer reversals and more accelerations than adult animals. There appeared to be two phases in the development of the reflex: one, the transition from newly hatched L1 animal to subsequent larval stages; the other, the transition from larval stage L4 to young adult, when the pattern of responding to taps changes from the larval to the adult pattern. These periods correlated with periods of known neuronal change in the neural circuit. It was hypothesized that the transition from larval to adult patterns of responding might be the result of the addition of a specific sensory neuron, AVM, and its associated connections.

Text

2010-09-02

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http://hdl.handle.net/2429/28162

Psychology

University of British Columbia

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