An integrated and automated system for assaying touch sensation in freely moving C. elegans [electronic resource]
Keywords: Electrical Engineering.

The sense of touch is the human interface with the mechanical world, enabling everything from walking and holding a cup of coffee, to recognizing painful, potentially life- threatening objects. The sensory neurons embedded in our skin are responsible for our sense of touch, but we know little about their activation mechanism. How do touch sensitive neurons detect a mechanical perturbation and transduce that stimulus into an electro-chemical signal the rest of our nervous system can interpret? In humans and other mammals, touch sensation depends on multiple types of somatosensory neurons distributed throughout our skin. The detection of stimuli is interpreted by our brain and consequently linked to diverse, complex behaviors. By contrast, Caenorhabditis elegans, a species of soil dwelling nematodes, have exactly six touch sensitive neurons that evoke simple behaviors, making them a well-suited model for characterizing touch sensitive neurons. The classical touch assay employs an eyebrow hair to stimulate the body of freely moving C. elegans, evoking an evasive behavioral response. If the animal senses the stimulus, it will react by moving away from the stimulus by either speeding up or reversing. Though this common assay has led to the discovery of genes required for touch sensation, it is limited in spatial precision, stimulus application control, and behavioral response characterization. In this thesis, I present a system for performing automated, quantitative touch assays that circumvents these limitations. We integrated optical technologies with real time image processing, computer vision, and control systems to apply a known force profile to a user-defined target on a freely moving animal. Additionally, I developed behavioral analysis software to objectively quantify and score the behavioral response to the stimulus. By adding a spatial domain to touch assay experiments, we 1) improve upon previous methods by reducing the probability of a false negative result incurred by stimulating outside the animal's receptive field, 2) map the spatial touch sensitivity of the animal along its anterior-posterior axis, and 3) characterize the behavioral response, revealing insights into the mechanosensation signaling pathway. We used the system to stimulate the animal along its anterior-posterior axis in wild type animals and spc-1(dn) transgenic animals, which have a previously identified defect in touch sensation. We found that the probability of evoking a response varies with both the stimulus strength and location. Animals reversed direction when touched on the anterior half of their bodies, accelerated when touched on the posterior half, and failed to respond when touched in the middle. Additionally, we observed that, once initiated, the characteristics of the animal's reversal were independent of the stimulus location, stimulus magnitude, and spc- 1(dn) transgene. These results imply that both stimulus strength and location govern the activation of a defined motor program and that C. elegans body consists of two distinct receptive fields, separated by a gap.