The basis of neuron-specific pathogenesis, resulting from the expression of misfolded proteins, is poorly understood and of central importance to an understanding of the cell-type specificity of neurodegenerative disease. In this study, we developed a new model for neuron-specific polyQ pathogenesis in Caenorhabditis elegans by pan-neuronal expression that exhibits polyQ length-dependent aggregation, neurotoxicity, and a pathogenic threshold at a length of 35-40 glutamines. Analysis of specific neurons in C. elegans revealed that only at the threshold length, but not at shorter or longer lengths, polyQ proteins can exist in a soluble state in certain lateral neurons or in an aggregated state in motor neurons of the same animal. These results provide direct experimental evidence that the expression of a single species of a toxic misfolded protein can exhibit a range of neuronal consequences.