A dual mechanosensory and chemosensory neuron in Caenorhabditis elegans.

After light touch to its nose, the nematode Caenorhabditis elegans halts forward locomotion and initiates backing. Here we show that three classes of neurons (ASH, FLP, and OLQ) sense touch to the nose and hence are required for this avoidance response. ASH, FLP, and OLQ have sensory endings that contain axonemal cilia. Mutant animals that have defective ciliated sensory endings as well as laser-operated animals that lack ASH, FLP, and OLQ fail to respond to touch to the nose. Together with the previous work of others, these results demonstrate that C. elegans has at least five morphologically distinct classes of mechanosensory neurons. Interestingly, the ASH neuron also acts as a chemosensory neuron; it mediates the avoidance of noxious chemicals. Since ASH possesses both chemosensory and mechanosensory modalities, this neuron might be functionally analogous to vertebrate nociceptors, which mediate the sensation of pain.