Simplified wiring diagram of the C. elegans chemosensory nervous system. Most amphid sensory neurons synapse to the locomotory circuit via multiple layers of connecting interneurons. However, some sensory neurons (e.g., ASH) synapse directly onto the command interneurons, and, similarly, some layer 1 interneurons also synapse directly onto the command interneurons. There are also substantial synapses between neurons in the same layers of organization, as well as gap junction connections (not shown). In this diagram, only neurons that have been discussed in some detail in this chapter are labeled. For a complete list of chemosensory neurons, see Table 1.1. (Adapted from Gray, J.M. et al. Proc. Natl. Acad. Sci. USA, 102, 3184–91, 2005.)