Neurotransmitter receptors, neurotransmitter synthesis and release pathways, and heterotrimeric GTP-binding protein (G protein)-coupled second messenger pathways are highly conserved between Caenorhabditis elegans and mammals, but gap junctions and chemosensory receptors have independent origins in vertebrates and nematodes. Most ion channels are similar to vertebrate channels but there are no predicted voltage-activated sodium channels. The C. elegans genome encodes at least 80 potassium channels, 90 neurotransmitter-gated ion channels, 50 peptide receptors, and up to 1000 orphan receptors that may be chemoreceptors. For many gene families, C. elegans has both conventional members and divergent outliers with weak homology to known genes; these outliers may provide insights into previously unknown functions of conserved protein families.