Central pattern generators (CPGs) are networks of interneurons that produce rhythmic outputs, controlling motor activities such as feeding and locomotion. Studying how the rhythms of activity are produced can be difficult, because it is hard to distinguish between intrinsic properties of individual neurons and properties that depend on the rest of the network. Straub et al. have isolated the neurons that make up the feeding CPG of the snail Lymnaea, and have found that the cycles of firing are initiated by the intrinsic properties of one key interneuron.

There are three main classes of interneuron in the Lymnaea CPG — N1, N2 and N3 — and each is active during a different phase of the feeding cycle. Each class has two subtypes, and recordings from the intact network have indicated that several of the subtypes have endogenous properties, such as bursting or generating plateau potentials, that might contribute to pattern generation. However, when Straub et al. studied each type of interneuron in isolation, they found that only one — the N1M interneuron — could generate plateau potentials intrinsically. Another, the N2v interneuron, generated plateau potentials in the presence of acetylcholine, and the rest showed no significant patterns of intrinsic activity. Instead, it seems that the rhythmic firing of these interneurons is driven by synaptic inputs from the rest of the CPG network.

These synaptic inputs could be mimicked in the culture system by the application of glutamate or acetylcholine, the two main neurotransmitters found in the CPG. By studying the electrical and pharmacological properties of the interneurons, both in isolation and in situ, the authors were able to present a new model of the generation of the feeding pattern by the circuit. Similar studies in crustacean CPGs have indicated that, in these networks, most interneurons have intrinsic properties that contribute to pattern generation. The Lymnaea feeding CPG seems to offer a different way of doing things, in which almost all of the interneurons rely on network interactions for their rhythmic activity.