A positive feedback loop promotes the active internal state of the C. elegans egg-laying circuit

Neural circuits generate different outputs by adopting distinct activity patterns, known as internal states, but what generates such states remains poorly understood. We analyzed how the deeply studied C. elegans egg-laying circuit switches from inactive to active states to produce egg-laying behavior. We found that posterior ventral process W (PVW) neurons make previously uncharacterized branches that terminate near the egg-laying muscles and that exhibit calcium transients when mechanically activated by egg-laying muscle contractions. Conversely, optogenetically activating PVWs induces egg-laying muscle activity. Silencing PVWs reduces the circuit's ability to enter an active state or respond to external stimuli that normally induce egg laying. PVWs appear to promote circuit activity without using any known classical neurotransmitter or biogenic amine. Our results suggest that PVWs mediate one of four feedback loops that collaborate to initiate, maintain, and terminate the active state, providing an understanding of how a model neural circuit switches between two persistent internal states.