The sex-specific VC neurons are mechanically activated motor neurons that facilitate serotonin-induced egg laying in C. elegans

Successful execution of behavior requires coordinated activity and communication between multiple cell types. Studies using the relatively simple neural circuits of invertebrates have helped to uncover how conserved molecular and cellular signaling events shape animal behavior. Here we show that the sex-specific, ventral C (VC) motor neurons drive vulval muscle contractility and serotonin-induced egg laying. Ca²⁺ imaging shows the VCs are active during vulval opening, and optogenetic stimulation of the VCs promotes vulval muscle Ca²⁺ activity. Blocking VC neurotransmission inhibits egg laying and increases failed attempts, indicating VC signaling facilitates full vulval muscle contraction. We also find the VCs are mechanically activated by vulval opening: stimulating vulval muscles drives VC Ca²⁺ activity and requires muscle contractility, showing presynaptic VCs and postsynaptic muscles mutually excite each other. Together, VC neurons coordinate muscle contractility in response to serotonin and mechanosensory feedback to enable efficient egg laying.