Figure 7. Working model of how circuit connectivity, signaling, and activity contribute to the observed rhythms that accompany the active and inactive egg-laying behavior states.
VA and VB motor neuron synapses release acetylcholine (ACh) that rhythmically excites the body wall muscles (bwm) and vm1 vulval muscles during each locomotion body bend, every ~10 s. Eggs are produced from each gonad arm every 10 min, and we propose the accumulation of 2–3 eggs mechanically excites the uterine muscles (um) facilitating exit from the inactive state. Sustained bursts of HSN command motor neuron activity trigger serotonin release that drives ~2 min active states. HSN also makes synapses and gap junctions with the cholinergic VC motor neurons, and VC makes synapses and gap junctions with the vm2 muscles whose contraction allows egg release. Passage of eggs mechanically activates the uv1 cells triggering release of tyramine and neuropeptides, which inhibit HSN activity to terminate the active state. See text for further details.