Episodic swimming behavior in the nematode C. elegans.

Controlling the choice of behavioral output is a central function of the nervous system. Here we document a novel spontaneous behavioral transition in C. elegans locomotion. Upon transfer of the nematode from a solid surface into a liquid environment, swimming occurs in two phases: an initial, 1-2 h phase of continuous swimming, followed by a second phase during which swimming is episodic. During the second, episodic phase, periods of active swimming alternate in a highly regular fashion with a quiescent state lasting for several minutes. We analyzed the nature of the quiescent state and the basis for spontaneous switching between swimming and quiescence. The transition from swimming to quiescence is promoted by acetylcholine signaling and initially during quiescence body wall muscles are in a state of contraction. After the first minute, quiescent worms respond to prodding and resume swimming normally. The major command interneurons that control the locomotory circuits are not necessary for quiescence since swimming-quiescence cycling occurs after ablation of command interneurons. However, when subsets of neurons including the command interneurons are killed, the switching pattern becomes less regular, suggesting that a timer governing switching may lie within circuitry controlling motor neurons. The results show that the motor circuits have a tendency to switch spontaneously between active and inactive behavioral states. This property might be important to the animal in a uniform environment where sensory input is invariant.