Episodic swimming behavior in the nematode C. elegans
2008
SUMMARY 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.
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