Background: Rhythmic motor behaviors can be generated continuously (e.g., breathing) or episodically (e.g., locomotion, swallowing), when short or long bouts of rhythmic activity are interspersed with periods of quiescence. Although the mechanisms of rhythm generation are known in detail in many systems, there is very little understanding of how the episodic nature of rhythmic behavior is produced at the neuronal level.
Results: Using a well-established episodic rhythm-generating neural circuit controlling molluscan feeding, we demonstrate that quiescence between bouts of activity arises from active, maintained inhibition of an otherwise rhythmically active network. We show that the source of the suppressive drive is within the circuit itself; a single central pattern generator (CPG) interneuron type that fires tonically to inhibit feeding during quiescence. Suppression of the tonic activity of this neuron by food is sufficient to change the network from an inactive to a rhythmically active state, with the cell switching function to fire phasically as part of the food-evoked rhythmogenesis. Furthermore, the absolute level of intrinsic suppressive control is modulated extrinsically by the animal's behavioral state (e.g., hunger/satiety), increasing the probability of episodes of feeding when the animal is hungry.
Conclusions: By utilizing the same intrinsic member of a CPG network in both rhythm-generation and suppression, this system has developed a simple and efficient mechanism for generating a variable level of response to suit the animal's changing behavioral demands.