Modulation and dynamic specification of motor rythm-generating circuits in crustacea.

The operation of central pattern generators (CPGs), oscillatory neural circuits responsible for rhythmic motor behavior, is now known to depend both on the synaptic interactions between constituent neurons and their intrinsic membrane properties (oscillatory, plateauing, etc). Moreover, these synaptic and cellular properties are not invariant, but are subject to a wide range of neuromodulatory influences that, by modifying the bioelectrical character of individual neurons and/or the strength of their synapses, are able to adapt the output of a given CPG circuit to the changing needs of the animal. Despite this ability to produce different functional configurations, however, the assumption remains of a CPG as a predefined assemblage of interconnected neurons dedicated to a particular behavior and functionally distinguishable from other circuits responsible for other tasks. However, our recent studies on the stomatogastric nervous system (STNS) of crustacea have begun to question this concept of the CPG as a discrete and identifiable entity within the central nervous system. Here we review evidence showing that under neuromodulatory instruction, individual neurons can participate in different oscillatory motor circuits and hence more than one rhythmic behaviour, and even more profoundly, preexisting networks can be dismantled to specify dynamically a new circuit for an entirely different behaviour. This de novo network construction is achieved again by neuromodulatory-induced alterations in the oscillatory and synaptic properties of individual target neurons. On this basis, therefore, a functional CPG network must be seen in a more dynamic context than previously thought since it may exist only in a particular behavioural situation dictated by modulatory influences.