MODULATION AND DYNAMIC SPECIFICATION OF MOTOR RHYTHM-GENERATING CIRCUITS IN CRUSTACEA

The operation of central pattern generators (CPGs), oscillatory neural circuit responsible for rhythmic motor behavior, is now known to depe nd both on the synaptic interactions between constituent neurons and t heir intrinsic membrane properties (oscillatory, plateauing, etc). Mor eover, these synaptic and cellular properties are not invariant, but a re subject to a wide range of neuromodulatory influences that, by modi fying the bioelectrical character of individual neurone and/or the str ength of their synapses. are able to adapt the output of a given CPG c ircuit to the changing needs of the animal. Despite this ability to pr oduce different Functional configurations, however, the assumption rem ains of a CPG as a predefined assemblage of interconnected neurons ded icated to a. particular behavior and functionally distinguishable from other circuits responsible for other tasks. However, our recent studi es on the stomatogastric nervous system (STNS) of crustacea have begun to question this concept of the CPG as a discrete and identifiable en tity within the central nervous system. Here we review evidence showin g that under neuromodulatory instruction, individual neurons can parti cipate in different oscillatory motor circuits and hence more than one rhythmic behaviour, an?even more profoundly, preexisting networks can be dismantled to specify dynamically a new circuit Tor an entirely di fferent behaviour This de novo network construction is achieved again by neuromodulatory gamma-induced alterations in the oscillatory and sy naptic properties of individual target neurons. On this basis, therefo re, a functional CPG network must be seen in a more dynamic context th an previously thought since it may exist only in a particular behaviou ral situation dictated by modulatory influences.