The Contributions of Motor Neuronal and Muscle Modulation to Behavioral Flexibility in the Stomatogastric System

The stomatogastric nervous system of crustaceans, which controlsthe four parts ofthe foregut, is subject to modulation at alllevels, sensory, central and motor. Modulation of the centralpattern generators, which are themselves made up largely ofmotor neurons, providesfor increased behavioral flexibilityin a variety of ways. First, each of the pattern generatorscan be reconfigured to give multiple outputs. Second, the "boundaries"of the different pattern generators are in fact somewhat fluid,so that the neuronal composition of the pattern generators canbe altered. For example, neurons can switch from one patterngenerator toanother, or two or more pattern generators can fuseto generate an entirely new pattern and thereby produce a newbehavior. The mechanisms responsible for many of these modulationsinclude alterations of both intrinsic properties and synapticinteractions between neurons. In addition, the alteration ofmembrane properties contributes more directly to the behavioraloutput by changing action potential frequency. Finally, themuscles of the stomatogastric system can themselves be modulated,with the cpvl muscle, for example, becoming an endogenous oscillatorin the presence of either dopamine or the peptide FMRFamide.