Motor pattern specification by dual descending pathways to a lobster rhythm-generating network

In the European lobster Homarus gammarus, rhythmic masticatory movements of the three foregut gastric mill teeth are generated by antagonistic sets of striated muscles that are driven by a neural network in the stomatogastric ganglion. In vitro, this circuit can spontaneously generate a single (type I) motor program, unlike in vivo in which gastric mill patterns with diffe rent phase relationships are found. By using paired intrasomatic recordings , all elements of the gastric mill network, which consists mainly of motone urons, have been identified and their synaptic relationships established. T he gastric mill circuit of Homarus is similar to that of other decapod crus taceans, although some differences in neuron number and synaptic connectivi ty were found. Moreover, specific members of the lobster network receive in put from two identified interneurons, one excitatory and one inhibitory, th at project from each rostral commissural ganglion. Integration of input fro m these projection elements is mediated by synaptic interactions within the gastric mill network itself. In arrhythmic preparations, direct phasic sti mulation of the previously identified commissural gastric (CG) interneuron evokes gastric mill output similar to the type I pattern spontaneously expr essed in vitro and in vivo. The newly identified gastric inhibitor interneu ron makes inhibitory synapses onto a different subset of gastric mill neuro ns and, when activated with the CG neuron, drives gastric mill output simil ar to the type II pattern that is only observed in the intact animal. Thus, two distinct phenotypes of gastric mill network activity can be specified by the concerted actions of parallel input pathways and synaptic connectivi ty within a target central pattern generator.