MOTOR PATTERN SELECTION VIA INHIBITION OF PARALLEL PATHWAYS

Motor pattern selection from a multifunctional neural network often re sults from direct synaptic and modulatory actions of different project ion neurons onto neural network components. Less well documented is th e presence and function of interactions among distinct projection neur ons innervating the same network. In the stomatogastric nervous system of the crab Cancer borealis, several distinct projection neurons that influence the pyloric and gastric mill rhythms have been studied. The se rhythms are generated by overlapping subsets of identified neurons in the stomatogastric ganglion (STG). One oi these identified projecti on neurons is the modulatory proctolin neuron (MPN). We showed previou sly that MPN stimulation excites the pyloric rhythm by its excitatory actions on STG neurons. In contrast to its excitatory actions on the p yloric rhythm, we have now found that MPN inhibits the gastric mill rh ythm. This inhibition does not occur within the STG, but instead resul ts from MPN-mediated inhibition of two previously identified projectio n neurons within the commissural ganglia. These projection neurons inn ervate the STG and, via their actions on STG neurons, they elicit the gastric mill rhythm as well as modify the pyloric rhythm in a manner d istinct from MPN. By inhibiting these projection neurons, MPN removes excitatory drive to gastric mill neurons and elicits an MPN-specific p yloric rhythm. Motor pattern selection by MPN therefore results from b oth a direct modulation of STG network activity and an inhibition of c ompeting pathways.