We are studying how distinct sensory pathways elicit different motor patterns from the multifunctional circuits in the stomatogastric ganglion (STG) of the crab Cancer borealis. Activation of one such pathway, the gastro-pyloric stretch receptor neurons (GPRs), directly modulates the pyloric rhythm and influences gastric mill neurons in the STG (Katz & Harris-Warrick, J Neurosci, 1991). The GPRs also indirectly influence these systems by activating the modulatory projection neurons MCN1 & CPN2 in the commissural ganglia (CoGs) (Blitz & Nusbaum, SFN Abstr, 1996). When GPR is stimulated rhythmically in a behaviorally-relevant pattern, it activates MCN1/CPN2 and thereby elicits a gastric mill rhythm with a cycle period of ~15 seconds, even though this rhythm can cycle faster. We are now determining how GPR influences ongoing gastric mill rhythms, such as the one activated by the mechanosensory pathway projecting through the dorsal posterior oesophageal nerve (DPON: Beenhakker et al, SFN Abstr, 2000). During a...