Fig. 4. Schematic drawing of the interactions between identified interneurones, sensory neurones and motor networks of the stomatogastric ganglion (STG). (Left) In the crab Cancer borealis, three pairs of proctolin (Proc)-like immunoreactive neurones are present (coloured in different shades of grey), which each elicit a different motor pattern. The two modulatory proctolin neurones (MPN) are located in the oesophageal ganglion (OG) or the oesophageal nerve and elicit a pyloric motor pattern (pyloric patterns coloured in different shades of orange) via excitatory synapses (symbolised by triangles). MPN inhibits, via the release of -aminobutyric acid (GABA; inhibitory synapses symbolised by small circles), two pairs of modulatory neurones located in the commissural ganglia, which are called commissural projection neurones 2 (CPN2) and modulatory commissural neurones 1 (MCN1), thereby preventing a gastric mill rhythm, which the latter neurones normally initiate. Stimulating MCN1 (containing proctolin, Cancer borealis tachykinin-related peptide, CabTRP, and GABA) alone elicits a gastropyloric motor pattern (gastric mill motor patterns are coloured in different shades of green, gastropyloric motor patterns are drawn in stripes of orange and green). After blocking the action of CabTRP, MCN1 does not elicit a gastric mill rhythm and the pyloric rhythm it initiates is more similar but still not identical to that elicited by MPN. Co-stimulation of the MCN1 and CPN2 elicits a different type of gastropyloric pattern. MCN1 receives rhythmic inhibition from the lateral gastric neurone (LG) in the STG. This does not influence the MCN1 synapses in the commissural ganglia (CoGs), demonstrating that activity of synapses can vary with the output region. Modulatory commissural neurones 7 (MCN7) also elicit a pyloric motor pattern that differs from that elicited by the MPNs. (Right) In the lobster Homarus gammarus, the anterior gastric receptor (AGR) excites two pairs of modulatory interneurones in the CoGs: the commissural gastric (CG) neurones and the gastric inhibitor (GI) neurones. AGR, which is a mechanoreceptor activated by the movements of the gastric mill muscle 1 (gm1), has its soma in the dorsal ventricular nerve (dvn) and projects through the STG without any arborization to innervate the CoGs. When AGR fires weakly, one gastric mill pattern is elicited. When AGR fires strongly, a second gastric mill pattern is elicited, demonstrating that the activity of a feedback loop is able to select different motor patterns (modified from Blitz et al., 1999; Blitz and Nusbaum, 1997; Coleman and Nusbaum, 1994; Coleman et al., 1995; Combes et al., 1999a; Combes et al., 1999b).