1. Stretching the pyloric region of the lobster's stomach in a manner that resembles pyloric dilation triggers a prolonged burst of impulses in two interneurones with axons in the inferior ventricular nerve (IVN). The burst is activated in the oesophageal ganglion by sensory axons that traverse the lateral ventricular nerves, the dorsal ventricular nerve and the stomatogastric nerve. These sensory axons do not appear to make synaptic contacts in the stomatogastric ganglion. 2. Electrical

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... tion of sensory branches of the pyloric nerve triggers similar bursts in the IVN interneurones. 3. The burst of impulses in the IVN interneurones lasts from 2 to 30 s and the impulse frequency ranges from 10 to 80 Hz in different parts of the burst. Once triggered, burst structure and burst duration are independent of the intensity or duration of stimuli applied to the sensory nerves. 4. These bursts alter both the gastric and pyloric motor patterns. The IVN interneurones make a complex pattern of synapses with stomatogastric neurones. These are: pyloric dilators (PD) - excitation and slow inhibition; ventricular dilator (VD) - excitation; gastric mill (GM) neurones - inhibition; lateral posterior gastric neurones (LPGN) - inhibition; and Interneurone I (Int I) - excitation and slow inhibition. The size of the p.s.p.s at each of these synapses depends on the duration and impulse-frequency of the burst in the presynaptic neurones, which in turn alters the firing patterns of the stomatogastric neurones in various ways.