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The projections of neurones that produce inhibitory junction potentials (i.j.p.s) have been analysed in the small intestine of the guinea-pig. Intracellular microelectrodes were used to record i.j.p.s in circular muscle cells in response to stimulation with pairs of electrodes placed parallel to or across the length of the intestine (longitudinal and transverse electrodes, respectively). In preparations of circular muscle with the myenteric plexus attached, the mean amplitude of i.j.p.s recorded 9 mm lateral to longitudinal electrodes was about 70% of that seen at 1 mm; however, in the presence of hexamethonium (400 microM) the amplitude fell more rapidly to about 40% at 9 mm. The decline was even steeper in preparations of circular muscle from which the myenteric plexus had been removed, with the amplitude falling to 10% at 9 mm. I.j.p.s were detected up to 30 mm anal to stimuli applied with transverse electrodes in the presence of hexamethonium, but the decline was more rapid on the oral side, with i.j.p.s being detected only up to 12 mm oral to the stimulus. Nerves that travel through or originate in the myenteric plexus were severed or removed in operations performed 2-7 days before tissue was taken. 2-3 days after removal of a circumferential strip of myenteric plexus, i.j.p.s could not be recorded from the underlying circular muscle. I.j.p.s evoked by longitudinal electrodes were recorded in an area between two circumferential cuts through the myenteric plexus, 1.5-8 mm apart, 3-7 days after an operation. In this region, the i.j.p.s were significantly smaller than normal although amplitudes of up to 95% of control were measured. The consequences of single circumferential cuts through the myenteric plexus were examined 3 and 7 days after operation. Longitudinal electrodes were used to stimulate surviving fibres. Small i.j.p.s were recorded on the anal edge of the lesion; the amplitude recovered to 56% of normal in the first 2 mm on the anal side and the amplitude slowly returned to normal at 20 mm anal. On the oral side there was slight reduction in amplitude at 0.5 and 1 mm, but at greater distances amplitudes were normal. It is concluded that the simplest explanation of these results is that two populations of i.j.p.-producing neurones have their cell bodies in the myenteric plexus. Many axons supply the circular muscle close to or just anal to their cell bodies, and others run for various distances up to 30 mm in the anal direction.(ABSTRACT TRUNCATED AT 400 WORDS)

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