Topographical and electrophysiological characteristics of highly excitable S neurones in the myenteric plexus of the guinea-pig ileum.

1. Most intracellular electrical recordings from myenteric neurones have been made from the centre of large ganglia. In this study, we examined the electrophysiological properties of neurones at the corners of large ganglia close to internodal strands and in microganglia. 2. Of 150 neurones in these locations: 111 were tonic S neurones; 9 were phasic S neurones and 30 were AH neurones. 3. Tonic S neurones were characterized by: (i) low resting membrane potentials (-50 +/- 1 mV, mean +/- s.e.m.); (ii) high input impedance (522 +/- 23 MOmega); (iii) low threshold for action potential (AP) generation (0.012 +/- 0.004 nA); (iv) firing of APs throughout a depolarizing pulse (duration <= 1 s) and one to four APs following a hyperpolarizing pulse and (v) spontaneous fast excitatory postsynaptic potentials (FEPSPs). A substantial proportion of tonic S neurones (43 %) also fired APs spontaneously (7.6 +/- 0.6 Hz; range, 0.3-19 Hz). All APs were blocked by tetrodotoxin (1 microM). 4. Tonic S neurones were subclassified, according to their post-stimulus responses, as SAH or SAD neurones. Following a burst of APs, SAH neurones exhibited a prominent after-hyperpolarization (duration, 711 +/- 10 ms) and SAD neurones an after-depolarization (duration, 170 +/- 10 ms). The after-hyperpolarization was reduced in four of ten neurones by apamin (0.3 microM). 5. FEPSPs were evoked in 20 of 38 S neurones by electrical stimulation applied both oral and anal to the recording site. Repetitive stimuli evoked slow excitatory postsynaptic potentials (SEPSPs) in some tonic S neurones. 6. Three functional classes of S neurones were identified after injection of neurobiotin through the recording microelectrode: (i) longitudinal muscle motor neurones, (ii) short circular muscle motor neurones, and (iii) ascending interneurones. 7. In conclusion, there appears to be topographical organization of highly excitable, tonic S neurones within the myenteric plexus, since, in contrast to other S neurones, they can be readily impaled in myenteric ganglia close to internodal strands and in microganglia.