1. Micro-electrodes were used to examine transmission from intrinsic and extrinsic nerves to single cells of the longitudinal and circular muscle coats of colons from guinea-pigs and rabbits.2. When intrinsic nerves were stimulated, inhibitory junction potentials (IJPs) were observed in over 90% of the cells of the longitudinal coat and nearly all of the cells of the circular coat. The characteristics of IJPs in the guinea-pig and rabbit were similar. Facilitation of two or three successive IJP's was sometimes observed. The IJPs were unaffected by guanethedine but were blocked by procaine. IJPs could not be elicited by stimulation of either the pelvic parasympathetic or periarterial sympathetic nerves. In some cells, the IJP was preceded by a brief depolarization, which probably results from the stimulation of excitatory nerves to the same cell.3. Excitatory junction potentials (EJPs) were observed in about 5% of cells of the longitudinal coat in response to stimulation of the intramural nerves. When the pelvic nerves were stimulated, EJPs were observed in most cells of both the longitudinal and circular muscle coats. The EJP in the guinea-pig colon was blocked by hyoscine.4. The interaction of the IJP evoked by intrinsic nerve stimulation and the EJP evoked by pelvic nerve stimulation was examined. When both nerve supplies were stimulated together, only an IJP was observed.5. The nature of the innervation of the colon and the origin of intrinsic non-adrenergic inhibitory neurones is discussed.
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