Transmission from intramural inhibitory nerves to the smooth muscle of the guinea-pig taenia coli.

1. Membrane potential changes of smooth muscle cells were recorded during stimulation of the intramural inhibitory nerves to the taenia coli.2. Stimulation across the taenia coli with single pulses of 200 musec duration excites the intramural nerves and not the muscle directly.3. The membrane potential changes due to stimulation of the intramural inhibitory nerves were different from those produced by perivascular inhibitory nerve stimulation in the following ways: hyperpolarizations (i.j.p.'s) of up to 25 mV were produced in response to single pulses; the latency, i.e. the time taken for the membrane to hyperpolarize after a stimulus of maximal strength, was as short as 80 msec; when the nerves were stimulated repetitively the membrane was hyperpolarized by up to 35 mV and all spontaneous activity was abolished; the mean hyperpolarization due to repetitive stimulation increased with the frequency of stimulation up to 10 pulses/sec and then remained constant; the hyperpolarization due to stimulation at frequencies greater than 5 pulses/sec was not maintained but decreased after 3-5 sec of stimulation; and finally when stimulation had ceased action potentials commenced firing at frequencies greater than normal.4. The amplitude and rate of hyperpolarization of the i.j.p. increased with increasing strength of stimulation until a maximum amplitude and rate of hyperpolarization was reached. The recovery or depolarizing phase of the i.j.p. was exponential with a time constant which varied from about 250 msec to 500 msec and could not therefore be due to the discharge of the membrane capacitance. In some cases there was an inflexion on this depolarizing phase and in these cases recovery led directly into an action potential.5. Spontaneous hyperpolarizations of the membrane were seen in some cells, and these hyperpolarizations were similar to those recorded on submaximal stimulation of the intramural nerves.6. There were no changes in the characteristics of the i.j.p. in the presence of guanethidine or bretylium.