Properties of the inhibitory potential of smooth muscle as observed in the response to field stimulation of the guinea-pig taenia coli.

1. The inhibitory potential evoked by field stimulation of the guineapig taenia coli was studied in hypertonic solution in which the spontaneous electrical and mechanical activity was slowed or abolished.2. There was no direct correlation between the inhibitory potential and the muscle membrane polarization in response to the stimulating current. The chronaxie for the inhibitory potential was less than 1 msec and that for the muscle spike 20-30 msec.3. The inhibitory potential decreased in size along the tissue when one end of the tissue was stimulated, but its spatial decay was slower than that of the electrotonic potential in the muscle bundle.4. Tetrodotoxin (5 x 10(-7) g/ml.) abolished the inhibitory potential without affecting the electrical properties of the muscle membrane. It was concluded that the inhibitory potential was the result of stimulation of intrinsic inhibitory nerves.5. The following properties of the intrinsic nerve fibres were deduced from the experimental results: The length of the nerve fibres is probably only a few mm and the space constant of the order of 0.1 mm. No repetitive firing is produced by a long current pulse. The absolute refractory period is 3-4 msec.