Impulse transmission in the myenteric plexus-longitudinal muscle preparation of the guinea-pig ileum.

1. In a preparation consisting of the myenteric plexus and the longitudinal muscle layer removed from a segment of guinea-pig ileum, spontaneous action potentials occurred which were unaffected by tetrodotoxin but suppressed by Mn(2+) and were therefore myogenic.2. A single current pulse of 0.1 ms duration evoked a response consisting of an early action potential followed after a delay of about 200 ms by a complex of biphasic spikes. The first action potential was conducted for no more than 15 mm and the second complex for 30-70 mm.3. Since the first action potential was unaffected by hyoscine or Mn(2+) but abolished by tetrodotoxin, it was due to excitation of nerve fibres. The later complex of spikes was suppressed by hyoscine and Mn(2+) and therefore due to excitation of smooth muscle. It was also inhibited by adrenaline or morphine, compounds which depress acetylcholine release. The evoked smooth muscle response was followed by absence of spontaneous electrical activity for 2-4 seconds.4. The nerves travelling in a longitudinal direction had a mean maximum conduction velocity of 0.65 m/s, an absolute refractory period of 2.8 ms and a relative refractory period of about 20 ms.5. The conduction velocity of the smooth muscle response evoked by stimulation of the nerve with a single pulse was 0.16 m/second. After a single pulse the muscle was inexcitable for 0.7-1.3 s; the delay of transmission from nerve to muscle was 210 ms. When instead of a single pulse a train of two-five pulses at 20 ms intervals was applied, the size, conduction distance and conduction velocity of the evoked smooth muscle response were increased.