Excitatory ascending and descending motor responses in the guinea pig small intestine: a comparative study of longitudinal and circular muscles by a triple bath method.

A triple organ bath was developed to study the ascending and descending reflexes in a guinea pig small intestine model, allowing synchronous recording of the motor activity of the longitudinal and circular muscle layers belonging to the oral and anal part of segment preparations. Field electrical stimulation (0.8 msec. 40 V. 5 Hz. 10 see) applied either to the anal or oral part of the segments elicited both tetrodotoxin (1 μM)-sensitive contractile local motor responses of muscle layers belonging to the stimulated part and ascending or descending motor responses of the contralateral, nonstimulated part at a distance of 20 mm. Simultaneous ascending and descending contractions of both muscle layers at a distance of 10 mm were observed when the electrical stimulation was applied to the middle part of the segments. Local responses of the circular muscle layer were considerably higher. The ascending motor responses of both muscle layers were more pronounced as compared to the descending responses and the motor responses of the longitudinal muscle layer were expressed more than those of the circular one. It is concluded that locally induced nerve stimulation propagated via intrinsic ascending or descending neural pathways could be synchronously coactivated by one and the same stimulus. Prominent ascending motor responses and contractility of the longitudinal muscle layer were observed, suggesting a predominant activity of the ascending reflex pathways and an essential role of the longitudinal muscle layer in orally directed reflexes.