Spontaneous and electrically-elicited motor activity was recorded by triple organ bath in rat segment-model preparation as display of excitation of local nerve networks and ascending or descending reflex pathways underlying contractile potency and functional coordination of colonic longitudinal and circular muscles. Spontaneous high-amplitude contractions, but not relaxations, appeared synchronously in both muscles. Electrical field stimulation applied to proximal or distal part of segments elicited both tetrodotoxin (0.1 µM)-sensitive local motor responses of the stimulated part and ascending or descending motor responses of the contralateral, nonstimulated part of the preparations. Contractions characterized the local response of longitudinal muscle. The circular muscle responded with relaxation followed by contraction. Synchronous ascending contractions and descending contraction of the longitudinal muscle and relaxation followed by contraction of the circular muscle were observed when the middle part of segments was stimulated, thus indicating that locally-induced nerve excitation propagated via intrinsic ascending or descending nerve pathways that could be synchronously coactivated by one and the same stimulus. The ascending motor responses were more pronounced and the motor responses of longitudinal muscle were expressed more than those of circular muscle suggesting an essential role of ascending reflex pathways and longitudinal muscle in the coordinated motor activity of colon.
We evaluated the motor responses in recto-anal preparations obtained from rats, in terms of the excitation displayed by modules of nerve networks and descending distally directed pathways, when subjected to the mechanographic on-line technique, a partitioned organ bath, electrical stimulation (EFS, 0.8 ms, 5 Hz) and distension. EFS elicited modular contractions, which increased in amplitude distally, in circular muscle rings isolated from the proximal, middle or distal rectum. The modular responses of the internal anal sphincter or anal canal were relaxation or contraction, respectively. The application of EFS to the distal rectum induced a descending contractile response in the anal canal (5.24±0.34 mN), while distension by balloon evoked a descending response consisting of contraction (1.72±0.20 mN) followed by relaxation (3.42±0.24 mN). The responses were sensitive to tetrodotoxin. Atropine considerably depressed the contractions in all preparations. Whether or not atropine was present, L-NNA increased the excitatory responses, while L-arginine decreased the contractions and extended the relaxation of internal anal sphincter and anal canal. The results suggest that excitatory neurotransmission(s) expressed in the distal rectum dominate modular nerve networks. Functionally-different descending pathways are involved in the motor activity of the anal canal. Stimulatory cholinergic pathways are dependent on the electrically-induced excitation, and inhibitory nitrergic pathways are sensitive to distension of rectal wall.
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