Three layers of the opossum stomach: responses to nerve stimulation

The three muscle layers of the opossum stomach were compared in their responses to electrical field stimulation. Strips were cut from the muscular wall of the stomach with their longer axes in the direction of the longitudinal, circular, and oblique muscle layers and stimulated at 1-min intervals with 20-sec trains of 1-msec pulses at 10 Hz and supramaximal strength at 500 ma. Strips either contracted, relaxed, or did not respond during the stimulus trains. Responses were abolished by tetrodotoxin, 10(-6) M. Contractions, but not relaxations, were abolished by atropine, 10(-6) M. Relaxations were not antagonized by propranolol, tolazoline, or phenoxybenzamine. Thus, the responses represent excitation of both cholinergic excitatory and nonadrenergic inhibitory nerves. In strips cut longitudinally, cholinergic contractions occurred more frequently than in the other two kinds of strips. After atropine treatment, the incidence of relaxation was greater in strips cut in the direction of the oblique layer than in the other two groups. The results suggest that, of the two kinds of nerves, the cholinergic excitatory innervation is usually dominant in the longitudinal muscle layer, and that the nonadrenergic inhibitory innervation is more complete in the oblique muscle layer.