Motoneurones of the submucous plexus regulate electrical activity of the circular muscle of canine proximal colon.

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Abstract
1. The hypothesis that the circular muscle of the canine proximal colon receives motor input from neurones in the submucous plexus was tested. Circular muscle cells were impaled with micro-electrodes and submucous plexus neurones were stimulated by electrical field stimulation and microejection of acetylcholine (Ach). 2. In the presence of atropine to block the direct muscarinic effects, microejection of ACh onto the submucosa where intact submucous ganglia were suspended evoked: (i) an inhibitory junction potential (i.j.p.) that reduced the amplitude, duration and rate of rise of the subsequent slow wave; (ii) a slow wave of increased duration following the initial inhibitory response. These responses were enhanced by increasing the volume of ACh administered. Responses to ACh were blocked by hexamethonium, 10-4 M; d-tubocurarine, 10-4 M; or tetrodotoxin (TTX), 10-6 M, suggesting they were neural in origin. 3. Both inhibitory and excitatory responses were the result of non-cholinergic and non-adrenergic nerves. The transmitters mediating these effects are unknown. 4. Removal of the longitudinal muscle, myenteric plexus, and the serosal portion of the circular muscle had no apparent effect on the responses to application of ACh to submucosal ganglia. In these preparations the responses to field stimulation were identical to those produced by ACh. 5. The submucous plexus also provides cholinergic input to the circular muscle. When ACh was discretely applied to the submucosa cholinergic responses were elicited at the muscle cell which were significantly reduced by hexamethonium or TTX. These findings suggest that the cholinergic responses were the result of ACh release by neurones at the effector and not by overflow of the exogenous ACh. Cholinergic responses were also elicited in preparations in which the myenteric plexus had been removed. 6. Slow waves in circular muscle of the proximal colon yield excitation-contraction coupling in the absence of Ca2+ action potentials. Therefore the influence of submucous neurones on electrical slow waves has direct consequences on motor activity. Reduction in the amplitude and duration of slow wave by i.j.p.s results in reduction in the amplitude and duration of phasic contractions. Excitatory inputs enhance contractions. 7. The data support a new concept: motoneurones emanating from submucous ganglia innervate the circular muscle and provide inhibitory and excitatory inputs to regulate slow wave activity. There appear to be at least three types of motoneurones activated by nicotniic stimulation of the ganglia or by field stimulation: (i) non-adrenergic inhibitory nerves, (ii) cholinergic excitatory nerves, and (iii) non-cholinergic, non-adrenergic excitatory nerves. These nerves may regulate colonic motility patterns.