ELECTROPHYSIOLOGICAL STUDIES ON THE INTESTINAL INTRINSIC REFLEX

The role of enteric inhibitory neurons in intestinal motility

The enteric nervous system controls much of the mixing and propulsion of nutrients along the digestive tract. Enteric neural circuits involve intrinsic sensory neurons, interneurons and motor neurons. While the role of the excitatory motor neurons is well established, the role of the enteric inhibitory motor neurons (IMNs) is less clear. The discovery of inhibitory transmission in the intestine in the 1960's in the laboratory of Geoff Burnstock triggered the search for the unknown neurotransmitter. It has since emerged that most neurons including the IMNs contain and may utilise more than one transmitter substances; for IMNs these include ATP, the neuropeptide VIP/PACAP and nitric oxide. This review distinguishes the enteric neural pathways underlying the 'standing reflexes' from the pathways operating physiologically during propulsive and non-propulsive movements. Morphological evidence in small laboratory animals indicates that the IMNs are located in the myenteric plexus and project aborally to the circular muscle, where they act by relaxing the muscle. There is ongoing 'tonic' activity of these IMNs to keep the intestinal muscle relaxed. Accommodatory responses to content further activate enteric pathways that involve the IMNs as the final neural element. IMNs are activated by mechanical and chemical stimulation induced by luminal contents, which activate intrinsic sensory enteric neurons and the polarised interneuronal ascending excitatory and descending inhibitory reflex pathways. The latter relaxes the muscle ahead of the advancing bolus, thus facilitating propulsion.

Reflex changes in circular muscle activity elicited by stroking the mucosa: an electrophysiological analysis in the isolated guinea-pig ileum

A preparation of isolated small intestine from the guinea-pig was studied in which reflex responses of the circular muscle were recorded intracellularly when sensory receptors in the mucosa were stimulated mechanically. This preparation was used to examine the properties of mucosa to muscle reflexes that involve non-cholinergic motor neurons innervating the circular muscle. Reproducible stimulation of the mucosa was achieved by stroking with a motor-driven brush. Gentle brush-strokes applied to the mucosa typically evoked inhibitory junction potentials anal to the stimulus and excitatory junction potentials at recording sites oral to the stimulus. Both events were rapid in onset and up to 25 mV in amplitude. The reflexes were blocked by tetrodotoxin (0.5 microM). Junction potentials declined in amplitude with distance from the stimulus, the amplitude of the excitation 15 mm oral to the stimulus was half that at 5 mm from the stimulus, whereas the amplitude of the inhibitory potential at 40-45 mm was about 60% of that at 5-10 mm anal to the stimulus. Hexamethonium (100-200 microM) blocked the ascending excitation but only slightly reduced the descending inhibition. Ascending excitation was blocked by antagonists for substance P receptors in the muscle, and inhibition was substantially reduced by apamin (0.2 microM), both before and after hexamethonium. Both responses were abolished by removal of the mucosa from the stimulus site and when lesions were made through the myenteric plexus between the stimulation and recording sites, but persisted when similar lesions were made through the submucous plexus. It is concluded that there are neurons with mechanoreceptive nerve endings in the mucosa. Stimulation of such sensory neurons leads to activation of pathways in the myenteric plexus that excite motor neurons to the muscle both oral and anal to the stimulation site. The demonstration that mucosa to muscle reflexes can be consistently evoked in the small intestine in vitro provides an opportunity for close analysis of the reflex pathways. Such analysis is not so readily achieved when reflexes are initiated by distension that, by moving the intestine, can dislodge the recording electrode.

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

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). 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. Both inhibitory and excitatory responses were the result of non-cholinergic and non-adrenergic nerves. The transmitters mediating these effects are unknown. 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. 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. 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. 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of jejunal flow by solutions of varying osmolality and acidity in the intact canine jejunum

Because gastric emptying is modulated by the osmolality or hydrogen ion concentration of intraduodenal solutions, we hypothesized that similar mechanisms exist to regulate flow within the small intestine and that the altered flow is associated with changes in intestinal motility. Sodium chloride solutions with higher and lower osmolality than plasma and both isotonic and hypotonic hydrochloric acid solutions flowed more slowly into segments of jejunum in conscious dogs than isotonic neutral salt solutions. The altered flow was associated with increased frequency of spike bursts in the segment with the salt solutions of varied osmolality, but with only increased force of contractions with both isotonic and hypotonic acid solutions. Some evidence exists to suggest that the changes in motility are mediated by a neural mechanism. If similar mechanisms exist in the duodenum, they may play a role in the regulation of gastric emptying.

Gastrointestinal myoelectrical activity in idiopathic intestinal pseudo-obstruction

We evaluated the myoelectrical and motor function of the esophagus, small intestine, colon and anal sphincter in four patients with chronic idiopathic intestinal pseudo-obstruction. All patients had aperistalsis of the esophagus, with incomplete relaxation of the lower esophageal sphincter after swallowing or balloon distension. Duodenal slow-wave frequency was normal at 11.4+/-0.3 (+/-S.E.M.) cycles per minute. The patients did not have a normal increase in duodenal spike or motor activity after intestinal distension, but duodenal activity increased after stimulation with intravenous secretin. Colonic slow-wave activities were present at two frequencies, 6.2+/-0.3 and 3.3+/-0.1 cycles per minute. Neostigmine administration increased both colonic spike and motor activity normally. These studies suggest that in this disorder, physiologic neural responses to swallowing or intestinal distension are impaired but the intestinal smooth-muscle slow-wave activity and the spike and motor responses to exogenous neurohormonal stimulation are intact.

Effect of quinidine on electrical and motor activity of canine small bowel

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