Mucosal enteroceptors with vagal afferent fibres in the proximal duodenum of sheep.

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Abstract
Single-unit afferent activity was recorded from the hepatic-duodenal nerve of chloralose-anesthetized sheep using electrophysiological techniques. Impulse activity was examined while the duodenal mucosa was irrigated with different chemical solutions and explored mechanically. Sixty-one mucosal mechanoreceptors were studied and these were divided into 3 classes of afferent response. Thirty-four U had no background activity. These silent units had a mechanical threshold of 132 mg and a slowly adapting response to maintained probing. They had non-myelinated axons (mean conduction velocity 0.93 m s-1). Five U adapted rapidly to mucosal probing. Twenty-one units had spontaneous background activity. These spontaneous units had a threshold to probing of 750 mg, and a characteristic sustained after-discharge and periods of mechanical insensitivity; 75% of them were non-myelinated (mean conduction velocity 0.73 m s-1). The other units had a mean conduction velocity of 3.3 m s-1. The behavior of most sensory units remained within these classes with the exception of 10 otherwise silent units whose activity became spontaneous after the application of prostaglandin F2.alpha. or NaOH solutions. Eighty percent of all units were sensitive to a variety of chemical solutions. Five silent U and 4 spontaneous U were insensitive to the chemical solution applied. The responses to chemical solutions were sustained for up to 90 s but units became refractory to repetitive treatments, often becoming insensitive after the 2nd application. A chemically specific population was subdivided into 2 groups: subgroup 1, 32 U were excited by KCl solutions (5 rapid adapting, 20 silent, 7 spontaneous); subgroup 2, 12 U were excited by volatile fatty acid solutions (8 silent U, 4 spontaneous). Three U were excited by both KCl and volatile fatty acid solutions (2 silent, 1 spontaneous). None of the units tested was sensitive to the osmolality of the solutions applied. Units were not excited by thermal changes over the range 10-45.degree. C and all were mechanically sensitive. No chemoreceptor or thermoreceptor with absolute specificity was found. The conclusions of this study are as follows: with 5 exceptions, all duodenal mucosal units had sustained responses to probing the mucosa. They therefore appear to differ from gastric mucosal units described as rapidly adapting mechanoreceptors with chemical sensitivity. The differences reported previously may be due to the difficulty of exciting sustained responses in conditions of poor mechanical stability. Because units became refractory to repetitive applications of hydrochloric acid it is concluded that they do not continuously monitor the acidity of duodenal chyme. For this, a sustained afferent input to the CNS is required. The refractoriness may, in part, be due to duodenal mucosal secretion. The low mechanical thresholds to probing and the insensitivity to stretch do not suit mucosal units to initiate reflexes requiring large mechanical thresholds such as occur during intestinal impaction and distension. Duodenal mucosal receptors have similar mechanical thresholds to rumen epithelial receptors, but unlike these, the mucosal receptors found were not osmosensitive over the range of solutions applied. Their threshold to alkaline solutions containing NaOH was the same as that required for experimentally induced inhibition of abomasal emptying in calves.