Tension receptors with vagal afferent fibres in the proximal duodenum and pyloric sphincter of sheep.

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Abstract
Single-unit afferent activity was recorded from the hepatic-duodenal branch of the vagus nerve of chloralose-anesthetized sheep during acute electrophysiological experiments. The impulse activity of 67 slowly adapting mechanoreceptors situated in the muscularis externa of the proximal duodenum and pyloric sphincter was synchronous with alterations in electromyographic and tension records. Afferent units were excited during passive distension, compression and drug-induced increases in muscle tension, satisfying the criteria for in series tension receptors. From the responses to compression some evidence was found for the existence of separate populations of tension receptors with different mechanical thresholds. Two fiber populations were found: non-myelinated (0.70 .+-. 0.26 SD m s-1) and myelinated (7.6 .+-. 1.6 SD m s-1). Mucosal application of solutions of hydrochloric acid, volatile fatty acids, alkali and amino acids, and mucosal probing modified the activity of most units. These changes were reduced by anesthesia of the mucosa. Tension receptors in the sheep duodenum occupy a position in series with longitudinal muscle, and their activity can be modified by the particulate and chemical composition of chyme by a mechanism involving local nerve plexuses. The activity of tension receptors is compared with that of 2 other mechanoreceptor classes located serosally (5 U) and in the lesser omentum (11 U). Receptors in neither of these 2 classes were directly excited by active contraction of the duodenum.