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

Single-unit afferent activity was recorded from the hepatic-duodenal branch of the vagus nerve of chloralose-anaesthetized sheep during acute electrophysiological experiments. The impulse activity of sixty-seven 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, thus satisfying the criteria for 'in series' tension receptors (Iggo, 1955). From the responses to compression some evidence was found for the existence of separate populations of tension receptors with different mechanical thresholds. Two fibre populations were found: non-myelinated (0.70 +/- 0.26 S.D. ms-1) and myelinated (7.6 +/- 1.6 S.D. 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 anaesthesia of the mucosa. It is concluded that tension receptors in the sheep duodenum occupy a position 'in series' with longitudinal muscle, and that 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 two other mechanoreceptor classes located serosally (five units) and in the lesser omentum (eleven units). Receptors in neither of these two classes were directly excited by active contraction of the duodenum.