Vagal innervation of the rat duodenum

Electrophysiologic and anterograde tract tracing studies have demonstrated that the vagus nerve innervates the duodenum. These studies, however, have provided little information regarding the finer anatomic topography within the vagal complex. In this study, the retrograde neuronal tracers WGA-HRP or DiI, applied to the duodenum, were used to characterize the vagal afferent and efferent innervation of this portion of the gastrointestinal tract. This approach labeled a substantial number of motor neurons in both the medial and lateral columns of the dorsal motor nucleus of the vagus (DMNV). Vagal motor neurons innervating the duodenum were seen across the medial-lateral extent of the DMNV and between 600 microm rostral to obex and 1600 microm caudal to obex. The three branches of the vagus nerve contained efferent fibers to the duodenum. The gastric branch of the vagus nerve was the pathway that connected the majority of DMNV neurons with the duodenum. These neurons were located in the medial and middle thirds of the DMNV. The celiac branch to the duodenum was composed of axons from the majority of lateral column neurons but also contained axons from neurons in the medial column. The hepatic branch of the vagus nerve contained only a small number of cell axons. Some neurons were located medially whereas others were in the lateral third of the duodenum. Although central terminations of vagal primary afferents from the duodenum were not found in previous tract tracing studies, we observed a large number of terminals in the subpostremal/commissural region of the nucleus of the solitary tract. Similar to the motor fibers, most afferent fibers from the duodenum were located in the gastric branch of the vagus nerve, although the hepatic and celiac branches also contained afferent neurons. These results demonstrate that the vagal innervation of the duodenum is unique, being an amalgam of what would be expected following labeling of more proximal and distal portions of the GI tract. The uniqueness of the sensory and motor innervation to the duodenum has implications for hypotheses regarding the organization of vagovagal reflexes controlling gastrointestinal function.