Direct vagal input to the gastrointestinal tract and other viscera: Re-definition of autonomic neuroscience or experimental artifacts?

The canonical autonomic nervous system pathway to peripheral targets consists of two neurons in series, i.e., preganglionic and postganglionic. However, several recent reports claim a direct innervation of some sub-diaphragmatic organs by neurons of the vagal dorsal motor nucleus without intercalated postganglionic neurons. The aim of this minireview is to summarize these studies e.g. on direct vagal preganglionic innervation of Brunner's glands, liver and spleen and to critically discuss their unorthodox findings and discrepancies with previous literature. We conclude that rigorous re-evaluation by complementary state-of-the-art strategies is obligatory.