Musings on the wanderer: what's new in our understanding of vago-vagal reflexes? I. Morphology and topography of vagal afferents innervating the GI tract.

An understanding of the events initiating vago-vagal reflexes requires knowledge of mechanisms of transduction by vagal afferents. Such information presumes an understanding of receptor morphology and location. Anatomic studies have recently characterized two types of vagal afferents, both putative mechanoreceptors distributed in gastrointestinal (GI) smooth muscle. These two receptors are highly specialized in that they 1) are morphologically distinct, 2) have different smooth muscle targets, 3) form complexes with dissimilar accessory cells, and 4) vary in their regional distributions throughout the GI tract. By comparison, information on the architecture and regional distributions of other classes of vagal afferents, notably chemoreceptors, has only begun to accumulate. Progress on the study of the two mechanoreceptors, however, illustrates general principles and delineates experimental issues that may apply to other submodalities of vagal afferents. By extension from morphological and physiological observations on the two species of smooth muscle endings, it is reasonable to hypothesize that additional classes of vagal receptors are also differentiated morphologically and that they vary in structure, accessory cells, regional distributions, and other features. A full appreciation of vago-vagal reflexes will require thorough structural and regional analyses of each of the types of vagal receptors within the GI tract.