Functional and chemical anatomy of the afferent vagal system

The results of neural tracing studies suggest that vagal afferent fibers in cervical and thoracic branches innervate the esophagus, lower airways, hea rt, aorta, and possibly the thymus, and via abdominal branches the entire g astrointestinal tract, Liver, portal vein, billiary system, pancreas, but n ot the spleen. In addition, vagal afferents innervate numerous thoracic and abdominal paraganglia associated with the vagus nerves. Specific terminal structures such as flower basket terminals, intraganglionic laminar endings and intramuscular arrays have been identified in the various organs and or gan compartments, suggesting functional specializations. Electrophysiologic al recording studies have identified mechano- and chemo-receptors, as well as temperature- and osmo-sensors. In the rat and several other species, mos tly polymodal units, while in the cat more specialized units have been repo rted. Few details of the peripheral transduction cascades and the transmitt ers for signal propagation in the CNS are known. Glutamate and its various receptors are likely to play an important role at the level of primary affe rent signaling to the solitary nucleus. The vagal afferent system is thus i n an excellent position to detect immune-related events in the periphery an d generate appropriate autonomic, endocrine, and behavioral responses via c entral reflex pathways. There is also good evidence for a role of vagal aff erents in nociception, as manifested by affective-emotional responses such as increased blood pressure and tachycardia, typically associated with the perception of pain, and mediated via central reflex pathways involving the amygdala and other parts of the limbic system. The massive central projecti ons are likely to be responsible for the antiepileptic properties of affere nt vagal stimulation in humans. Furthermore, these functions are in line wi th a general defensive character ascribed to the vagal afferent, paraventri cular system in lower vertebrates. (C) 2000 Elsevier Science B.V. All right s reserved.