The results of neural tracing studies suggest that vagal afferent fibers in cervical
and thoracic branches innervate the esophagus, lower airways, heart, aorta, and possibly
the thymus, and via abdominal branches the entire gastrointestinal tract, liver, portal
vein, billiary system, pancreas, but not 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 organ
compartments, suggesting functional specializations. Electrophysiological recording
studies have identified mechano- and chemo-receptors, as well as temperature- and
osmo-sensors. In the rat and several other species, mostly polymodal units, while
in the cat more specialized units have been reported. Few details of the peripheral
transduction cascades and the transmitters 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 afferent signaling to the solitary nucleus. The vagal afferent system is
thus in an excellent position to detect immune-related events in the periphery and
generate appropriate autonomic, endocrine, and behavioral responses via central reflex
pathways. There is also good evidence for a role of vagal afferents 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 projections are likely to be responsible for the antiepileptic properties
of afferent vagal stimulation in humans. Furthermore, these functions are in line
with a general defensive character ascribed to the vagal afferent, paraventricular
system in lower vertebrates.