Abstract
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.
Keywords
Vagus nerve, Visceral afferents, Paraganglia, Nodose ganglion, Nucleus of the solitary tract, Immunosensing, Nociception, Defensive mechanisms
Introduction
The vagus nerve is different from the other cranial nerves in that it predominantly supplies the thoracic and abdominal cavities. It got the name from the ‘wandering’ character of its complex course along the esophagus and major arteries. Considering the theme of the present volume, the goal of this brief review is to provide a sound background of the anatomical aspects of this ‘wanderer’ and relate them to specific functions as they are known, in particular to its potential role in thermoregulation and immune-brain communication. Clearly the nerve itself is merely a cable, and the most interesting parts are its peripheral interface with specific sensors and effectors, as well as its central interface with the rest of the brain. Nevertheless, knowledge of the branching pattern of the nerve and its variations in individual animals is important for selective vagotomy experiments. In addition, the nerve trunks themselves contain more than just axons and glial cells. The presence of dendritic cells belonging to the immune defense system has been recently demonstrated in cervical and subdiaphragmatic vagal trunks (Goehler et al., 1999). In addition, paraganglia are often embedded in the major trunks near branch points (Prechtl and Powley, 1985; Kummer and Neuhuber, 1989; Dahlqvist et al., 1994). Therefore, we will first have a look at the gross anatomy and branching pattern of the vagus nerves and then focus on the peripheral and central interfaces.
Acknowledgements
We thank Laurel Patterson for editorial assistance. Research of the authors is partially funded by National Institute of Health Grants DK 47348 and DK 57242, and the Pennington Foundation (HRB), and Deutsche For- schungsgemeinschaft SFB 353/B9 (WLN).