Brain stem projections of sensory and motor components of the vagus complex in the cat: II. Laryngeal, tracheobronchial, pulmonary, cardiac, and gastrointestinal branches

Abstract

The central sensory and motor connections of various respiratory, cardiovascular, and gastrointestinal viscera were analyzed using the transganglionic and retrograde transport of horseradish peroxidase (HRP). In 42 adult cats, we examined the brain stem and peripheral ganglia following microinjections of HRP (10 μl) into individual visceral organs—larynx, extrathoracic trachea, intrathoracic trachea, right main bronchus, right lung (upper lobe), heart, and stomach. Comparison of individual cases led to the conclusion that distinct patterns of sensory and motor projections to the medulla exist for each visceral organ studied. The nucleus of the tractus solitarius (nTS) receives the sensory projections from all the viscera listed above, with two exceptions: (1) a few sensory fibers from the larynx terminate in the ipsilateral spinal tract of the trigeminal nerve (spV), and (2) some sensory fibers from the bronchus, lung, and stomach terminate in the area postrema (ap). Within the nTS, the sensory fibers from each visceral organ terminate in a number of subnuclei. The dnTS, mnTS, and ncom receive sensory projections from all the viscera studied. The remaining five subnuclei (dlnTS, ni, nI, vlnTS, vnTS) of the nTS are not connected to all viscera, and the density of projections to these regions varies for different viscera. However, there does not seem to be any specific region of the medulla which is devoted entirely to receiving the sensory fibers from a particular visceral organ. Rather, the rostrocaudal extent of sensory fibers, from most of the viscera studied, spans the entire length of the medulla. Differences in the central representation of viscera were found to be subtle and to lie within the organization of the nuclear subgroups of the nTS. The central representation of unpaired or midline viscera (e.g., trachea and heart) is bilateral for both sensory and motor innervation. However, for unilateral, paired viscera (e.g., bronchi and lungs), it was consistently found that over one third of the sensory and motor representation is contralateral. Control experiments involving vascular injections of HRP excluded the possibility that this contralateral labeling could have been due to vascular uptake of the enzyme. The localization of sensory perikarya of visceral afferents in the “principal visceral ganglion” of the vagus—the nodose ganglion—was overlapping, and no well‐demarcated regions in the nodose ganglion could be identified that received projections primarily from a given visceral organ. The motor nuclei providing parasympathetic (preganglionic) and somatic motor innervation to the viscera were primarily the dmnX, nA, and nRA. The entire dmnX (extending over 10–15 mm rostrocaudally), contributed fibers to each area injected with HRP, with the exception of the extrathoracic trachea. No region in the dmnX was found where preference was given to a specific viscus. The nA contributed efferents to all the viscera studied, and this contribution came from the entire 6 mm of nA contributing vagal efferents. Visceral containing smooth muscle as well as skeletal muscle were innervated by the nA. The caudal nRA provided motor fibers to the larynx, trachea, and stomach, and again no preferred rostrocaudal representation of motoneurons to a given viscus was found. Postganglionic sympathetic innervation to the viscera studied was found to be localized to the stellate and superior cervical ganglia. Within these sympathetic ganglia, some regional preference for different viscera was detected. In the case of motor innervation, unilateral paired viscera received motor fibers from both ipsilateral and contralateral sides of the medulla via both vagus nerves.