Ultrastructural characterization of pharyngeal and esophageal motoneurons in the nucleus ambiguus of the rat

Abstract

The viscerotopic organization of the upper alimentary tract has been established in the nucleus ambiguus, but there is little information about the morphology of the individual neurons innervating the pharynx and esophagus. We studied the ultrastructure of pharyngeal (PH), cervical esophageal (CE), and subdiaphragmatic esophageal (SDE) motoneurons labeled by retrogradely transported wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) in the compact formation of the nucleus ambiguus. WGA-HRP was injected into the lower pharynx, or the cervical and subdiaphragmatic esophagus of male rats. The retrogradely labeled PH neurons in the rostral portion of the compact formation were large (26.1 x 50.1 microns, 906.7 microns2), polygonal, and contained well-developed cell organelles with a round nucleus. Subsurface cisterns connected with rough endoplastic reticulum were often present near the postsynaptic membrane. Both CE and SDE neurons in the compact formation were medium-sized, round or oval, and contained well-developed cell organelles, although the SDE neuron was significantly larger than the CE neuron (24.9 x 33.6 microns, 593.0 microns2 in the SDE neuron, and 19.5 x 30.2 microns, 440.3 microns2 in the CE neuron). The average number of axosomatic terminals in a sectional plane was largest in PH neurons (29.0), smaller in CE neurons (7.9), and smallest in SDE neurons (4.2). The number of axosomatic terminals containing round vesicles (Gray's type I) was almost equal to that of terminals containing pleomorphic vesicles (Gray's type II) in PH and CE neurons, but there were few Gray's type II axosomatic terminals in SDE neurons. Desmosome-like junctions at somato-somatic or somato-dendritic apposition were often present in the area surrounding SDE neurons. There were also small unlabeled neurons (9.5 x 18.1 microns, 131.8 microns2) in the compact formation of the nucleus ambiguus. The small neurons contained poorly developed cell organelles and an irregular shaped nucleus with invaginated nuclear membrane, and had no Nissl bodies. These results indicate that PH neurons have the characteristics of somatic motoneurons, and that CE and SDE neurons are similar to visceral motoneurons.

Ambiguus motoneurons innervating laryngeal and esophageal muscles are malpositioned in the Reelin-deficient mutant rat, Shaking Rat Kawasaki

“…This figure was obtained from a representative case of injection. Previous reports using the retrograde tracing method have shown the viscerotopic representation of laryngeal and esophageal motoneurons in the ambiguus nucleus of the normal adult rats [14,17]. In the P21 normal rat, motoneurons innervating different muscles may be intermingled, and therefore, in the present study, the viscerotopic representation of the laryngeal and esophageal muscles could not be confirmed in a strict sense (Figure 3, left column).…”

Ultrastructure of the central subnucleus of the nucleus tractus solitarii and the esophageal afferent terminals in the rat

“…The esophageal afferent terminals often form synaptic glomeruli with small dendrites and unlabeled terminals just like the gustatory and the superior laryngeal terminals, while, the esophageal terminals contacted unlabeled Gray's type I terminals by adherent junctions without forming chemical synaptic contacts. Similar contacts by adherent junctions have been reported at dendrodendritic, or somatodendritic contacts in the compact formation of the nucleus ambiguus and the inferior olivary nucleus (Llinµs et al 1974;Hayakawa et al 1996). These structures are thought to be associated with electronic coupling between neurons, though they are different from the electrical synapse (Llinµs et al 1974;Lewis 1994).…”

Calcitonin gene-related peptide immunoreactive neurons innervating the soft palate, the root of tongue, and the pharynx in the superior glossopharyngeal ganglion of the rat

“…The intermediate subnucleus projects directly to the motor neurons of the semicompact formation of the nucleus ambiguus (Bao et al, 1995;Hayakawa et al, 1998), which projects to the soft palate and the pharynx (Bieger and Hopkins, 1987;Hayakawa et al, 1996). Thus, the nociceptive information from the soft palate and the pharynx is thought to participate in a neuronal circuitry that includes the superior glossopharyngeal ganglion, the intermediate subnucleus of the NTS, and the semicompact formation of the nucleus ambiguus.…”