[Localization of the rabbit pharyngeal motoneurons and peripheral courses of their axons: a study by means of the retrograde HRP or fluorescent labeling technique].

The localization of the rabbit pharyngeal montoneurons in the nucleus ambiguus and peripheral courses of their axons were investigated using injection of retrograde labeling tracers, i.e., HRP and nuclear yellow, into the individual pharyngeal muscles or using the injection in conjunction with intracranial severing of either the vagal or glossopharyngeal rootlets. The nucleus ambiguus of the rabbit was divided into four groups of neuron, of which the following two, the compact cell group (CoG) and the medial scattered group (SGm), were pertinent to the pharyngeal montoneurons. The CoG is a group formed by a compact arrangement of the smallest neurons and situated in the rostral half of the nucleus ambiguus. The SGm is a group formed by a scattered arrangement of slightly larger neurons in the rostral one-third of the nucleus and located medial to the CoG. The labeled stylopharyngeal motoneurons occupied the rostral portion of the SGm at a level from about 2800 to 3100 microns rostral to the obex. They were completely abolished by severing the glossopharyngeal rootlets, in contrast to no change of labeled neuron number when severing the vagal rootlets. Thus, all axons of the stylopharyngeal motoneurons were concluded to traverse the glossopharyngeal rootlets. The labeled palatopharyngeal motoneurons were found in the caudal portion of the CoG at a level from 500 to 1900 microns rostral to the obex, with their number being numerous at the caudal one-third of CoG, 500 to 1300 microns. All their axons traversed the vagal rootlets. The labeled pharyngeal constrictor motoneurons were found at almost all rostrocaudal levels of CoG, 500 to 2900 microns rostral to the obex, with their number being numerous in its middle one-third level. Although motoneurons of the superior constrictor, those of the middle constrictor, and those of the thyro and cricopharyngeal muscles composing the inferior constrictor overlapped rostrocaudally in location, their ranges of appearance had a tendency of arranging rostrocaudally in that order. At the middle one-third level of CoG, in which the CoG was subdivided into two subgroups, dorsomedial and ventrolateral, the superior and middle constrictor motoneurons were located in its entire portion. The majority of axons of the pharyngeal constrictor motoneurons traversed the vagal rootlets, though a few axons whose somata lay in the rostral portion of the CoG traversed the glossopharyngeal rootlets.