Nucleus ambiguus of the rabbit: cytoarchitectural subdivision and myotopical and neurotopic representations.

The cytoarchitectural subdivisions of the nucleus ambiguus of the rabbit and its myotopical and neurotopical representations were investigated with HRP labeling. The nucleus was subdivided into the compact cell group (CoG), the medial and lateral scattered cell groups (SGm and SGl), and the diffuse cell group (DiG). The CoG was formed by esophageal, pharyngeal constrictor, and palatal motoneurons in the rostral half of the nucleus. The SGm and SGl were located medial and lateral to the CoG, respectively, in the rostral one-third of the nucleus. Stylopharyngeal and cricothyroid motoneurons were located in the most rostral one-fifth of the SGm and the remaining four-fifths, respectively, whereas the SGl was not labeled with HRP injections into the palatal, pharyngeal, esophageal, and laryngeal muscles. The DiG was formed by recurrent laryngeal motoneurons in the caudal two-thirds of the nucleus. Neurons of origin for the glossopharyngeal nerve occupied the stylopharyngeal region, with a few of them scattered in the CoG and SGl. Neurons giving rise to axons in the superior laryngeal nerve occupied the cricothyroid region, with a few of them scattered in the pharyngeal constrictor region; whereas the pharyngeal vagal branch originated from the pharyngeal constrictor and palatal regions. Neurons of the DiG, SGl, and esophageal region contributed to the infranodosal vagus nerve; esophageal fibers of the recurrent laryngeal nerve originated from the dorsal esophageal region. Laryngeal fibers of the recurrent laryngeal nerve originated from the DiG, the caudal neurons of which had axons traversing the cranial accessory root.