Localization of motoneurons for each muscle innervated by the nucleus ambiguus (AM) was studied by means of the horseradish peroxidase (HRP) method' in 201 cats. Also, the exact extent of AM in the rostrocaudal direction was determined using the same technique. HRP labeled neurons were observed ipsilaterally for all the muscles except for the esophageal (E) muscle. The motoneurons of the stylopharyngeus (STP) were recognized in the retrofacial nucleus (RF). The motoneurons of the cephalopharyngeal (CeP), the hyopharyngeal (HP), the thyropharyngeal (TP), the cricopharyngeal (CP), E and the cricothyroid (CT) muscles were found in RF and the rostral part of AM. For the levator veli palatini (LVP) muscle, the motoneurons were observed in the rostral part of AM. The motoneurons of the posterior cricoarytenoid (PCA), thyroarytenoid (TA), lateral cricoarytenoid (LCA) and interarytenoid (IA) muscles were found in the caudal part of AM.
Beginning at the level immediately caudal to the superior olivary nucleus, the cell column of AM extended caudally to the level of the caudal end of the inferior olivary nucleus. The length of the cell column was 5.5-6.0 mm in adult cats. The cell column were divided into three sections cytoarchitecturally: the rostral one sixth which was loose, the next two sixths which was compact and the caudal half which was loose in formation. The rostral loose formation contained most of the cells innervating STP and a part of the cells innervating CeP and HP. The compact formation consisted of two portions: the large cell group which occupied large part of AM and was located ventrolaterally and the small compact cell group which was located dorsomedially. The former innervated E and CT, whereas the latter innervated Cep, HP and LVP. However, the motoneurons for TP and CP were supplied from both of them. The caudal loose formation was connected to those groups of the compact formation. It contained the cells innervating of PCA, TA, LCA and IA.
Following HRP injection into the nodose ganglion, the recurrent laryngeal nerve and the external branch of the accessory nerve, the junction between the caudal end of the cells column of AM and the motoneurons of the accessory nerve was clearly differentiated. There was no connection between these two groups of neurons.