Kimio Ogata

Localization of rabbit laryngeal motoneurons in the nucleus ambiguus

The localization of rabbit laryngeal motoneurons in nucleus ambiguus was investigated using injection of a fluorescent labeling substance, i.e., nuclear yellow, into the individual laryngeal muscles. The nucleus ambiguus of the rabbit comprises four subnuclei, CoG, SGm, SGl, and DiG. The CoG is a group of compactly arranged neurons, and is situated in the rostral one-half of the nucleus. The SG, situated in its rostral one-third, is scattered around the CoG, with a subdivision into SGm and SGl. These subdivisions are medial and lateral to the CoG, respectively. The DiG is formed by diffusely arranged neurons, and is located in the caudal two-thirds of the nucleus. All labeled motoneurons were found in the ipsilateral nucleus ambiguus. The motoneurons supplying the cricothyroid muscle, which is innervated by the superior laryngeal nerve, were present in the SGm, with a clear rostralward segregation from the other motoneurons. The motoneurons supplying the muscles innervated by the inferior laryngeal nerve were located in the DiG, where they displayed a rostrocaudal myotopical arrangement in the order posterior cricoarytenoid, thyroarytenoid, and lateral cricoarytenoid. The posterior cricoarytenoid motoneurons were intermingled with the thyroarytenoid motoneurons in the rostral two-thirds of the DiG, and the former tended to be concentrated more rostrally than the latter. The lateral cricoarytenoid motoneurons were confined to the most caudal one-third of the DiG.

Fibers supplying the laryngeal musculature in the cranial root of the rabbit accessory nerve : Nucleus of origin, peripheral course and innervated muscles

The location of the rabbit laryngeal motoneurons whose axons traverse the cranial root of the accessory nerve was studied with injection of HRP or nuclear yellow into the laryngeal muscles in combination with the intracranial severing of either the rootlets of the vagus nerve or those of the cranial root. The motoneurons were located in the diffuse cell group that forms a subnucleus occupying the caudal two-thirds of the nucleus ambiguus and sending fibers to the inferior laryngeal nerve. The caudal one-third of the diffuse cell group supplying the lateral cricoarytenoid muscle, was occupied only by these motoneurons, whereas in its rostral two-thirds, they were intermingled with motoneurons having axons that traversed the vagal rootlets. The thyroarytenoid and posterior cricoarytenoid motoneurons are present in the rostral two-thirds of the diffuse cell group; axons of the former traversed the rootlets of the cranial root, and of the latter traversed the vagal rootlets. On the other hand, the medial scattered cell group, located in the rostral one-third of the nucleus ambiguus and sending fibers to the cricothyroid muscle via the superior laryngeal nerve, contained only motoneurons with axons traversing the vagal rootlets. The above findings clarified that fibers of the cranial root enter the inferior laryngeal nerve after joining the vagus, and then reach the adductor muscles for the vocal fold, with their neurons of origin in a caudal portion of the nucleus ambiguus. The vagal rootlet fibers, with their neurons of origin situated more rostrally in the nucleus, innervate the tensor and abductor muscles via the superior and inferior laryngeal nerve, respectively.