AUTONOMIC NEURONS SENDING FIBERS INTO THE CANINE LARYNGEAL NERVES

Abstract

The distribution of the autonomic neurons sending fibers into the canine superior and inferior laryngeal nerves was investigated by an immunohistochemical technique with cholera toxin B subunit as a retrograde tracer. Cholera toxin was applied to the right internal branch of the superior laryngeal nerve (SLNI), external branch of the superior laryngeal nerve (SLNE) or inferior laryngeal nerve (ILN) in 2-month-old dogs. In every case of cholera toxin application, labeled neurons were seen mainly in the caudal portion of the ipsilateral superior cervical ganglion, whereas none were seen in the stellate ganglion. The number of labeled neurons in the superior cervical ganglion after application of cholera toxin to the SLNI was over 20 times that observed after application to the SLNE or ILN. When cholera toxin was applied to the SLNI or SLNE, labeled neurons were found mainly in the ipsilateral dorsal motor nucleus of the vagus (DMNV), and a few labeled neurons were found in the ipsilateral reticular formation. The neurons in the DMNV were localized, on the average, between 2.7 mm and 5.3 mm rostral to the obex. The number of neurons in the DMNV labeled retrogradely from the SLNI was much larger than that from the SLNE. In contrast, no labeled neurons were detected in the DMNV after application to the ILN. These results demonstrate the following: 1) The sympathetic neurons innervating the canine larynx are distributed mainly in the caudal portion of the superior cervical ganglion and they send their fibers mainly into the SLNI. 2) The parasympathetic neurons innervating the canine larynx have a limited distribution in the dorsal motor nucleus of the vagus and their main pathway is also the SLNI.