Localization of laryngeal motor neurons in the kitten

In a series of 12 newborn kittens, horseradish peroxidase (HRP) was used to trace retrograde axoplasmic flow in the motor neurons to laryngeal muscles. The animals were sacrificed 24 hours after injection of HRP into a specific laryngeal muscle, and the brain stems were stained for peroxidase. This clear-cut colorimetric method permitted the localization of the motor neurons in two nuclei of the ipsilateral brain stem. These are the nucleus ambiguus and the retrofacial nucleus. The primary source of laryngeal motor supply is the nucleus ambiguus. All the laryngeal muscles were represented here in two divisions. Adductor neurons were located in the dorsal division and were more loosely arranged in the lateral reticular formation. The abductor neurons of the posterior cricoarytenoid muscle formed the compact ventral division of the nucleus ambiguus and were fewer in number than adductor neurons by a factor of four to one. Since the expiratory and inspiratory centers are also located dorsally and ventrally in the brain stem reticular formation, the motor cells of the nucleus ambiguus are conveniently arranged to receive their afferent input. This arrangement is probably the result of phylogenetic development of abductor laryngeal function and pulmonary function in lower forms. A second source of laryngeal innervation is the retrofacial nucleus. This small nucleus is situated rostral to the nucleus ambiguus and is made up of small and medium-sized neurons of at least two types. Only the cricothyroid (CT) and posterior cricoarytenoid (PCA) muscles were shown to have significant innervation from this nucleus. The CT neurons were located peripherally while the PCA cells occupied the central portion of the nucleus. The functional significance of this nucleus is unknown, but it is suggested that it may have something to do with the various types of muscle units that have been demonstrated physiologically in the CT and PCA muscles.