Monosynaptic inputs from the nucleus tractus solitarii to the laryngeal motoneurons in the nucleus ambiguus of the rat

The cricothyroid (CT) and the posterior cricoarytenoid (PCA) muscles in the larynx are activated by the laryngeal motoneurons located within the nucle us ambiguus; these motoneurons receive the laryngeal sensory information fr om the nucleus tractus solitarii (NTS) during respiration and swallowing. W e investigated whether the neurons in the NTS projected directly to the lar yngeal motoneurons, and what is the synaptic organization of their nerve te rminals on the laryngeal motoneurons using the electron microscope. When wh eat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) was injecte d into the NTS after cholera toxin subunit B-conjugated HRP (CT-HRP) was in jected into the CT muscle or the PCA muscle, the anterogradely WGA-HRP-labe led terminals from the NTS were found to directly contact the retrogradely CT-HRP-labeled dendrites and soma of both the CT and the PCA motoneurons, T he labeled NTS terminals comprised about 4% of the axosomatic terminals in a section through the CT motoneurons, and about 9% on both the small (PCA-A ) and the large (PCA-B) PCA motoneurons. The number of labeled axosomatic t erminals containing round vesicles and making asymmetric synaptic contacts (Gray's type I) was almost equal to that of the labeled terminals containin g pleomorphic vesicles and making symmetric synaptic contacts (Gray's type II) on the CT motoneurons. The labeled axosomatic terminals were mostly Gra y's type II on the PCA-A motoneurons, while the majority of them were Gray' s type I on the PCA-B motoneurons. These results indicate that the laryngea l CT and PCA motoneurons receive a few direct excitatory and inhibitory inp uts from the neurons in the NTS.