Ultrastructural studies of the nucleus ambiguus in cat and monkey following injection of HRP into the vagus nerve

Summary The fine structure of retrogradely labelled neurons in the nucleus ambiguus (NA) has been examined in cat and monkey (Macaca fascicularis) following injections of horseradish peroxidase (HRP) into the vagus nerve. Many retrogradely labelled neurons were observed in the NA although unlabelled neurons were also present within the boundaries of the NA as identified by the distribution of retrogradely labelled cells. In both species a wide range of sizes of labelled neurons (20–60 m in long axis) was observed from rostral to caudal levels of the NA. Large labelled neurons were generally oval or spindle-shaped while smaller neurons were oval in cross-section. Unlabelled neurons observed among labelled NA neurons tended to be smaller on average than the labelled neurons and ranged in size from 15 to 30 m in long axis. The unlabelled neurons typically had nuclei which were more invaginated than those of the labelled neurons.Quantitative analyses of the synaptic organization in the NA revealed high proportions of terminals containing round vesicles and making asymmetrical or symmetrical contact with the somata in both monkey and cat. Terminals containing pleomorphic vesicles and making symmetrical contact with somata were slightly less numerous. Retrogradely labelled neurons exhibited a positive correlation between the size of neuron and density of synapses on the surface. There tended to be a greater synaptic density on monkey NA neurons than on cat NA neurons of comparable size.