Ultrastructure and synaptology of the nucleus ambiguus in the rat: The semicompact and loose formations

The fine structure of the pharyngomotor semicompact and laryngomotor loose formations of the rat nucleus ambiguus was studied in single and serial sections by means of light and electron microscopy. Motoneurons and their dendrites were identified after retrograde labelling by injections of neuroanatomical tracers into pharyngeal and laryngeal muscles or nerves. Pharyngeal motoneurons measured 39 x 29 μm and had 2-25 axosomatic synapses per somatic profile, representing an estimated average of 182 synapses per soma. Laryngeal motoneurons measured 42 x 30 μm with 6-33 synapses per profile, or an average of 339 synapses per soma. In both subdivisions, axon terminals that contained round vesicles and formed asymmetric junctions and terminals that contained pleomorphic vesicles and formed symmetric junctions were distributed in approximately equal proportions on somata and dendrites, forming over 90% of the synapse population. A small percentage (2-8%) of synapses had a subsurface cistern situated below the axon terminal (C type). Small, atypical motoneurons measuring 15 x 5 μm with an invaginated nucleus were also present in both subdivisions. The ultrastructure and synaptology of pharyngeal and laryngeal motoneurons are characterized by similarities to those of spinal motoneurons and by their relatively large numbers of axosomatic synapses in comparison to esophageal motoneurons of the compact formation of the nucleus ambiguus.