ULTRASTRUCTURE AND SYNAPTOLOGY OF THE NUCLEUS AMBIGUUS IN THE RAT - THE SEMICOMPACT AND LOOSE FORMATIONS

The fine structure of the pharyngomotor semicompact and laryngomotor l oose 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 inj ections of neuroanatomical tracers into pharyngeal and laryngeal muscl es or nerves. Pharyngeal motoneurons measured 39 x 29 mu m and had 2-2 5 axosomatic synapses per somatic profile, representing an estimated a verage of 182 synapses per soma. Laryngeal motoneurons measured 42 x 3 0 mu m with 6-33 synapses per profile, or an average of 339 synapses p er soma. In both subdivisions, axon terminals that contained round ves icles and formed asymmetric junctions and terminals that contained ple omorphic vesicles and formed symmetric junctions were distributed in a pproximately equal proportions on somata and dendrites, forming over 9 0% of the synapse population. A small percentage (2-8%) of synapses ha d a subsurface cistern situated below the axon terminal (C type). Smal l, atypical motoneurons measuring 15 x 5 mu m with an invaginated nucl eus were also present in both subdivisions. The ultrastructure and syn aptology of pharyngeal and laryngeal motoneurons are characterized by similarities to those of spinal motoneurons and by their relatively la rge numbers of axosomatic synapses in comparison to esophageal motoneu rons of the compact formation of the nucleus ambiguus. (C) 1996 Wiley- Liss, Inc.