A light and electron microscope study of the connections between the preganglionic fibers and the intralingual ganglion cells in the rat.

The topographical distribution of the preganglionic neurons sending projection fibers to the tongue, and the connections between their fibers and the intralingual ganglion cells, were examined in the rat. When horseradish peroxidase injections were made into the anterior two-thirds of the tongue, labeled neuronal cell bodies were distributed mainly in the lateral reticular formation at the level between the rostral part of the facial nucleus and the caudal part of the superior olivary complex. On the other hand, after horseradish peroxidase injections into the posterior one-third of the tongue, labeled neuronal cell bodies were found mainly in the rostromedial part of the nucleus of the solitary tract, and additionally in the lateral reticular formation just ventral to the rostral part of the nucleus of the solitary tract. In both cases, labeled neuronal cell bodies were always found in the hypoglossal nucleus. The anterograde tracing study with Phaseolus vulgaris-leucoagglutinin or Fluoro-ruby confirmed the topographical organization suggested by the retrograde tracing study; when the tracer injections were centered on the lateral reticular formation at the level of the rostral part of the facial nucleus or on the rostral part of the nucleus of the solitary tract, labeled fibers distributed mainly in the anterior or posterior part of the tongue, respectively. It was also shown that the axon terminals of the preganglionic fibers labeled with Fluoro-ruby made close contacts with the intralingual ganglion cells immunopositive for neuron specific enolase. The electron microscopy combined with the anterograde tracing method with biotinylated dextran amine further indicated that the preganglionic fibers made synaptic contacts with the soma and dendritic processes of the intralingual ganglion cells.