Application of different fluorescent tracers to the right and left hypoglossal nerve of the frog revealed the extent of dendrites crossing the midline into the territory of contralateral hypoglossal motoneurons. By using confocal microscopy, a large number of close appositions were detected between hypoglossal motoneurons bilaterally, which formed dendrodendritic and dendrosomatic contacts. The distance between the neighboring profiles suggested close membrane appositions without interposing glial elements. Application of neurobiotin to one hypoglossal nerve resulted in labeling of perikarya exclusively on the ipsilateral side of tracer application, suggesting the absence of dye-coupled connections with contralateral hypoglossal motoneurons. At the ultrastructural level, the dendrodendritic and dendrosomatic contacts did not show any morphological specialization; the long membrane appositions may provide electrotonic interactions between the neighboring profiles. We propose that dendrites of hypoglossal motoneurons that cross the midline subserve one of the morphological substrates of co-activation, synchronization and timing of bilateral activity of tongue muscles during prey-catching behavior of the frog.
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