Projections from the medullary swallowing center to the hypoglossal motor nucleus: a neuroanatomical and electrophysiological study in sheep.

Neurons of the hypoglossal (XIIth) motor nucleus participate in swallowing, but nothing is known of the input to these cells from swallowing interneurons (SIN) belonging to the medullary swallowing center (SC). After electrophoretic injection of horseradish peroxidase within the XIIth motor nucleus in sheep, labeled neurons were found principally in the ipsilateral ventrolateral reticular formation, 1-4 mm rostral to the obex which corresponds to the ventral region of the SC. Labeled cells were observed in the region of the nucleus of the tractus solitarius (dorsal region of the SC), predominantly when the injection site of HRP extended beyond the XIIth nucleus. The projections of ventral SIN to the XIIth nucleus was confirmed electrophysiologically. Twenty-seven of 98 tested SIN were antidromically activated (latency: 2.7 +/- 1.5 ms) by stimulating the XIIth nucleus. Twenty-one of these were histologically localized in the ventral SC, and only one in the dorsal SC. The other 5 SIN were located in the ventral group according to their stereotaxic coordinates. Our data show that SIN in the ventral reticular formation, part of the medullary SC, project to the XIIth motor nucleus; we suggest that ventral SIN are command interneurons for the different pools of motoneurons involved in swallowing.