Connections between the ventral medullary swallowing area and the trigeminal motor nucleus of the sheep studied by tracing techniques.

The anatomical connections between two regions involved in swallowing, the medullary reticular formation around the nucleus ambiguous (ventral group of swallowing interneurons) and the trigeminal motor nucleus (NMV) were studied in sheep using anterograde and retrograde tracing techniques. [3H]leucine and HRP were injected into the ventral medulla and the NMV respectively. These injections were performed according to stereotaxic coordinates previously established by electrophysiological recording tests. The results show bilateral connections between the ventral medulla, 2-4 mm rostral to the obex, and the trigeminal motor nucleus. In addition, the same area of the ventral medulla is connected with the homologous contralateral region and with the facial, vagal and hypoglossal nuclei that are also involved in swallowing. Some connections between the ventral medulla and the NMV must be monosynaptic since aggregates of silver grains typical of axon terminals were observed on the cell bodies of trigeminal motoneurons. Obviously, the various connections shown to exist in this study may serve other functions than swallowing. Nevertheless, most of the results are consistent with the view that the ventral group of medullary swallowing interneurons are switching neurons that distribute the programmed swallowing excitation to the various motoneuron pools involved in deglutition.