Axonal branching of medullary swallowing neurons projecting on the trigeminal and hypoglossal motor nuclei: demonstration by electrophysiological and fluorescent double labeling techniques

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Abstract
The projections of ventral medullary reticular neurons on both trigeminal (Vth) and hypoglossal (XIIth) motor nucleus were studied in sheep anesthetized with halothane. In a first series of experiments, extracellular microelectrodes were used to record the activity of medullary swallowing interneurons (SINs) located in the ventral region (around the nucleus ambiguus) of the swallowing center. Antidromic activation after electrical stimulation of the Vth and XIIth nuclei was tested in 83 SINs. For 38 SINs a clear antidromic activation was observed and for 8 of them the response was triggered by stimulation of either nucleus. As confirmed by the reciprocal collision test, these 8 SINs had branched axons sending information to both nuclei tested. Average latencies for antidromic activation of branched SINs after stimulation of the XIIth and the Vth motor nucleus were 2.2±0.6 ms and 2.7±0.8 ms respectively. The axonal conduction velocity of these neurons was 4.4±1.3 m/s for the collateral to the Vth motor nucleus and 2.7±0.7 m/s for axons projecting to the XIIth motor nucleus. In a second series of experiments the double retrograde labeling technique was used to confirm the existence of neurons with branched axons in the medullary regions corresponding to the swallowing center. Small and well localized injections of Fast Blue (FB) and Diamidino Yellow (DY) fluorescent tracers were made in the Vth and in the XIIth motor nucleus respectively. A relatively large number of double-labeled cells was found in the ventral region of swallowing center (reticular formation around the nucleus ambiguus, 2–4 mm in front of obex). Such neurons (supplying both the XIIth and the Vth motor nucleus) appeared mixed with those innervating only either the XIIth or the Vth motor nucleus. Each type of neuron, i.e. single or double labeled, was shown to have bilateral distribution with an ipsilateral predominance.