A pontine primary relay for ascending projections of the superior laryngeal nerve,

1. In sheep anaesthetized with fluothane, electrical stimulation of the superior laryngeal nerve (SLN), which contains most of the afferent fibres for swallowing, evokes potentials in the medial part of the ipsilateral thalamic VPM (nucleus ventro-postero-medialis) within about 5 msec. This region constitutes the secondary synaptic relay for the laryngeal impulses projecting to the frontal cortex concerned with swallowing. 2. SLN fibres are synaptically connected with cells of the NTS (nucleus of the tractus solitarius), 2-4 mm rostral to the obex (see Car and Jean, 1971). Coagulation of this region abolishes reflexly and cortically induced swallowing, but does not influence the thalamic or cortical responses induced by SLN stimulation. 3. SLN stimulation evokes potentials with a short latency (2 msec) in a restricted pontine area localized 5 mm from the midline and above the trigeminal motor nucleus, just in front of the central emergence of the facial nerve (i.e; about 12 mm rostral to obex). Restricted coagulation of this pontine region eliminates both the thalamic and the cortical projection of SLN. 4. Repetitive stimulation (2 V; 0.2 msec; 20-30 Hz) of this same pontine region produces rhythmic swallowing with characteristics quite similar to those of swallowing induced by SLN or bulbar stimulation. 5. Other data show that SLN fibres, or at least part of them, bifurcate after entering the brain stem (about 6 mm in front of the obex), and give a caudal branch, which reaches the bulbar swallowing centre (3 mm rostral to the obex) by running through the tractus solitarius; and a rostral branch terminating in the pons where the primary synaptic relay for the ascending laryngeal pathway is localized.