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.

Animals↗

PubMed1975

Comparison of activity in pontine versus medullary neurones during swallowing.

1. On decerebellectomized sheep lightly anaesthetized with fluothane, the activity of 49 neurones in the pontine relay (see Car et al., 1975) was recorded with microelectrodes following stimulation of afferents in the superior laryngeal nerve (SLN), 2. These pontine neurones (PN) exhibited an "initial activity" (one or a few spikes) for stimulation either of the homolateral SLN (35 PN) or glossopharyngeal nerve (14 PN). This initial activity had a latency between 1.5 and 4 msec. When swallowing was induced by SLN stimulation, a later discharge appeared. This "swallowing activity" consisted of a variable burst of spikes. 3. The effect of curarization was tested for 13 PN. It always eliminated the "swallowing activity". 4. A clear antidromic response of 16 PN (26 tested PN) was induced by stimulating the thalamic VPM nucleus. This stimulation failed to elicit an antidromic response in medullary neurones (14 tested) located in the nucleus of the tractus solitarius. 5. It is concluded that PN are probably sensory relay neurones which inform higher nervous centres of the state of oropharyngeal receptors; whereas medullary swallowing neurones are really interneurones involved in the programming of the wallowing motor sequence.

Animals↗

PubMed1973