Inhibition of hypoglossal motoneurons by stimulation of the jaw-opening muscle afferents.

The synaptic linkage of jaw-opening muscle afferents with the retractive and the protrusive motoneurons (R-Mns and P-Mns) in the hypoglossal motor nucleus was explored in anesthetized cats. It was found that stimulation of low-threshold afferents arising in the jaw-opening muscles (m. mylohyoideus and m. digastricus) evoked IPSPs in both R-Mns and P-Mns with about 10 to 15 msec latencies. By injecting Cl ions into R-Mns or P-Mns, the hyperpolarizing postsynaptic potentials evoked by jaw-opening muscle afferent impulses were reversed to depolarizing potentials. The lingual-IPSPs of hypoglossal motoneurons were concluded to be dependent on increase of Cl conductance. Stimulation of low threshold lingual afferents evoked a reflex discharge in the retractive nerve fibers with a 6.0 msec latency and in the protrusive nerve fibers with a 5.5 msec latency. Inhibition of the linguohypoglossal reflex by stimulation of the jaw-opening muscle afferents was completely removed by an intravenous injection of strychnine (0.1 mg/kg) or picrotoxin (6 mg/kg). It is concluded that the low threshold afferents arising in the jaw-opening muscles are connected to both R-Mns and P-Mns and P-Mns through inhibitory synapses via a polysynaptic pathway.