Masseteric reflex inhibition induced by afferent impulses in the hypoglossal nerve

Summary o 1. Effects of stimulation of the central cut end of the hypoglossal nerve on the masseteric reflex were studied in cats (spinal cord sectioned between C 2 and C 3 ). Hypoglossal stimulation with 1–3 pulses induced a prolonged suppression of the ipsilateral and contralateral masseteric reflex, with or without a facilitatory phase following the peak of suppression. 2. The threshold of suppression was 2–3 times higher than the threshold for firing axons of hypoglossal motor neurons and maximal suppression was obtained when the hypoglossal nerve was stimulated with pulses supramaximal for these axons. Pulses of longer duration than necessary to induce suppression were usually required to induce facilitation. 3. Hypoglossal influences on the masseteric reflex were almost completely abolished following section of the hypoglossal roots on the stimulated side, whereas the effects still remained following section of the glossopharyngeal, vagal and accessory nerve roots and the dorsal roots of C 1 and C 2 when the hypoglossal roots were left intact. 4. Dihydro-β-erythroidine did not alter the hypoglossal effects on the masseteric reflex. 5. Neither precollicular nor medullo-spinal transection altered these hypoglossal influences, whereas ponto-medullary transection completely abolished them. 6. From these results it was concluded that afferent impulses in the hypoglossal nerve enter the brain stem primarily through the hypoglossal roots, ascend bilaterally in the brain stem via a polysynaptic route to the trigeminal motor nucleus and induce inhibition and facilitation of the masseteric monosynaptic reflex.