Masseteric reflex inhibition induced by afferent impulses in the hypoglossal nerve
Department of Anatomy and Brain Research Institute, University of California, Los Angeles, Calif. 90024 (U.S.A.)Brain Research (Impact Factor: 2.84). 04/1970; 18(2):241-55. DOI: 10.1016/0006-8993(70)90326-4
Summary1.Effects of stimulation of the central cut end of the hypoglossal nerve on the masseteric reflex were studied in cats (spinal cord sectioned between C2 and C3). 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 C1 and C2 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.
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ABSTRACT: The electrical activity of the masseter muscle was recorded with bipolar surface electrodes while subjects maintained a steady biting pressure on a wooden stick placed between the right or left first molars. Under these experimental conditions, tapping the labial surface of an upper central incisor, or electrical stimulation of the labial gingiva overlying the root of an incisor, evoked a short latency excitatory reflex in the masseter muscle. The latency of the reflex was consistently shorter than that of the monosynaptic jaw-jerk reflex. Following this excitatory reflex the tonic electrical activity in the masseter muscle was abolished for approximately 20 msec.The administration of a local anesthetic to the stimulus tooth markedly reduced both the excitatory and inhibitory reflexes evoked by the tooth tap and completely blocked the reflexes evoked by electrical stimulation of the gingiva.The hypothesis is put forth that synaptic interaction between the cell bodies of intra-oral and muscle spindle primary afferent neurons, located in the mesencephalic nucleus V, is the central mechanism responsible for the excitatory masseteric reflex evoked by periodontal and gingival receptor stimulation.
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