Role of the lateral reticular nucleus in suppressing the jaw-opening reflex following stimulation of the red nucleus.

We found in a previous study that stimulation of the red nucleus (RN) facilitated the low-threshold afferent-evoked jaw-opening reflex (JOR) and suppressed the high-threshold afferent-evoked JOR. It has been reported that the RN projections to the contralateral lateral reticular nucleus (LRt), and stimulation of the LRt inhibits the nociceptive JOR. These facts suggest that RN-induced modulation of the JOR is mediated via the LRt. We investigated whether electrically induced lesions of the LRt, or microinjection of muscimol into the LRt, affects RN-induced modulation of the JOR. The JOR was evoked by electrical stimulation of the inferior alveolar nerve (IAN), and was recorded as the electromyographic response of the anterior belly of the digastric muscle. The stimulus intensity was either 1.2 (low-threshold) or 4.0 (high-threshold) times the threshold. Electrically induced lesion of the LRt and microinjection of muscimol into the LRt reduced the RN-induced suppression of the high-threshold afferent-evoked JOR, but did not affect the RN-induced facilitation of the low-threshold afferent-evoked JOR. These results suggest that the RN-induced suppression of the high-threshold afferent-evoked JOR is mediated by a relay in the contralateral LRt.

Modulation of two types of jaw-opening reflex by stimulation of the red nucleus.

The red nucleus (RN) is divided cytoarchitecturally into two parts, the parvicellular part (RPC) and the magnocellular part (RMC). The present study aims, first, to compare the effects of RN stimulation between low- and high-threshold afferents-evoked jaw opening reflexes (JORs), and secondly to compare the size of these effects in the RPC and RMC. Experiments were performed on rats anesthetized with urethane-chloralose. The JOR was evoked by electrical stimulation of the inferior alveolar nerve and was recorded as the electromyographic response of the anterior belly of the digastric muscle. The stimulus intensity was either 1.2 (low-threshold) or 4.0 (high-threshold) times that necessary to elicit the JOR. Conditioning electrical stimulation of the RN significantly facilitated the JOR evoked by the low-threshold afferents. On the other hand, conditioning electrical stimulation of the RN significantly suppressed the JOR evoked by the high-threshold afferents. Microinjection of monosodium glutamate into the RN also facilitated the JOR evoked by the low-threshold afferents, but suppressed that evoked by high-threshold afferents. Facilitation did not differ between the RMC and the RPC. Suppression by the RMC stimulation was significantly greater than that by the RPC stimulation. These results suggest that the RN has distinct functional roles in the control of the JOR.

Suppression of the nociceptive jaw-opening reflex by stimulation of the red nucleus.

We studied the effect of stimulation of the red nucleus (RN) on the jaw-opening reflex (JOR) in anesthetized rats. The JOR was evoked by electrical stimulation of the tooth pulp of a lower incisor, and was recorded as the electromyographic responses of the anterior belly of the digastric muscle, bilaterally. Conditioning electrical stimulation of the RN was found to suppress the JOR bilaterally. Microinjection of monosodium glutamate into the RN also suppressed the JOR bilaterally. The suppressive effect of the magnocellular part of the RN was significantly larger than that of the parvicellular part of the RN. These results imply that the RN is involved in control of the JOR evoked by noxious stimulus.

Neuronal activities of the vestibular nuclear complex during mechanically induced rhythmic jaw movements in rats.

We studied the neuronal activities of the vestibular nuclear complex (VN) neurons during rhythmic jaw movements in rats anesthetized with urethane. Rhythmic jaw movements were induced by mechanical stimulation of the palate mucosa. The firing rate of approximately 25% of VN neurons increased significantly, and that of 10% of VN neurons decreased significantly, during these rhythmic jaw movements. There was no correlation between the change in the firing rate and the phase of the rhythmic jaw movements (jaw-opening and jaw-closing phases). The neurons that were affected were intermingled in the VN. These results suggest that the VN neurons are involved in controlling jaw movements.

Effects of neck muscle activities during rhythmic jaw movements by stimulation of the medial vestibular nucleus in rats.

This study first examines whether there is rhythmic activity of the neck muscles during cortically induced rhythmic jaw movements in rats anesthetized by urethane. Rhythmic jaw movements were induced by repetitive electrical stimulation of the orofacial motor cortex. An electromyogram in the splenius muscles (spEMG) showed rhythmic bursts during the jaw-opening phase, or during the transition from the jaw-opening phase to the jaw-closing phase. In the sternomastoid (stEMG), however, the electromyogram did not show any bursts during rhythmic jaw movements. A further study then examines whether stimulation of the medial vestibular nucleus (MVN) modulates the rhythmic activity of the neck muscles. Stimuli applied in the jaw-closing phase induced a transient burst in the stEMG, and the duration of activity in the spEMG was increased. Stimuli applied in the jaw-opening phase induced a transient burst in the stEMG and an inhibitory period in the spEMG. These results imply that the MVN is involved in the modulation of neck muscle activities during rhythmic jaw movements induced by stimulating the orofacial motor cortex.