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The present study aimed to examine whether the jaw-opening reflex (JOR) is modulated during swallowing, and if so, to compare the modulation between the low- and high-threshold afferent-evoked reflex responses. Experiments were carried out on 11 anesthetized rabbits. The inferior alveolar nerve was stimulated to evoke the JOR in the digastric muscle. The stimulus intensity was either 1.5 (low threshold) or 4.0 (high threshold) times the threshold for eliciting the JOR. As a conditioning stimulation, the superior laryngeal nerve (SLN) was repetitively stimulated to evoke the swallowing reflex. The stimulus intensity ranged from 0.6 to 8.0 times the threshold to evoke the swallowing reflex during SLN stimulation over 20s. Electromyographic (EMG) activities of the digastric and mylohyoid muscles were recorded, and the peak-to-peak EMG amplitude of the digastric muscle was measured and compared with and without SLN stimulation, as well as with and without swallowing. Comparisons were also made between low- and high-threshold afferent-evoked JORs. The JOR was strongly suppressed during SLN stimulation. The degree of suppression increased and the latency for the JOR was delayed when the stimulus current applied to the SLN was increased. Such modulation was apparent when the low-threshold afferent-evoked JOR was recorded. Effects of motor outputs of swallowing events and those of single-pulse stimulation of SLN on the inhibition of the JOR were not noted. These results suggest that the JOR evoked by both the low- and high-threshold afferents was inhibited during laryngeal sensory input and following swallowing, probably to prevent opposing jaw movements evoked by oral sensory input during swallowing., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Transthyretin (TTR) is mainly expressed in the liver and choroid plexus of the brain. The majority of familial amyloidotic polyneuropathy cases are caused by a mutant TTR gene. The origin of the TTR deposited in the peripheral nervous system is unknown. We studied the expression of TTR in the peripheral nerves of normal mice and transgenics bearing the human mutant TTR in a mouse Ttr-null background. Using RT-PCR, Ttr and TTR mRNA was observed in both dorsal root ganglia and sciatic nerves. Ttr mRNA was detected in cultured mouse Schwann cells and the immortalized mouse Schwann cell line, IMS32 cells. Human TTR mRNA and protein were detected in cultured Schwann cells derived from the transgenic mice. We conclude that the TTR gene is expressed in the Schwann cells of peripheral nerves., (Copyright 2010 Elsevier B.V. All rights reserved.)

The present study aimed to determine if sensory inputs from the intraoral mechanoreceptors similarly contributed to regulating the activity of the jaw-opening muscles throughout the masticatory sequence. We also aimed to determine if sensory inputs from the chewing and non-chewing sides equally regulated the activity of the jaw-opening muscles. Electromyographic (EMG) activities of jaw muscles (digastric and masseter) and jaw movements were recorded in awake rabbits. The entire masticatory sequence was divided into preparatory, rhythmic-chewing and preswallow periods, based on jaw muscles activity and jaw movements. The jaw-opening reflex (JOR) was evoked by unilateral low-intensity stimulation of the inferior alveolar nerve (IAN) on either the chewing or non-chewing side. Amplitude of the JOR was assessed by measuring peak-to-peak EMG activity in the digastric muscle, and was compared among the masticatory periods and between the chewing and non-chewing sides. The JOR was strongly suppressed during the jaw-closing phase in the rhythmic-chewing and preswallow periods, but this effect was transiently attenuated during the late part of the jaw-opening phase in these periods. However, modulation of the JOR varied from strong suppression to weak facilitation during the preparatory period. The patterns of JOR modulation were similar on the chewing and non-chewing sides in all masticatory periods. The results suggest that the sensory inputs from the intraoral mechanoreceptors regulate the activity of the jaw-opening muscles differently during the preparatory period compared with the other masticatory periods. Sensory inputs from both the chewing and non-chewing sides similarly regulate the activity of the jaw-opening muscles.

Changes in reflex responses in the genioglossus (GG) muscle evoked by electrical stimulation of the inferior alveolar nerve and GG muscle tone (background activity, BGA) were investigated during sleep-wakefulness stages in rabbits. The GG muscle showed two types of electromyographic activity patterns: a respiration-related phasic activity and non-respiration-related activity. GG reflex responses and BGA exhibited a stage-dependent decrease as they were constantly suppressed from quiet wakefulness to non-rapid eye movement sleep to rapid eye movement sleep (REMS). Degree of suppression of reflexes was much larger than that of BGA regardless of GG activity patterns. When amplitude of reflex responses was compared between with and without rapid eye movements during REMS, no difference between the conditions was noted. These results suggest that excitability of the GG muscle is affected by sleep stages by not only a modulation of excitability in motoneurons but also in interneurons involved in the reflex arc.

Previous studies have shown that jaw reflexes and activity patterns of the jaw muscles were modulated in the presence of jaw muscle pain. However, there is no study comparing the modulatory effects on the jaw reflexes induced by noxious stimulation to the jaw muscle. To clarify this, effects of the application of mustard oil (MO), an inflammatory irritant, into the temporalis (jaw-closing) muscle on (1) jaw-opening reflex evoked by tooth pulp stimulation (TP-evoked JOR) as a nociceptive reflex, (2) jaw-opening reflex evoked by inferior alveolar nerve stimulation as a non-nociceptive reflex and (3) jaw-closing reflex evoked by trigeminal mesencephalic nucleus stimulation as a proprioceptive reflex were investigated in anesthetized rats. The MO application induced suppression of all reflexes, and the effect on the TP-evoked JOR was more prominent than on the other reflexes. To elucidate the involvement of endogenous opioid system for the suppressive effect, a systemic administration of naloxone following the MO application was conducted. The MO-induced suppressive effect on the TP-evoked JOR was reversed by the naloxone administration. The results suggest that noxious stimulation to the jaw muscle modulate jaw reflexes particularly for the nociceptive jaw-opening reflex, and the modulatory effect includes both facilitatory and inhibitory aspects. The results also suggest that pain modulatory systems such as the endogenous opioid system play a crucial role in the suppression of the nociceptive transmissions related to nociceptive reflexes, and in some pathological states, defense reflexes may not be evoked properly.

To evaluate the coordination of tongue and suprahyoid muscle activities during natural mastication, electromyograms (EMGs) of jaw-closer, jaw-opener, suprahyoid (mylohyoid, MH), tongue-retractor (styloglossus, SG) and tongue-protractor (genioglossus, GG) muscles were recorded as well as the jaw-movement trajectories in vertical and horizontal axes in awake rabbits. Each masticatory cycle had three components including the fast-closing (FC), slow-closing (SC) and opening (Op) phases. The duration of the SC phase was much longer during pellet chewing while the durations of the FC and Op phases were much shorter during pellet chewing than bread or banana chewing. The jaw movements during banana chewing had a small amplitude of lateral excursion and a large amplitude of gape as compared with those during pellet and bread chewing. The MH muscle exhibited double-peaked EMG bursts during the Op phase. The MH bursts in the late part of the Op phase were dominant on the non-chewing side during pellet and bread chewing. The SG muscle also exhibited double-peaked EMG bursts. During pellet and bread chewing, the SG bursts during the SC phase were significantly larger on the chewing side than the non-chewing side. These bursts were also dominant during pellet chewing as compared with banana chewing. There was little difference in the GG bursts between the chewing and non-chewing sides or among the foods. Our results suggest that patterns of the MH and SG muscle activity are affected by the peripheral inputs and/or chewing patterns while those of the GG muscle activity was less modulated regardless of the consistency of foods.

To clarify the jaw-closer and tongue-retractor muscle activity patterns during mastication, electromyographic activity of the styloglossus (SG) as a tongue-retractor and masseter (Mass) as a jaw-closer muscles as well as jaw-movement trajectories were recorded during cortically evoked rhythmic jaw movements (CRJMs) in anesthetized rabbits. The SG and Mass muscles were mainly active during the jaw-closing (Cl) phase. The SG activity was composed of two bursts in one masticatory cycle; one had its peak during the jaw-opening (Op) phase (SG1 burst) and the other during the Cl phase (SG2 burst). The Mass activity during the Cl phase was dominant on the working side (opposite to the stimulating side) while the SG1 and SG2 bursts were not different between the sides. When the wooden stick was inserted between the molar teeth on the working side during CRJMs, the facilitatory effects on the SG1 and SG2 bursts on both sides were noted as well as those on the Mass bursts, but the effects on the SG1 burst seemed to be weak as compared with those on the Mass and SG2 bursts. The difference in the burst timing between the sides was noted only in the SG1 burst. When the trigeminal nerves were blocked, the peak and area of the SG and Mass burst decreased during CRJMs, and the facilitatory effects of the wooden stick application on the muscles were not noted. The results suggest that the jaw and tongue muscle activities may be adjusted to chew the food and make the food bolus.

To study peri-oral facial muscle activity patterns and coordination with jaw muscles during ingestive behavior, electromyographic (EMG) activities in the peri-oral facial (buccinator: BUC, orbicularis oris: ORB) and jaw (masseter, digastric) muscles along with jaw movement trajectories were recorded in awake rabbits. A standardized amount of apple in a cylindrical shape was used as the test food. The period from food intake to just before swallowing (the masticatory sequence) was divided into three masticatory periods (preparatory period, rhythmic chewing period and preswallow period) based on the activity pattern of jaw muscles and jaw movement trajectories, and jaw movements and EMG activities in both the jaw and facial muscles during each masticatory period were assessed. Both the jaw and facial muscles were active throughout the masticatory sequence, and the activity patterns of facial muscles and the pattern of coordination between the facial and jaw muscles varied for each masticatory period. No consistent pattern was noted for the BUC activity during the preparatory period, whereas the ORB showed tonic activity throughout this period. During the rhythmic chewing and preswallow periods, both the ORB and BUC showed jaw-movement-related rhythmic bursts. However, significant differences were noted in the burst properties in both facial muscles and their temporal correlations with the jaw muscle activities between these two periods. Results suggest that the neural mechanisms regulating facial muscle activities may differ between the masticatory periods, and such mechanisms may contribute to the well-coordinated orofacial movements required for smooth masticatory sequence.

The effects of stimulation of the cortical masticatory area (CMA) on swallowing evoked by superior laryngeal nerve (SLN) were studied in anesthetized rabbits. Electromyographic activity of the thyrohyoid, masseter, and digastric muscles and jaw-movement trajectories were recorded to monitor rhythmic jaw movements (RJMs) or swallowing. A systematic series of microelectrode penetrations within the CMA was made for each animal, and the effects of CMA stimulation on swallowing were tested by comparing the number of swallows evoked by stimulation of the CMA alone, the SLN alone, and simultaneous stimulation of the SLN and CMA. A significant facilitatory effect was observed in 49 (52%) of the 95 CMA loci tested. No significant effect was noted in the remaining 46 loci. Three different types of RJMs were evoked by CMA stimulation, and topographical organization was noted among CMA loci that evoked different types of RJMs. A high percentage of (77%) the CMA loci that evoked RJMs with a prominent horizontal excursion of the jaw facilitated swallowing and was located in the posterolateral and deep part of the CMA. A majority (88%) of the CMA loci that evoked RJMs with small circular jaw movements did not affect swallowing and was located in the anteromedial and shallow part of the CMA. The facilitatory effect of CMA stimulation on swallowing remained even after removal of peripheral sensory inputs by means of deafferentation of infraorbital and inferior alveolar nerves. Results suggest the existence of facilitatory descending pathways to the swallowing center from particular intracortical loci of CMA.

Genioglossus (Gg) reflexes elicited by electrical stimulation of the inferior alveolar nerve were examined in naturally chewing rabbits. To eliminate possible contaminations of the digastric (Dig) activity in the Gg responses, the Dig nerve was denervated bilaterally. Masticatory and tongue muscles were well coordinated during chewing after the denervation; i.e., there were no significant differences in the phase durations between before and after denervation. The Gg reflex measured was divided into three categories depending on the chewing phase (i.e., jaw-opening, OP; fast-closing, FC; and slow-closing, SC) in which the stimulus was delivered. The reflex amplitude was phasically modulated for the phases, in that the amplitude in the OP phase was larger than that in any other phase (P<0.05). On the other hand, the amplitude in the FC and SC phases was not significantly different to each other and from the control value obtained when the animal was awake and resting. The pattern of the modulation in the reflex amplitude was different from the previous report as to the Dig reflex in that OP

The effects of reversible cold block-induced bilateral inactivation of the face primary motor cortex (face MI) on mastication and swallowing were studied in awake monkeys. A warm or cold alcohol-water solution was pumped through thermodes placed bilaterally on the dura overlying the intracortical microstimulation-defined face MI while the monkey chewed and swallowed food during pre-cool (thermode temperature 37 degrees C), cold block (4 degrees C), and post-cool (37 degrees C) sessions. Vertical and horizontal jaw movements and electromyographic (EMG) activity of several muscles were monitored. Each masticatory sequence was divided into three masticatory phases (i.e. food preparatory, rhythmic chewing, preswallow). The cold block markedly affected the ability of the monkey to carry out mastication although it did not prevent mastication from occurring. The masticatory deficit was characterized by a significant elongation of the total masticatory time, including in particular elongation of the food preparatory phase. The coordination of the jaw- and tongue-muscle activities was severely disrupted during the food preparatory phase. Face MI cold block also significantly affected the duration of some masticatory-related EMG activities and had some limited effects on the temporal relationships of the EMG activities during mastication. Although cold block significantly affected the duration and some EMG parameters of the preswallow phase, it had no significant effect on swallow duration or the EMG parameters during swallowing. These findings provide further evidence that the primate face MI plays a critical role in the regulation of mastication and that it plays a role in the preparation for swallowing.

To study central and peripheral control mechanisms maintaining rhythmical jaw behaviors, chewing movements with different foods in texture were obtained in the freely behaving rabbit. Jaw movement trajectories and muscle activities (masseter, digastric, thyrohyoid) were recorded and the durations in total cycle, fast closing (FC), slow closing (SC), and opening (OP) phases were obtained as well as the burst duration in the muscles. Durations varied cycle-by-cycle and among the foods, however, the total cycle duration was found to have little differences among the foods tested. Regression analyses were applied to seek time relations to the change in total cycle duration with the duration in its constituent phases. Results suggested that changes in the total cycle duration may be due to those in the SC duration (power phase) with hard food (heavy load), but due to those in the OP duration (reverse phase) with soft food (light load). The duration of the FC was fairly constant for all the foods tested. In conclusion, the chewing rhythm may be controlled centrally to be independent of the load, and chewing cycles may begin at the middle of the opening phase.