Difference in the burst patterns of digastric and mylohyoid activities during feeding in the freely behaving rabbit

The functional role of the rabbit digastric muscle during mastication

Muscle spindle abundance is highly variable in vertebrates, but the functional determinants of this variation are unclear. Recent work has shown that human leg muscles with the lowest abundance of muscle spindles primarily function to lengthen and absorb energy, while muscles with a greater spindle abundance perform active-stretch-shorten cycles with no net work, suggesting that muscle spindle abundance may be underpinned by muscle function. Compared with other mammalian muscles, the digastric muscle contains the lowest abundance of muscle spindles and, therefore, might be expected to generate substantial negative work. However, it is widely hypothesised that as a jaw-opener (anatomically) the digastric muscle would primarily function to depress the jaw, and consequently do positive work. Through a combination of X-ray reconstruction of moving morphology (XROMM), electromyography and fluoromicrometry, we characterised the 3D kinematics of the jaw and digastric muscle during feeding in rabbits. Subsequently, the work loop technique was used to simulate in vivo muscle behaviour in situ, enabling muscle force to be quantified in relation to muscle strain and hence determine the muscle's function during mastication. When functioning on either the working or balancing side, the digastric muscle generates a large amount of positive work during jaw opening, and a large amount of negative work during jaw closing, on average producing a relatively small amount of net negative work. Our data therefore further support the hypothesis that muscle spindle abundance is linked to muscle function; specifically, muscles that absorb a relatively large amount of negative work have a low spindle abundance.

Mylohyoid Muscle: Current Understanding for Clinical Management-Part I: Anatomy and Embryology

The mylohyoid is one of the suprahyoid muscles, along with the geniohyoid, digastric, and stylohyoid muscles. It lies between the anterior belly of the digastric muscle inferiorly and the geniohyoid superiorly. In Part I, the anatomy and embryology of the mylohyoid muscle will be reviewed in preparation for the clinical discussion in Part II.

Regional Variation in Contractile Patterns and Muscle Activity in Infant Pig Feeding

At the level of the whole muscle, contractile patterns during activity are a critical and necessary source of variation in function. Understanding if a muscle is actively lengthening, shorting, or remaining isometric has implications for how it is working to power a given behavior. When feeding, the muscles associated with the tongue, jaws, pharynx, and hyoid act together to transport food through the oral cavity and into the esophagus. These muscles have highly coordinated firing patterns, yet also exhibit high levels of regional heterogeneity in both their timing of activity and their contractile characteristics when active. These high levels of variation make investigations into function challenging, especially in systems where muscles power multiple behaviors. We used infant pigs as a model system to systematically evaluate variation in muscle firing patterns in two muscles (mylohyoid and genioglossus) during two activities (sucking and swallowing). We also evaluated the contractile characteristics of mylohyoid during activity in the anterior and posterior regions of the muscle. We found that the posterior regions of both muscles had different patterns of activity during sucking versus swallowing, whereas the anterior regions of the muscles did not. Furthermore, the anterior portion of mylohyoid exhibited concentric contractions when active during sucking, whereas the posterior portion was isometric during sucking and swallowing. This difference suggests that the anterior portion of mylohyoid in infant pigs is functioning in concert with the tongue and jaws to generate suction, whereas the posterior portion is likely acting as a hyoid stabilizer during sucking and swallowing. Our results demonstrate the need to evaluate both the contractile characteristics and activity patterns of a muscle in order to understand its function, especially in cases where there is potential for variation in either factor within a single muscle.

Swallowing dysfunction following radiation to the rat mylohyoid muscle is associated with sensory neuron injury

Radiation-based treatments for oropharyngeal and hypopharyngeal cancers result in impairments in swallowing mobility, but the mechanisms behind the dysfunction are not clear. The purpose of this study was to determine if we could establish an animal model of radiation-induced dysphagia in which mechanisms could be examined. We hypothesized that 1) radiation focused at the depth of the mylohyoid muscle would alter normal bolus transport and bolus size and 2) radiation to the mylohyoid muscle will induce an injury/stress-like response in trigeminal sensory neurons whose input might modulate swallow. Rats were exposed to 48 or 64 Gy of radiation to the mylohyoid given 8 Gy in 6 or 8 fractions. Swallowing function was evaluated by videofluoroscopy 2 and 4 wk following treatment. Neuronal injury/stress was analyzed in trigeminal ganglion by assessing activating transcription factor (ATF)3 and GAP-43 mRNAs at 2, 4, and 8 wk post treatment. Irradiated rats exhibited decreases in bolus movement through the pharynx and alterations in bolus clearance. In addition, ATF3 and GAP-43 mRNAs were upregulated in trigeminal ganglion in irradiated rats, suggesting that radiation to mylohyoid muscle induced an injury/stress response in neurons with cell bodies that are remote from the irradiated tissue. These results suggest that radiation-induced dysphagia can be assessed in the rat and radiation induces injury/stress-like responses in sensory neurons.NEW & NOTEWORTHY Radiation-based treatments for head and neck cancer can cause significant impairments in swallowing mobility. This study provides new evidence supporting the possibility of a neural contribution to the mechanisms of swallowing dysfunction in postradiation dysphagia. Our data demonstrated that radiation to the mylohyoid muscle, which induces functional deficits in swallowing, also provokes an injury/stress-like response in the ganglion, innervating the irradiated muscle.

Effect of attention on chewing and swallowing behaviors in healthy humans

We examined how attention alters chewing and swallowing behaviors. Twenty-one healthy volunteers were asked to freely eat 8 g of steamed rice in three separate trials, and we obtained the average number of chewing cycles (N) and chewing duration (T) prior to the first swallow in each trial. We also conducted an N-limited test, in which participants chewed the food while independently counting the number of chewing cycles and swallowed the food when they reached N, and a T-limited test, in which they chewed the food for T sec and then swallowed. We recorded electromyograms (EMGs) from masseter and suprahyoid muscles and collected videoendoscopic images. In the N-limited test, chewing speed decreased, masseter muscle activity (area under the curve of the rectified EMG burst) per cycle increased, and suprahyoid muscle activity per cycle decreased. In the T-limited test, the chewing speed increased, muscle activities per cycle decreased, and the number of cycles increased. The occurrence frequency of bolus propulsion into the pharynx before swallowing was smaller in the N- and T-limited tests than in the free chewing test. Further, the whiteout time was longer in the T-limited test than in the free chewing test. Attentional chewing changes not only chewing but also swallowing behavior.

Modification of Masticatory Rhythmicity Leading to the Initiation of the Swallowing Reflex in Humans

Modification of movements by proprioceptive feedback during mastication has an important role in shifting from the oral to the pharyngeal phase of swallowing. The aim of this study was to investigate the kinetics of masticatory muscles throughout a sequence of oropharyngeal swallowing and to present a hypothetical model of the involvement of the nervous system in the transition from mastication to the swallowing reflex. Surface electromyographic signals were recorded from the jaw-closing masseter muscles and the jaw-opening suprahyoid muscle group when a piece of bread (3-5 g) was ingested. Participants were not provided any additional instruction regarding how to chew and swallow. In the final stage of mastication, compared with other stages of mastication, the duration between sequential peak times of rhythmic activity of the masseter muscles was prolonged. Electromyography revealed no significant change in the suprahyoid muscle group. Accordingly, contraction of the jaw-closing muscles and the jaw-opening muscles altered from out-of-phase to in-phase. We have presented a hypothetical model based on the results of the present study, in which mastication shifts to the swallowing reflex when feed-forward inputs from rhythm generators for the jaw-closing and the jaw-opening muscles converge onto an assumed "convertor" neuron group concurrently. This model should contribute to understanding the pathophysiology of dysphagia.

Transcranial direct current stimulation enhances sucking of a liquid bolus in healthy humans

BACKGROUND

Transcranial direct current stimulation (tDCS) is a non-invasive technique used for modulating cortical excitability in vivo in humans. Here we evaluated the effect of tDCS on behavioral and electrophysiological aspects of physiological sucking and swallowing.

METHODS

Twelve healthy subjects underwent three tDCS sessions (anodal, cathodal and sham stimulation) on separate days in a double-blind randomized order. The active electrode was placed over the right swallowing motor cortex. Repeated sucking and swallowing acts were performed at baseline and at 15 and 60 min after each tDCS session and the mean liquid bolus volume ingested at each time point was measured. We also calculated average values of the following electrophysiological parameters: 1) area and 2) duration of the rectified EMG signal from the suprahyoid/submental muscles related to the sucking and swallowing phases; 3) EMG peak amplitude for the sucking and swallowing phases; 4) area and peak amplitude of the laryngeal-pharyngeal mechanogram; 5) oropharyngeal delay.

RESULTS

The volume of the ingested bolus significantly increased (by an average of about 30% compared with the baseline value) both at 15 and at 60 min after the end of anodal tDCS. The electrophysiological evaluation after anodal tDCS showed a significant increase in area and duration of the sucking phase-related EMG signal.

CONCLUSIONS

Anodal tDCS leads to stronger sucking of a liquid bolus in healthy subjects, likely by increasing recruitment of cortical areas of the swallowing network. This finding might open up interesting perspectives for the treatment of patients suffering from dysphagia due to various pathological conditions.

EMG activity in hyoid muscles during pig suckling

Infant suckling is a complex behavior that includes cycles of rhythmic sucking as well as intermittent swallows. This behavior has three cycle types: 1) suck cycles, when milk is obtained from the teat and moved posteriorly into the valleculae in the oropharynx; 2) suck-swallow cycles, which include both a rhythmic suck and a pharyngeal swallow, where milk is moved out of the valleculae, past the larynx, and into the esophagus; and 3) postswallow suck cycles, immediately following the suck-swallow cycles. Because muscles controlling these behaviors are active in all three types of cycles, we tested the hypothesis that different patterns of electromyographic (EMG) activity in the mylohyoid, hyoglossus, stylohyoid, and thyrohyoid muscles of the pig characterized each cycle type. Anterior mylohyoid EMG activity occurred regularly in every cycle and was used as a cycle marker. Thyrohyoid activity, indicating the pharyngeal swallow, was immediately preceded by increased stylohyoid and hyoglossus activity; it divided the suck-swallow cycle into two phases. Timed from the onset of the suck-swallow cycle, the first phase had a relatively fixed duration while the duration of the second phase, timed from the thyrohyoid, varied directly with cycle duration. In short-duration cycles, the second phase could have a zero duration so that thyrohyoid activity extended into the postswallow cycle. In such cycles, all swallowing activity that occurred after the thyrohyoid EMG and was associated with bolus passage through the pharynx fell into the postswallow cycle. These data suggest that while the activity of some muscles, innervated by trigeminal and cervical plexus nerves, may be time locked to the cycle onset in swallowing, the cycle period itself is not. The postswallow cycle consequently contains variable amounts of pharyngeal swallowing EMG activity. The results exemplify the complexity of the relationship between rhythmic sucking and the swallow.

The digastric muscle is less involved in pharyngeal swallowing in rabbits

The swallowing reflex is centrally programmed by the lower brain stem, the so-called swallowing central pattern generator (CPG), and once the reflex is initiated, many muscles in the oral, pharyngeal, laryngeal, and esophageal regions are systematically activated. The mylohyoid (MH) muscle has been considered to be a "leading muscle" according to previous studies, but the functional role of the digastric (DIG) muscle in the swallowing reflex remains unclear. In the present study, therefore, the activities of single units of MH and DIG neurons were recorded extracellularly, and the functional involvement of these neurons in the swallowing reflex was investigated. The experiments were carried out on eight adult male Japanese white rabbits anesthetized with urethane. To identify DIG and MH neurons, the peripheral nerve (either DIG or MH) was stimulated to evoke action potentials of single motoneurons. Motoneurons were identified as such if they either (1) responded to antidromic nerve stimulation of DIG or MH in an all-or-none manner at threshold intensities and (2) followed stimulation frequencies of up to 0.5 kHz. As a result, all 11 MH neurons recorded were synchronously activated during the swallowing reflex, while there was no activity in any of the 7 DIG neurons recorded during the swallowing reflex. All neurons were anatomically localized ventromedially at the level of the caudal portion of the trigeminal motor nucleus, and there were no differences between the MH and DIG neuron sites. The present results strongly suggest that at least in the rabbit, DIG motoneurons are not tightly controlled by the swallowing CPG and, hence, the DIG muscle is less involved in the swallowing reflex.

Mastication-induced modulation of the jaw-opening reflex during different periods of mastication in awake rabbits

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.

Swallowing and dysphagia rehabilitation: translating principles of neural plasticity into clinically oriented evidence

PURPOSE

This review presents the state of swallowing rehabilitation science as it relates to evidence for neural plastic changes in the brain. The case is made for essential collaboration between clinical and basic scientists to expand the positive influences of dysphagia rehabilitation in synergy with growth in technology and knowledge. The intent is to stimulate thought and propose potential research directions.

METHOD

A working group of experts in swallowing and dysphagia reviews 10 principles of neural plasticity and integrates these advancing neural plastic concepts with swallowing and clinical dysphagia literature for translation into treatment paradigms. In this context, dysphagia refers to disordered swallowing associated with central and peripheral sensorimotor deficits associated with stroke, neurodegenerative disease, tumors of the head and neck, infection, or trauma.

RESULTS AND CONCLUSIONS

The optimal treatment parameters emerging from increased understanding of neural plastic principles and concepts will contribute to evidence-based practice. Integrating these principles will improve dysphagia rehabilitation directions, strategies, and outcomes. A strategic plan is discussed, including several experimental paradigms for the translation of these principles and concepts of neural plasticity into the clinical science of rehabilitation for oropharyngeal swallowing disorders, ultimately providing the evidence to substantiate their translation into clinical practice.

Oral behavior from food intake until terminal swallow

We analyzed oral behavior from food intake until terminal swallow for mastication and swallowing under a freely eating condition with a natural food. Measurements, including movement of the mandible and tongue, the size of the gape, different sequences involved in the oral aspect of the swallowing action, and bolus size and movement were carried out in five "freely eating subjects" using videofluorography. During food intake, the tongue moved forwards and backwards to introduce food into the mouth, to compress the food against the hard palate, and to transport food to the occlusal surface of the molar teeth. Most of the food was swallowed in the first swallow, and any residual food was aggregated by the tongue into a bolus and then swallowed in the last swallow. These findings suggest that 1) tongue manipulation plays an important role in recognizing and evaluating the volume of bite taken, 2) the intra-oral compression of food has a role in the recognition of food texture, 3) stage I transport is closely bound to the texture recognition process, 4) humans need at least two swallows, even with one bite of food, when ingesting food freely, and 5) the duration time of the oral stage of swallowing may depend on the bolus volume and be longer for smaller volumes unlike those measured under the command swallow.

Electromyographic activity during the reflex pharyngeal swallow in the pig: Doty and Bosma (1956) revisited

The currently accepted description of the pattern of electromyographic (EMG) activity in the pharyngeal swallow is that reported by Doty and Bosma in 1956; however, those authors describe high levels of intramuscle and of interindividual EMG variation. We reinvestigated this pattern, testing two hypotheses concerning EMG variation: 1) that it could be reduced with modern methodology and 2) that it could be explained by selective detection of different types of motor units. In eight decerebrate infant pigs, we elicited radiographically verified pharyngeal swallows and recorded EMG activity from a total of 16 muscles. Synchronization signals from the video-radiographic system allowed the EMG activity associated with each swallow to be aligned directly with epiglottal movement. The movements were highly stereotyped, but the recorded EMG signals were variable at both the intramuscle and interanimal level. During swallowing, some muscles subserved multiple functions and contained different task units; there were also intramuscle differences in EMG latencies. In this situation, statistical methods were essential to characterize the overall patterns of EMG activity. The statistically derived multimuscle pattern approximated to the classical description by Doty and Bosma (Doty RW, Bosma JF. J Neurophysiol 19: 44-60, 1956) with a leading complex of muscle activities. However, the mylohyoid was not active earlier than other muscles, and the geniohyoid muscle was not part of the leading complex. Some muscles, classically considered inactive, were active during the pharyngeal swallow.

Coordination of cranial motoneurons during mastication

Mastication is the first stage of digestion and involves several motor processes such as food intake, intra-oral food transport, bolus formation and chewing in its broad sense. These complicated motor functions can be accomplished by the well-coordinated activities in various cranial motoneurons innervating the jaw, hyoid, tongue and facial muscles. The brainstem masticatory central pattern generator (CPG) plays a crucial role in generating basic activity patterns of these cranial motoneuron groups. However, descending inputs from higher brain (e.g., cerebral cortex) and mastication-generated peripheral sensory inputs also play important roles in modulating the activity pattern of each motoneuron so that the final motor outputs fit the environmental demand. In this review, we focus on the coordination of the trigeminal, facial and hypoglossal motoneurons during mastication. We first summarize findings showing the activity patterns of muscles innervated by these motoneurons during natural mastication, and then discuss the possible neural mechanisms underlying their coordinated activities during mastication.

Extrinsic tongue and suprahyoid muscle activities during mastication in freely feeding rabbits

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.

Coordination of jaw and extrinsic tongue muscle activity during rhythmic jaw movements in anesthetized rabbits

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.

A 5-HT2C agonist elicits hyperactivity and oral dyskinesia with hypophagia in rabbits

Serotonergic 5-HT2C and 5-HT1B receptors mediate inhibitory controls of eating. Questions have arisen about potential behavioral and neurological toxicity of drugs that stimulate the 2C site. We evaluated eating and other motor responses in male Dutch-belted rabbits after administration of m-chlorophenylpiperazine (mCPP). Studies conducted in vitro and in vivo assessed the pharmacological specificity of the ingestive actions of this agent. mCPP (0.15-10 micromol/kg sc) reduced consumption of chow and 20% sucrose solution with equal potencies (ED50 approximately equal 0.6 micromol/kg). In radioligand binding to rabbit cortex, mCPP displayed 15-fold higher affinity for 5-HT2C than for 5-HT1B receptors. The serotonin antagonist mesulergine (7000-fold selective for 5-HT2C) reversed the hypophagic action of mCPP, but the 5-HT1B/1D antagonist GR127,935 did not. GR127,935 (0.5 micromol/kg) did prevent hypophagia produced by the highly selective 5-HT1B/1D agonist GR46,611. Observational methods demonstrated that mCPP decreased the frequency of eating chow but increased other motor activities. When rabbits consumed sucrose, videoanalysis revealed that mCPP reduced total time licking and the duration of individual bouts, but not bout frequency or the actual rate of consumption. mCPP increased locomotor and other activities, and greatly increased vacuous oromotor stereotypies and tongue protrusions. Nonetheless, rabbits licked accurately at the spout for sucrose. When sucrose was infused intraorally through a cheek catheter, mCPP actually increased the peak amplitude and overall magnitude of jaw movements. We conclude that mCPP stimulates 5-HT2C receptors to reduce food intake in rabbits. This hypophagia involves disruption of appetitive components of eating and is accompanied by adverse motor actions. This profile raises questions about the use of the 5-HT2C receptor as a target for novel therapeutic agents for obesity.

Investigations into the role of the thyrohyoid muscles in the pathogenesis of dorsal displacement of the soft palate in horses

REASONS FOR PERFORMING STUDY

Contributes to the understanding of the pathogenesis of dorsal displacement of the soft palate during exercise so that management of this condition could be enhanced.

HYPOTHESIS

That the thyrohyoid muscles play an important role in the stability of the laryngo-palatal relationship and that dysfunction of these muscles leads to dorsal displacement of the soft palate (DDSP) during exercise.

METHODS

Ten horses were exercised on a high-speed treadmill under 4 different treatment conditions: control conditions (n = 10), after resection of thyrohyoid muscles (TH, n = 10), after sham-treatment (n = 5), or after restoration of function of the thyrohyoid muscles with surgical sutures (prosthesis-treatment, n = 6). During trials, the following determinations were made: videoendoscopy of the upper airway, gait frequency and pharyngeal and tracheal static pressures.

RESULTS

None of the 10 horses developed DDSP during 2 separate treadmill-exercise trials under the control conditions. Seven of the 10 horses developed DDSP after resection of the TH muscles, 4 of 5 of these horses still experienced DDSP after sham-treatment, but 5 of 6 horses no longer experienced DDSP at exercise after the prosthesis-treatment. There were significant anomalies in airway pressures, respiratory frequency, and occurrence of DDSP in both the TH resection and sham-treatment conditions compared to control conditions. In contrast, no statistical differences were noted in any of the parameters measured between the prosthesis-treatment and control conditions.

CONCLUSIONS

That the function of the TH muscles is important to the stability of the laryngo-palatal relationship and plays a role in the pathophysiology of exercise-induced DDSP.

POTENTIAL RELEVANCE

Management of horses with DDSP could be enhanced by restoring the function of the TH muscles.

Comparison of SLN-evoked swallows during rest and chewing in the freely behaving rabbit

Interactions between the swallowing central neural pathway and the chewing central neural pathway were examined in freely behaving, unanesthetized rabbits. Pharyngeal swallows were elicited by electrical stimulation of the superior laryngeal nerve (SLN) and defined by thyrohyoid muscle (TH) activity in the electromyogram (EMG). Recordings were obtained from rabbits at rest and during chewing. The number of swallows elicited by the SLN stimulation was significantly increased (P<0.001) during quiet oral function (at rest) and during chewing. The increased number of swallows from each baseline was similar, signifying that the effect of the SLN stimulation was similar in generating swallowing in both groups. The swallows induced with SLN stimulation were very similar to natural swallows as defined by the temporal pattern of the EMG duration and the timing of EMG activities. Our results suggest that: (1). the peripheral inputs to the swallowing pathway may rarely be modulated by the chewing pathway in the generation of swallows; (2). the swallowing pathway and the chewing pathway may interact at the level of the rhythm generators; (3). each animal has its own threshold for eliciting pharyngeal swallowing, and the threshold may be independent of the number of chews.

Effects of the inferior alveolar nerve stimulation on tongue muscle activity during mastication in freely behaving rabbits

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<FC approximately SC<control was obtained. The results suggest that the modulatory mode in the Gg and Dig reflexes may be different in the pattern of the modulation under the natural chewing behavior and the Gg reflex is independent of the masticatory muscles in the nature. The reflex could be more sensitive to control the tongue movements collecting food bolus in the OP phase during chewing than in the jaw-closing phase.

Coordination between the masticatory and tongue muscles as seen with different foods in consistency and in reflex activities during natural chewing

To study the coordination between the masticatory and extrinsic tongue muscles during natural chewing, electromyographic activities in the digastric (Dig) as a jaw opener, the masseter (Mas) as a jaw closer, the genioglossus (Gg) as a tongue protruder, and the styloglossus (Sg) as a tongue retractor as well as jaw movement trajectories were recorded while rabbits chewed soft, hard, and very hard foods. The Dig and Gg were active in the jaw-opening phase (OP active group), and the Mas and Sg were active in the jaw-closing phase (CL active group). Food consistency affected differently on the duration of burst activities between the muscle groups, i.e. in the CL active group, the duration was longer for the harder food, while there was no difference in the duration of the OP active group among the foods. During hard food chewing in particular, we confirmed our recent findings that reflexly-induced short but large bursts of activity could be documented in the Dig during the jaw-closing phase. Similar short bursts were also documented in the Gg as with the Dig in this study. Inhibitory periods were often observed in the Mas with the Dig short burst and were also observed in the Sg along with the Gg short burst; however the inhibitory effect in the Sg was less pronounced. These findings suggest that: (1) both masticatory and extrinsic tongue muscles are active in a well-coordinated manner during stable chewing, but that (2) reflex effects on antagonistic muscles (i.e. Dig vs. Mas in the masticatory muscles, Gg vs. Sg in the tongue muscles) evoked by tooth contact during chewing may not be analogous between the two muscle groups.

Tongue and jaw muscle activities during chewing and swallowing in freely behaving rabbits

To study the function of the tongue and the coordination among jaw, tongue, and hyoid muscles during chewing and swallowing, we recorded the electromyographic activities from the masseter (Mas), digastric (Dig), mylohyoid (Myl), thyrohyoid (Thy), genioglossus (Gg) and styloglossus (Sg) muscles as well as jaw movement trajectories in the freely behaving rabbit. Three phases were identified in the chewing cycle (fast- and slow-closing and opening phases). The Gg (main tongue protruder) was active synchronously with the Dig during opening. The Sg (tongue retractor) showed two peaks in each cycle, one in the opening phase and the other in the closing phase. The latter may have a role in retracting the tongue during jaw closing. The co-contraction of the antagonists (i.e. Gg and Sg) during opening may contribute to shape the tongue to be appropriate to gather the foodstuff. In the swallowing cycle, five phases were identified, two in the closing phase and three in the opening phase. Regression analysis revealed that swallowing cycles had a longer cycle duration than that of the chewing cycles due to an extra phase (a pause) inserted in the opening phase, where there was a small co-activation in the jaw opening and closing muscles. The findings suggest that the swallowing center affects masticatory center in the central nervous system, and may also support the view that the masticatory burst timing begins with the Dig activities in the middle of the opening phase.