Short-term and long-term effects of unilateral external carotid artery ligation on orofacial functions in rats.

The external carotid artery (ECA) plays a major role in supplying blood to the head and neck. Although impeded blood flow in the ECA is expected to affect orofacial functions, few studies have shown how blood flow obstruction in the ECA contributes to impairment of these functions, including chewing and swallowing. This study was performed to investigate the effects of ECA ligation (ECAL) on immediate and long-term changes in masticatory and swallowing functions as well as the jaw-opening reflex evoked in the digastric muscle. The experiments were carried out using male Sprague-Dawley rats. In the acute experiment, the digastric reflex evoked by low-threshold electrical stimulation of the inferior alveolar nerve and the swallow reflex, identified by digastric and thyrohyoid electromyographic (EMG) bursts, were compared between before and 1 h after ECAL. The chronic experiment was conducted on freely moving rats. EMGs of the masseter, digastric, and thyrohyoid muscles were chronically recorded. The long-term effects of ECAL on behavior and muscle histology were compared between rats with an intact ECA and rats with ECAL. In the acute experiment, the peak amplitude of the digastric reflex on the ECAL side was significantly decreased 1 h after ECAL. In the chronic experiment, although most parameters of the masticatory and swallowing EMGs were not significantly different between the groups, the results suggest wide variation of the effect of ECAL on the muscles. Blood supply compensation from collaterals of the internal carotid artery may be permanent in some animals.NEW & NOTEWORTHY The inhibitory effect of unilateral external carotid artery ligation (ECAL) on the ipsilateral digastric reflex was small but evident. Most parameters of masticatory and swallowing muscle activity were not significantly different after ECAL. Wide variation was noted in the effect of ECAL on the ipsilateral muscle activity. Blood supply compensation from collaterals of the internal carotid artery may occur in response to the impaired blood flow.

Impact of salivary flow inhibition on masticatory behaviours in healthy individuals.

BACKGROUND: It remains unclear how the salivary flow and the fat content of food affect bolus formation during mastication. OBJECTIVES: We aimed to clarify: (1) how hyposalivation affects jaw-closing and hyoid-elevating muscle activities in bolus formation, and (2) if the effect of hyposalivation on muscle activity depends on the fat content of food. METHODS: Eighteen healthy male volunteers were instructed to freely ingest four test foods: Plain, Fat without seasoning, Fat with seasoning, and Soft rice crackers. Masseter and suprahyoid electromyographic activities were recorded before and 30 min after the administration of atropine sulfate, a muscarinic receptor antagonist that induces hyposalivation. RESULTS: Hyposalivation extended the masticatory duration significantly in all the test foods except Fat with seasoning. Masticatory cycle time was significantly longer with vs without hyposalivation for the Soft (p = .011). Suprahyoid activity/cycle was significantly greater with vs without hyposalivation (p = .013). Masticatory cycle time was significantly longer at the late stage with vs without hyposalivation for the Soft (p < .001). Suprahyoid activity/cycle was significantly greater at the middle (p = .045) and late stages (p = .002) with vs without hyposalivation for the Soft and greater at the late stage with vs without hyposalivation for the Plain (p = .043). Changes in masticatory cycle time and suprahyoid activity/cycle for these foods had significantly positive relationship (p < .001). CONCLUSION: Hyposalivation-induced changes in masticatory behaviours resulted from the middle and late stage suprahyoid activity. Fat content and seasoning compensate for salivary flow inhibition.

Modulation of reflex responses of the anterior and posterior bellies of the digastric muscle in freely moving rats.

BACKGROUND: Chewing and licking are primarily activated by central pattern generator (CPG) neuronal circuits in the brainstem and when activated trigger repetitive rhythmic orofacial movements such as chewing, licking and swallowing. These CPGs are reported to modulate orofacial reflex responses in functions such as chewing. OBJECTIVE: This study explored the modulation of reflex responses in the anterior and posterior bellies (ant-Dig and post-Dig, respectively) of the digastric muscle evoked by low-intensity trigeminal stimulation in conscious rats. METHODS: The ant-Dig and post-Dig reflexes were evoked by using low-intensity electrical stimulation applied to either the right or left inferior alveolar nerve. Peak-to-peak amplitudes and onset latencies were measured. RESULTS: No difference was observed between threshold and onset latency for evoking ant-Dig and post-Dig reflexes, suggesting that the latter was also evoked disynaptically. The peak-to-peak amplitude of both reflexes was significantly reduced during chewing, licking and swallowing as compared to resting period and was lowest during the jaw-closing phase of chewing and licking. Onset latency was significantly largest during the jaw-closing phase. Inhibitory level was similar between the ant-Dig and post-Dig reflex responses and between the ipsilateral and contralateral sides. CONCLUSION: These results suggest that both the ant-Dig and post-Dig reflex responses were significantly inhibited, probably due to CPG activation during feeding behaviours to maintain coordination of jaw and hyoid movements and hence ensure smooth feeding mechanics.

Evaluating the effect of management on patients with oral hypofunction: A longitudinal study.

INTRODUCTION: Declining oral function may affect subsequent physical frailty in the older population. The aim of this longitudinal study was to summarise data on patients who underwent functional oral examination and evaluate the impact of management on patients with oral hypofunction (OHF). METHODS: Dental outpatients aged over 65 years at their initial visit to the Niigata University Hospital received detailed assessment of seven oral function items to diagnose oral hypofunction using diagnostic criteria defined by the Japanese Society of Gerodontology. Patients with OHF at the first assessment received management including oral health guidance for low function and dental treatment. They were re-evaluated approximately 6 months later and the two assessments were compared. According to the results of the second assessment, the patients were divided into two groups: OHF improved and OHF re-diagnosed. RESULTS: Of the 273 patients who underwent the first assessment, 86 (31.5%) were diagnosed with OHF and received management. Of those, 42 (48.8%) completed the second assessment. Comparing the first and second assessment, significant improvement was observed in oral hygiene, occlusal force, tongue-lip motor function of /pa/, mastication and swallowing. The change in values from the first assessment demonstrated a significant difference between the OHF improved and re-diagnosed groups only in occlusal force. CONCLUSION: Management for patients with OHF can contribute to the improvement of poor oral function, and an increase in occlusal force was notable in the recovery from OHF.

Relationships Between Survival and Oral Status, Swallowing Function, and Oral Intake Level in Older Patients with Aspiration Pneumonia.

The factors affecting the survival of patients with aspiration pneumonia (AP) remain unclear. This study aimed to determine whether factors, including oral status, swallowing function, and oral intake level, were related to survival outcomes in older patients hospitalized for AP. The study enrolled patients with AP who were admitted to our hospital between February 2017 and November 2019. Patients were divided into two groups based on the 90-day mortality after the first swallowing function evaluation: survivors and deceased. The data were compared between the two groups. A total of 29 patients were diagnosed with AP. Of these patients, 13 died within 90 days. The numbers of patients who could not use removable dentures and required sputum suctioning and had cough reflex at rest were significantly higher in the deceased than in the survivors. The salivary pooling and pharyngeal clearance scores evaluated by videoendoscopy, International Dysphagia Diet Standardisation Initiative Functional Diet Scale score determined after swallowing function evaluation, and consciousness level were significantly worse in the deceased than in the survivors. There were significant differences in patients' oral status, swallowing function, oral intake level, and consciousness level between the survivors and deceased.

Functional evaluation of jaw and suprahyoid muscle activities during chewing.

BACKGROUND: It has not yet been clarified how the type of the chewing task affects related muscle activity and how the suprahyoid muscles contribute to masticatory function in humans. OBJECTIVES: This study aimed to investigate the difference in the suprahyoid muscle activity between the freely and unilaterally chewing tasks and between the working and non-working sides during chewing. MATERIALS AND METHODS: Twenty healthy volunteers were instructed to chew peanuts and two different types of rice crackers in two ways: freely and unilaterally while surface electromyograms of the masseter and suprahyoid muscles were recorded. The chewing duration, number of chewing cycles and chewing rate were compared between the tasks. Furthermore, the masseter and suprahyoid muscle activities per chewing cycle were compared between the sides. RESULTS: The chewing duration was significantly longer, and the chewing rate was significantly higher during unilaterally chewing than freely chewing. The chewing duration differed significantly among the different foods; the harder the food, the longer the duration. Chewing rate and suprahyoid activity were significantly higher during soft rice cracker chewing. Masseter activity was higher on the chewing side than on the non-chewing side while there was no difference in suprahyoid activity between the sides. CONCLUSION: The current results demonstrate a difference in the masticatory efficacy between the chewing tasks and a functional role of the suprahyoid muscles during chewing, which does not differ between the chewing and non-chewing sides.

Effects of Carbonation and Temperature on Voluntary Swallowing in Healthy Humans.

We investigated how swallowing behaviors are affected by the temperature and carbonation of water in healthy humans. Twenty-nine healthy volunteers were instructed to drink as much natural water, carbonated water, or cider as they wanted, and we recorded the volume of solution swallowed and electromyographic (EMG) activity of the masseter and suprahyoid muscles. Sensory tests regarding the ease of holding the solution in the mouth and ease of swallowing were also performed. The volume of carbonated water swallowed was significantly lower than that of natural water and cider. The ease of holding and swallowing the solution significantly differed between solution types such that natural water was the easiest solution to hold and swallow, followed by cider and then carbonated water in both tests. EMG activity was also affected by the solution type. Masseter EMG activity was significantly lower when swallowing natural water compared with carbonated water. Suprahyoid EMG activity was significantly lower when swallowing natural water compared with carbonated water and cider. The volume of solution swallowed was significantly correlated with the ease of holding and swallowing the solution, but not with masseter or suprahyoid EMG activities. The ease of holding and swallowing the solution significantly affected masseter and suprahyoid EMG activities. The results suggested that when participants experienced difficulty holding and swallowing the solution, masseter and suprahyoid EMG activity increased. Considering our findings that mechanical stimulation with bubbles decreased the volume of solution swallowed and increased EMG activities, carbonated water swallowing may be useful in treating deglutition disorders.

Changes of bolus properties and the triggering of swallowing in healthy humans.

There is wide variation in chewing behaviours, even among healthy humans. Further, the way in which humans determine swallowing initiation when chewing solid foods remains unclear. The current study sought to investigate how the bolus properties change over time during chewing, and to clarify which factors affect chewing and swallowing behaviours, including swallowing initiation, in healthy humans. Twenty-four healthy volunteers were instructed to chew 8 g of steamed rice and spit it out at 50%, 100% and 150% of their own chewing duration, defined as the time of chewing from onset of the first chewing cycle to onset of the first swallow. Chewing and swallowing behaviours were monitored and determined by visual inspection of video recordings. The physical properties such as hardness, cohesiveness and adhesiveness as well as water content of the bolus were measured. In each subject, maximum bite force, tongue pressure and stimulated salivary flow rate were also measured. Hardness gradually decreased, and the cohesiveness and water content of the bolus did not change up to 50% of chewing duration, followed by a slight but significant increase. The adhesiveness of the bolus rapidly decreased at the beginning of chewing. Chewing duration was significantly related to stimulated salivary flow rate; greater salivary flow rate was associated with shorter chewing duration. Variation of chewing duration and swallowing initiation was not dependent on bolus properties during the chewing of steamed rice, but mainly depended on the surface lubrication of the bolus.

Survey of oral hypofunction in older outpatients at a dental hospital.

BACKGROUND: Oral hypofunction is defined as the reversible stage preceding oral dysfunction. However, its assessment and management need further examination and consideration. OBJECTIVE: The present study aimed to examine the current state of oral hypofunction among outpatients at the university hospital. METHODS: New outpatients underwent medical interviews and detailed assessment of oral hypofunction at their initial visit to our dental department. Oral hypofunction was diagnosed if the results of three of the following seven assessment items were below cut-off values: poor oral hygiene, oral dryness, reduced occlusal force, decreased tongue-lip motor function, decreased tongue pressure, decreased masticatory function and deterioration of swallowing function. The relationships between factors used to diagnose oral hypofunction as well as the distributions of assessment values were clarified. RESULTS: Seventy-five of 209 patients (35.9%) were finally diagnosed with oral hypofunction. Diagnosis of oral hypofunction was significantly related to patient age, Eichner index, a chief complaint of dental or periodontal disease or ill-fitting dentures and a history of neurodegenerative disease. The lowest quintile values for poor oral hygiene, oral dryness and decreased masticatory performance were all above current cut-off values. Reduced occlusal force contributed strongly to the diagnosis of oral hypofunction. CONCLUSION: Oral hypofunction in dental outpatients at a university hospital was associated with patient age, medical history and chief complaint at presentation. Diagnosis of oral hypofunction can be closely related to reduced occlusal force. The validity of cut-off values for assessments of oral hypofunction needs further consideration.

Inter-individual variation of bolus properties in triggering swallowing during chewing in healthy humans.

BACKGROUND: There is a wide variation of chewing behaviours even in healthy humans. The present study was aimed to clarify how the rheological properties of the bolus during chewing are different among individuals. We also investigated whether the swallowing threshold was consistent among the individuals and whether the difference in the bolus properties at the swallowing affected swallowing movement. METHODS: Twenty-nine healthy volunteers were asked to ingest 8 g of steamed rice. Based on the chewing duration defined by the time until first swallow, participants were asked to chew the food and then either spit it out or swallow it at 50%, 100% and 150% of chewing duration. In seven volunteers, the maximum bite force was measured. RESULTS: The hardness of the bolus gradually decreased throughout recording. Although the chewing duration varied widely, there was a significant negative correlation between time and hardness, regardless of the difference in individual chewing duration. The cohesiveness of the bolus was unchanged at the 100% time point, followed by a slight but significant increase. There was no significant correlation between the chewing duration and adhesiveness of the bolus. Swallowing-related bursts of Supra and Infra EMGs were not related to the chewing duration or bolus properties, and chewing duration did not affect swallowing function. CONCLUSION: The current results suggest that the bolus properties such as the hardness and cohesiveness during chewing are just dependent on the chewing duration and the difference in the bolus properties does not affect the swallowing initiation and movement.

Functional Role of Suprahyoid Muscles in Bolus Formation During Mastication.

It still remains unclear how the suprahyoid muscles function in bolus formation during mastication. This study aimed to investigate the contributory role of the suprahyoid muscles during mastication. A total of 20 healthy young volunteers were asked to perform tongue pressure generation tasks and unilateral mastication tasks using peanuts and two different types of rice crackers. Surface electromyographic (EMG) activity of the masseter and suprahyoid muscles and mandibular kinematics were recorded. Suprahyoid activity increased with increasing tongue pressure. Masticatory duration until the first deglutition differed significantly among the different foods; the harder the food, the longer the duration. This was also the case in masseter activity per masticatory cycle. Masticatory rate and suprahyoid activity per masticatory cycle were significantly higher during soft rice cracker mastication. Masseter activity was higher on the masticatory side than on the non-masticatory side, however, there was no difference in suprahyoid activity between the sides. Suprahyoid activity and jaw gape showed significant positive correlation in the early stage on both the masticatory and non-masticatory sides. The suprahyoid muscles functioned dominantly for jaw-opening during peanut mastication, and for bolus formation, especially in the late stage during soft rice cracker mastication. Bolus formation was performed dominantly on the masticatory side during rice cracker mastication. These findings clearly demonstrate a functional role of the suprahyoid muscles during mastication of solid foods from assessments using both EMG activity and mandibular kinematic recordings.

Effect of peripherally and cortically evoked swallows on jaw reflex responses in anesthetized rabbits.

This study aimed to investigate whether the jaw-opening (JOR) and jaw-closing reflexes (JCR) are modulated during not only peripherally, but also centrally, evoked swallowing. Experiments were carried out on 24 adult male Japanese white rabbits. JORs were evoked by trigeminal stimulation at 1 Hz for 30 s. In the middle 10 s, either the superior laryngeal nerve (SLN) or cortical swallowing area (Cx) was simultaneously stimulated to evoke swallowing. The peak-to-peak JOR amplitude was reduced during the middle and late 10-s periods (i.e., during and after SLN or Cx stimulation), and the reduction was dependent on the current intensity of SLN/Cx stimulation: greater SLN/Cx stimulus current resulted in greater JOR inhibition. The reduction rate was significantly greater during Cx stimulation than during SLN stimulation. The amplitude returned to baseline 2 min after 10-s SLN/Cx stimulation. The effect of co-stimulation of SLN and Cx was significantly greater than that of SLN stimulation alone. There were no significant differences in any parameters of the JCR between conditions. These results clearly showed that JOR responses were significantly suppressed, not only during peripherally evoked swallowing but also during centrally evoked swallowing, and that the inhibitory effect is likely to be larger during centrally compared with peripherally evoked swallowing. The functional implications of these results are discussed.

Changes in jaw muscle activity and the physical properties of foods with different textures during chewing behaviors.

This study aimed to investigate how the activity of the masseter (Mas) and suprahyoid (Hyoid) muscles is influenced by the physical properties of food, how changes in the rheological properties of food differ between different foods during the process of food reduction, and how different salivary flow rates affect bolus-making capability during masticatory behavior in healthy humans. Ten healthy adults participated in this study. Electromyographic (EMG) recordings were obtained from the Mas and Hyoid muscles, and 15 g of steamed rice and rice cake was prepared as test foods. In the ingestion test, the subjects were asked to eat each food in their usual manner. The chewing duration, number of chewing cycles before the first swallow, Mas and Hyoid EMG activity, and chewing cycle time were compared between the foods. Total chewing duration was divided into three substages: early, middle, and late; chewing cycle time and EMG activity per chewing cycle of each substage were compared between the foods and among the substages. In the spitting test, the rheological properties of the bolus at the end of each substage were compared between the foods and among the substages. Finally, stimulated salivary flow rates were measured and the relationships between salivary flow rate and chewing duration, EMG activity, and changes in physical food characteristics were investigated. There were significant differences in total chewing duration and the number of chewing cycles, but not in chewing cycle time, between the foods, which had similar hardness values. The EMG activity levels of the Mas and Hyoid per chewing cycle for the rice cake were significantly greater than for the steamed rice throughout the recording periods. While Mas activity did not change among the substages during chewing, Hyoid EMG activity decreased as chewing progressed. Chewing cycle time also gradually decreased as chewing progressed. The hardness of both foods initially increased, then gradually decreased back to baseline. The adhesiveness of the rice cake initially increased, and did not fall throughout the recording period; the adhesiveness of the steamed rice did not significantly change. Cohesiveness barely changed in either of the two foods during chewing, but was significantly greater for the rice cake than for the steamed rice. Finally, a correlation between the stimulated salivary flow rate and chewing performance was evident only in a change in Mas EMG activity. The current results demonstrate that the activities of the Mas and Hyoid muscles changed as chewing progressed, and were affected by hardness, adhesiveness, and cohesiveness. Salivary flow rate may affect the changes in Mas activity during the process of bolus formation.

How do tablet properties influence swallowing behaviours?

OBJECTIVES: Behavioural performance of tablet swallowing was evaluated with different tablet conditions in terms of size, number and surface coating. METHODS: Four different types of tablets were prepared: small or large, and with or without a surface coating. Fourteen normal male adults were instructed to swallow the prepared tablets with 15 ml of water. The number of tablets in one trial was changed from one to three. To evaluate swallowing and tablet transport, electromyographic activity was recorded in the left suprahyoid muscles, and videofluorographic images were examined. KEY FINDINGS: All tablet conditions (size, number and surface coating) affected the swallowing performance in terms of total number of swallows, electromyographic burst patterns and location of remaining tablets. Increases in the size and number of tablets increased the number of swallows and electromyographic burst area and duration. In addition, all of these parameters increased while swallowing tablets without a coating compared with tablets with a coating. Location of the remaining tablets was mainly within the mouth. CONCLUSIONS: This study only clarified the normal pattern of tablet swallowing under several conditions in healthy subjects, but the results may facilitate comprehensive evaluation and treatment planning in terms of administering medication to dysphagic patients.

Differences in Chewing Behavior during Mastication of Foods with Different Textures.

The present study was designed to evaluate how typical Japanese staple foods - rice and rice cake - with different physical properties influence the patterns of muscle activity during chewing. Six normal adults were asked to eat rice and rice cake freely in a natural manner. Surface electromyographic recordings of the masseter and suprahyoid muscles were performed. There were large differences between the foods in terms of masticatory duration. The masseter activity per cycle was significantly greater for rice cake than for rice. The masseter activity gradually decreased as chewing progressed, although the suprahyoid activity did not change throughout the sequence of rice cake chewing. The results suggest that the process of bolus formation with rice cake is very different from that with rice in terms of jaw-closing and jaw-opening muscle activities. PRACTICAL APPLICATIONS: This study aimed to examine how the masticatory sequence adapts to variations in the initial properties of foods. This research may enhance our understanding of the oral physiology of bolus formation and changes in the physical properties of foods during chewing.

Effects of chewing and swallowing behavior on jaw opening reflex responses in freely feeding rabbits.

It has been reported that the jaw opening reflex (JOR) evoked by intra-oral innocuous stimulation was suppressed during a reflex swallow in anesthetized animals only. However, the mechanism of JOR inhibition during swallowing has not yet been elucidated. The aim of the present study was to investigate the effects of peripheral nerve stimulation on masticatory behaviors, as well as the modulation of low threshold afferent evoked JOR responses during chewing and swallowing in freely feeding animals. The JOR in the digastric muscle was evoked by low threshold electrical stimulation of the inferior alveolar nerve (IAN). Changes in the peak-to-peak amplitude of digastric electromyographic responses were compared among the phases of chewing and swallowing. IAN stimulation did not produce any differences in cycle duration, gape of the jaw in one cycle, or swallowing interval, suggesting a minimal effect on feeding behaviors. The JOR amplitude during the fast-closing, slow-closing, and slow-opening phases of chewing was significantly smaller than that of the control (recorded when the animal was at rest) and fast-opening phase. During swallowing, the JOR amplitude was significantly less than the control. Inhibition of the JOR during swallowing is assumed to prevent unnecessary opposing jaw opening motion.

Differential involvement of two cortical masticatory areas in modulation of the swallowing reflex in rats.

To clarify the functional role of cortical descending inputs involved in the swallowing reflex, the effect of electrical stimulation of two cortical masticatory areas (CMAs: A- and P-area) on rhythmic jaw movements (RJMs) and superior laryngeal nerve (SLN)-evoked swallows were studied. RJMs and swallowing reflex were elicited by repetitive electrical stimulation of CMAs and the SLN, respectively. The electromyographic activities of jaw-closer (masseter), jaw-opener (digastric), and laryngeal-elevator (thyrohyoid) muscles were recorded to identify the RJMs and swallowing reflex. The number of evoked swallows was significantly lower, and swallowing interval was significantly longer during A-area stimulation compared with those without stimulation. Conversely, these parameters were not significantly altered during P-area stimulation. The inhibition of swallows by A-area stimulation was not affected by an increase in sensory input by wooden stick application between upper and lower teeth, or A-area stimulation preceding SLN stimulation. The present findings suggest that the swallowing reflex is inhibited by activation of the A-area, but not the P-area. Since no changes in swallows were seen after the increase in intraoral sensory input and prior activation of masticatory central pattern generator (CPG), swallowing inhibition may be mediated by direct inputs from the A-area or inputs via the masticatory CPG into the swallowing CPG.

Effects of electrical stimulation of the superior laryngeal nerve on the jaw-opening reflex.

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.