Do muscle-spindle afferents act as interneurons during mastication?
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Animals↗
Biomedical subjects
J P Lund
Publications and source records attributed to J P Lund.
Modification of rhythmical jaw movements by noxious pressure applied to the periosteum of the zygoma in decerebrate rabbits.
The purpose of this study was to describe and quantify changes in the movement and EMG patterns caused by tonic noxious pressure on the periosteum of the zygoma during electrically induced rhythmic jaw movements in the decerebrate rabbit. Eight New Zealand rabbits were anesthetized with urethane. EMG electrodes were placed in the masseter and digastric muscles and a light was attached to the mandibular symphysis to track jaw movements. The animals were decerebrated and the anesthetic was discontinued. Rhythmic jaw movements were evoked by electrically stimulating the corticobulbar tracts (1-msec rectangular pulses, 50 Hz), in the absence and presence of tonic noxious pressure applied bilaterally to the zygomatic periosteum (range: 400-1500 kPa). The overall response to tonic noxious pressure was a statistically significant decrease in the frequency and amplitude of the rhythmic jaw movements and in the mandibular velocity during opening and closing. The slowing of the frequency was associated with a significant increase in the duration between muscle bursts. In those animals in which the jaw closing muscles were most active, there was a significant decrease in the area of the masseter muscle bursts during jaw closure. The changes in motor activity in response to the application of tonic noxious pressure are similar to those that have been reported for patients experiencing musculoskeletal pain, suggesting that pain modifies motor programs at the segmental level. Our data support the pain-adaptation model.
Animals↗
Validity of diagnostic and monitoring tests used for temporomandibular disorders.
Currently, diagnosis of temporomandibular disorders (TMD) depends on a comprehensive history and physical examination, supplemented, when indicated, by images of hard and soft tissues. However, there are electronic diagnostic devices being marketed to acquire other measures described as relevant to TMD and to use these for diagnosis of TMD and for monitoring the effects of treatment. This paper reviews the capacity of several devices to measure these variables accurately and reliably and to assess the theoretical basis of each of these tests. Diagnostic ability was established, when possible, according to the commonly accepted measures of sensitivity, specificity, and positive predictive values. It was found that many tests lack theoretical validity, that measurement validity tends to be poor, and that diagnostic ability can be even worse than chance, because of a high percentage of false-positive diagnoses. Based on these findings, the use of these instruments in clinical practice is inappropriate at this time and may lead to the treatment of large numbers of subjects who have no disorder.
Auscultation↗
Jaw pain prevalence among French-speaking Canadians in Québec and related symptoms of temporomandibular disorders.
The purpose of this study was to assess the prevalence and pattern of self-reported TMD jaw pain in a randomized stratified sample from the general population living in the Province of Québec, Canada. Through a telephone survey, standardized questions were asked to 897 French-speaking respondents, aged 18 years old and over, regarding frequency, severity, daily pattern of jaw pain, presence of difficulty in opening, joint clicking, and sleeping problems. All prevalence estimates were adjusted to the sociodemographic distribution of the non-institutionalized population. The results indicate that TMD jaw pain is self-reported by 30% of the general population; however, the prevalence of cases reporting frequent episodes (quite often or very often) is estimated at 7%, with more than two-thirds (69%) of the respondents in this subgroup experiencing moderate to severe pain. The prevalence rates of frequent difficulty in opening and joint clicking were estimated at 9% and 4%, respectively. Approximately one in four subjects with frequent episodes of jaw pain also reported frequent joint clicking or difficulty in opening, and a strong association (Gamma coefficient > 0.6) was found among all three TMD symptoms. Our data suggest that the prevalence of clinically significant TMD-related jaw pain (frequent jaw pain of moderate to severe intensity) is approximately 5% in the general population of the Province of Québec. In the nine months preceding the survey, about 2% of the total population sought treatment for a TMD symptom.
Adolescent↗
Clinical diagnosis of orofacial pain: impact of recent FDA ruling on electronic devices.
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Device Approval↗
Cortical reorganization and deafferentation in adult macaques.
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Afferent Pathways↗
Effects of posture on the coordination of respiration and swallowing.
1. Although a pause in respiration during swallowing is a feature common to all mammals, inhibition usually occurs during expiration in adult humans and during inspiration in most other species. We tested the hypothesis that this difference is due, at least in part, to the position of the body while feeding. 2. The coordination of respiration and swallowing was studied in adult human subjects in two body positions; upright, which is typically human, and on hands and knees, which is similar to the feeding posture of most other animal species. 3. Our major finding was that the respiratory phase in which swallowing occurred was significantly related to posture. Swallows tended to occur late in the expiratory phase while feeding upright, but during early expiration while on all fours. 4. We speculate that the phase of respiration in which swallowing occurs changes with posture to compensate for the alterations in the mechanical properties of the upper body.
Animals↗
Within-subject comparisons of implant-supported mandibular prostheses: psychometric evaluation.
In a within-subject cross-over clinical trial, psychometric and functional measurements were taken while 15 completely edentulous subjects wore mandibular fixed prostheses and long-bar removable implant-supported prostheses. In this paper, the results of a psychometric assessment are presented. Eight subjects first received the fixed bridge and seven the removable type. After having worn a prosthesis for a minimum of two months, subjects responded to psychometric scales that measured their perceptions of various factors associated with prostheses. They also chewed test foods while masticatory activity was recorded. The prostheses were then changed and the procedures repeated. At the end of the study, patients were asked to choose the prosthesis that they wished to keep. Patients assigned significantly higher scores, on visual analogue scales, to both types of implant-supported prostheses than to their original conventional prostheses for all factors tested, including general satisfaction. However, no statistically significant differences between the two implant-supported prostheses were detected except for the difficulty of chewing carrot, apple, and sausage. For these foods, the fixed prostheses were rated higher. Subjects' responses to category scales were consistent with their responses to the visual analogue scales. These results suggest that, although patients find the fixed bridge to be significantly better for chewing harder foods, there is no difference in their general satisfaction with the two types of prostheses.
Adaptation, Psychological↗
Within-subject comparisons of implant-supported mandibular prostheses: choice of prosthesis.
Although previous studies have demonstrated that implant-supported prostheses are more satisfactory and efficient for edentulous patients than are conventional prostheses, until now no investigation has directly compared different types of implant-supported prostheses. We carried out a within-subject cross-over clinical trial with fixed and long-bar removable implant-supported mandibular prostheses. Fifteen subjects were randomly divided into two groups. One group received the fixed prosthesis first, while the other first received the removable. After a two-month adaptation period, psychometric measurements of various aspects of the prostheses and physiological tests of masticatory efficiency were carried out. The prostheses were then changed, and the procedures repeated. At the end of the study, subjects chose the prosthesis they wished to keep. In this paper, we report on the data gathered at this last appointment. Eight subjects chose the fixed (F group), and seven chose the removable (R group). Both groups rated stability and ability to chew with the fixed as significantly better than with the removable. However, the R group rated ease of cleaning as the most important factor governing their decision, followed by esthetics and stability. The F group considered stability to be the most important factor in their decision, followed by chewing ability and ability to clean. There was a tendency for the removable to be chosen by older subjects (+50 years). These results suggest that patients choose fixed or removable implant-supported prostheses for specific reasons, and that patient attitudes should be considered when the design of a prosthesis is being planned for an individual patient.
Adaptation, Psychological↗
Pain responses to experimental chewing in myofascial pain patients.
Although patients with myofascial pain of the masticatory muscles often report that chewing exacerbates their pain, this has never been verified experimentally. In this study, pain was assessed before and after chewing in 20 asymptomatic subjects and in 61 patients with muscle pain. First, self-reports of pain were obtained with a checklist and on five-point category scales (CAT) at the screening visit. None of the asymptomatic subjects reported that mastication or other jaw movements caused pain. On the other hand, the majority of patients reported that movements were painful (67.2%, checklist; 78.7%, CAT). Afterward, pain intensity at rest and after chewing on wax for 3 min was reported on 100-mm Visual Analogue Scales (VAS). No asymptomatic subjects had pain before or after the chewing test, while about 50% of the patients reported an increase of pain after chewing. In this subgroup, mean pain intensity increased by 102.6%. However, mean pain intensity after chewing decreased by 56.6% in about 30% of the patient sample. These patients had significantly higher resting pain than the first subgroup. These data show that a short chewing test can exacerbate pain in most myofascial pain patients but has no effect in asymptomatic subjects. Surprisingly, the exercise decreased pain in an important subgroup of patients. These results suggest that two subgroups of myofascial pain patients may exist with opposite reactions to exercise. It remains to be seen if these reactions are due to two different pathologies or to the fact that the pre-exercise pain levels were significantly different in the two groups.
Adolescent↗
Within-subject comparisons of implant-supported mandibular prostheses: evaluation of masticatory function.
In the past, fixed prostheses were believed to be more efficient implant-supported devices than removable types for edentulous patients. However, this hypothesis was never properly tested. Therefore, a within-subject crossover clinical trial was designed in which 145 completely edentulous subjects were tested wearing implant-supported mandibular fixed prostheses and long-bar overdentures. Eight subjects received the fixed appliance first and seven the removable type. The patients' perceptions of various characteristics of the implant-supported prostheses were measured after a minimum of two months' adaptation. Mandibular movements and jaw muscle electromyographic activity were recorded while the patients chewed five standard-sized test foods: bread, apple, hard cheese, sausage, and raw carrot. The prostheses were then changed, worn for the same period of adaptation, and the procedures repeated. There were three test sessions per prosthesis, and each included five trials per food. The measurements were repeated three times at one-week intervals. Mastication time was found to be shorter for three foods (bread, cheese, and sausage) when subjects wore the long-bar overdenture. The vertical amplitude of the masticatory strokes was significantly less with the overdenture for all foods except carrot. Cycle duration was significantly longer with the overdenture for sausage and carrot. Contrary to what might be expected, the long-bar overdenture appears to be no less efficient than the fixed prosthesis. Furthermore, these data suggest that patients are capable of adapting their masticatory movements to the characteristics of the two prostheses.
Adult↗
Comparison of pain and quality of life in bruxers and patients with myofascial pain of the masticatory muscles.
Although it has been suggested that bruxism is a cause or a risk factor in myofascial pain of the masticatory muscles, the prevalence of pain in bruxers and its characteristics have not been assessed or compared to those of myofascial pain patients in general. In this study, self-reports of pain and quality of life were recorded on 100-mm visual analogue and five-point category scales from two research populations: (1) 19 nocturnal bruxers who participated in a polysomnographic study and (2) 61 patients with myofascial pain of the masticatory muscles with no evidence of bruxism who participated in a controlled clinical trial on the efficacy of oral splints. The data show that pain was more intense in those bruxers who reported pain than among the myofascial pain patients, even though pain was not the chief complaint of bruxers. Both conditions reduced the patient's quality of life, although pain patients (either bruxism or myofascial pain) appeared to be much more affected than bruxers who were pain-free. The fact that pain from bruxism was worst in the morning suggests that it is possibly a form of postexercise muscle soreness. Myofascial pain, which was worst late in the day, is likely to have a different etiology.
Adolescent↗
An immunocytochemical and autoradiographic investigation of the serotoninergic innervation of trigeminal mesencephalic and motor nuclei in the rabbit.
The results of a previous experiment suggest that the cell bodies of many jaw closing muscle spindle afferents in the trigeminal mesencephalic nucleus of the rabbit are phasically inhibited during fictive mastication. The aim of this study was to investigate one possible neurotransmitter system that could be involved in this modulation, serotonin, by use of receptor autoradiography techniques and immunofluorescence combined with retrograde labelling of masseteric spindle afferents and motoneurons. A second objective was to compare the serotonin innervation of neurons in the trigeminal mesencephalic nucleus with that of masseteric motoneurons. Serotoninergic fibres were seen surrounding labelled masseteric spindle afferents, as well as unlabelled neurons, in the trigeminal mesencephalic nucleus. These fibres were close to the cell bodies and sometimes to the axon hillocks of the neurons. Although it has been reported that many neurons of the trigeminal nucleus are multipolar in some species, none of the labelled spindle afferent in this study had more than one process. Throughout the motor trigeminal nucleus, serotonin fibres were found in close proximity with cell bodies and with the proximal portions of axons and dendrites of labelled and unlabelled motoneurons. Serotonin fibres were also seen adjacent to cell bodies and processes of efferent neurons in cell group k. Autoradiography with several tritiated ligands was used to reveal the presence of receptors for serotonin as well as its uptake sites. Only serotonin2 receptors were found to be abundant in the trigeminal mesencephalic nucleus. The motor nucleus and cell group k contained serotonin2 and serotonin3 receptors, as well as serotonin uptake sites. Serotonin1A receptors appear to be absent from both nuclei. The findings suggest that release of serotonin from fibres in close proximity to trigeminal primary afferent somata could modify the transmission of action potentials from muscle spindle receptors during mastication through an action on serotonin2 receptors. In the motor nucleus and cell group k, serotonin may alter neuronal properties through actions on at least two receptor subtypes (serotonin2 and serotonin3).
Action Potentials↗
An investigation of the coupling between respiration, mastication, and swallowing in the awake rabbit.
1. The interrelationships between respiration, mastication, and swallowing were studied in awake rabbits previously prepared for the chronic recording of jaw movements and the electromyographic activity of jaw and throat muscles. These signals were recorded before, during, and after the mastication of rabbit chow, together with respiratory movements. 2. The onset of activity in the thyrohyoid muscle was used as a marker for swallowing. Measurements were made of cycle duration, and the phase relationship between the masticatory and respiratory rhythms was calculated. Deviations in masticatory and respiratory movements during swallowing were detected; the phases of the masticatory and respiratory cycles in which the deviations occurred were determined, and the intervals between the deviations and the swallowing marker were calculated. 3. Two characteristic swallowing patterns were observed that were called Interposed and Terminal swallows. Interposed swallows occurred within a masticatory sequence, while Terminal swallows ended the sequence. 4. In most rabbits, respiratory rate slowed during mastication. This brought the average frequency of the two movements closer together, and there was a weak but significant correlation between the masticatory and respiratory rates. However, respiration and mastication were not linked on a cycle-by-cycle basis, and the change in respiratory rate was found to be unrelated to masticatory rate. Further, there was no tendency for the masticatory and respiratory rhythms to adopt any particular phase relationship as swallowing was approached. Some weak and transient phase coupling of the two systems occurred after swallowing. 5. During swallowing, respiration was most often inhibited soon after the start of the inspiratory phase. Swallows within a masticatory sequence occurred most frequently during the early opening phase of the masticatory cycle, and swallows after the end of a sequence occurred with the jaw in the resting position. There was a strong tendency for the durations of the masticatory and respiratory cycles containing deviations to be longer than preceding control cycles, and the effect on respiration lasted several cycles. 6. For both Interposed and Terminal swallows, a tight temporal relationship was observed between deviations in respiration and the swallowing marker: most deviations tended to occur before swallowing. In contrast, the time of deviations in mastication relative to the swallowing marker depended on swallow type and was more variable. Finally, there was no link between the start of the pauses in the two rhythms. 7. We conclude that respiratory and masticatory processes are not strongly coupled.(ABSTRACT TRUNCATED AT 400 WORDS)
Animals↗
Evidence that the masticatory muscles receive a direct innervation from cell group k in the rabbit.
These experiments have shown that a group of neurons lateral to the trigeminal motor nucleus innervates the muscles of mastication. The work began to describe the location of digastric last-order interneurons, using the technique of transneuronal labeling with wheatgerm agglutinin-conjugated horseradish peroxide injected into the left digastric muscle of rabbits under general anaesthesia. Four to eight days later, the animals were killed with an overdose of anaesthetic and perfused. Coronal sections of the frozen brainstem were cut at 20 microns thickness and processed for peroxidase activity. Motoneurons in the ventral and caudal divisions of the trigeminal motor nucleus were labeled in all animals as expected. An additional population of neurons located ventrolaterally to the motor nucleus in cell group k were also found to be labeled if the survival time was five days or more. In an attempt to determine whether cell group k neurons were labeled transynaptically, two series of control experiments were carried out. In the first, crystals of fluorescein- and rhodamine-conjugated dextran amines and horseradish peroxidase were applied directly to central ends of cut digastric nerves. In the second, central ends of cut digastric nerves were enclosed in cuffs containing 40-60% horseradish peroxidase solutions. Again, neurons in both the trigeminal motor nucleus and cell group k were labeled suggesting that neurons within cell group k project to the digastric muscle. Similar experiments using dextran amines and wheatgerm peroxidase were carried out on the masseter muscle. Motoneurons in the dorsomedial and rostral half of the trigeminal motor nucleus, as well as primary afferent cell bodies in the mesencephalic nucleus of the trigeminal nerve, were labeled in all experiments. In addition, a population of neurons in cell group k, dorsal to those associated with the digastric muscle, were found to contain each one of the reaction products. Since it is thought that only the wheatgerm agglutinin-conjugated horseradish peroxidase transferred from one neuron to another, we conclude that cell group k neurons provide an additional innervation to the digastric and masseter muscles.
Animals↗
The pain-adaptation model: a discussion of the relationship between chronic musculoskeletal pain and motor activity.
Articles describing motor function in five chronic musculoskeletal pain conditions (temporomandibular disorders, muscle tension headache, fibromyalgia, chronic lower back pain, and postexercise muscle soreness) were reviewed. It was concluded that the data do not support the commonly held view that the pain of these conditions is maintained by some form of tonic muscular hyperactivity. Instead, it seems clear that in these conditions the activity of agonist muscles is often reduced by pain, even when this does not arise from the muscle itself. On the other hand, pain causes small increases in the level of activity of the antagonist. As a consequence of these changes, force production and the range and velocity of movement of the affected body part are often reduced. To explain how such changes in the behaviour come about, we propose a neurophysiological model based on the phasic modulation of excitatory and inhibitory interneurons supplied by high-threshold sensory afferents. We suggest that the "dysfunction" that is characteristic of several types of chronic musculoskeletal pain is a normal protective adaptation and is not a cause of pain.
Adaptation, Physiological↗
Discharge patterns of trigeminal commissural last-order interneurons during fictive mastication in the rabbit.
1. The aim of these experiments was to examine the physiological properties and patterns of firing of trigeminal interneurons during fictive mastication in anesthetized and paralyzed rabbits. Antidromic stimulation was used to show that the 82 interneurons projected to the area of the contralateral fifth nerve motor nucleus (NVmot). 2. Straight-line conduction velocities calculated from stereotaxic coordinates of the stimulating and recording electrodes for 63 interneurons were found to range between 3.7 and 16.3 m/s (mean, 9.5 m/s). 3. Histological reconstructions of recording electrode tracks showed that the interneurons observed in this study were located in the lateral brain stem in or just medial to the rostral trigeminal sensory nuclei, including the intertrigeminal (NVint) and supratrigeminal (NVs) areas, the main sensory nucleus of the fifth nerve (NVsnpr), the rostral subdivision of the oral nucleus of the spinal trigeminal tract (NVor tau), and the rostral part of the parvocellular reticular nucleus (NRpc alpha). 4. Forty-six interneurons were shown to have low-threshold (LT) peripheral receptive fields, and 41 of these (88%) were in the oral cavity. Most of the responses were rapidly adapting. 5. Twenty-eight interneurons changed their pattern of firing during cortically induced fictive mastication. The discharge frequency of 20 neurons varied in phase with the fictive masticatory motor output, which was recorded from central ends of cut hypoglossal nerves (XII) and/or from the NVmot. Others were briefly excited and then inhibited (n = 2), only inhibited (n = 4), or tonically excited during fictive mastication (n = 2). Fifteen others were unaffected by this test. 6. It was found that the rhythmically active neurons could be further subdivided into two categories: those receiving short-latency excitatory input from the masticatory area of the cortex (n = 11) and those that did not (n = 9). No obvious differences in peripheral receptive fields for neurons in these categories were found. 7. We suggest that these phasically active premotor neurons are part of the circuitry generating the rhythmic masticatory pattern, specifically those that directly control the bursts of firing of the trigeminal motoneurons (burst generators, BGs). Their properties allow them to integrate sensory information and descending commands with the masticatory rhythm that is probably generated in midline brainstem reticular nuclei.
Animals↗
Power and sample size calculations for clinical trials of myofascial pain of jaw muscles.
When a clinical trial is planned, the approximate number of subjects needed for significant differences between/among groups to be detected must be estimated. Sample-size calculations provide the investigator with this information. This paper discusses the choice of outcome measures and describes the steps used to estimate the numbers of subjects necessary for a study that compares treatments for patients with chronic myofascial pain of jaw muscles. Within- and between-subject variances were estimated for the chosen variables, the subjects' pain ratings on visual analogue scales. Sample sizes were then calculated for theoretical differences between groups by pre-treatment means and overall standard deviations (Cohen, 1977). The results of this analysis can be used by other researchers when planning studies involving these types of patients.
Adolescent↗