Masseter muscle excitation evoked by mechanical stimulation of upper incisor teeth in relaxed human subjects

Masseter length determines muscle spindle reflex excitability during jaw-closing movements

INTRODUCTION

The masticatory muscles are considered to be important determinants of facial form, but little is known of the muscle spindle reflex characteristics and their relationship, if any, to face height. The aim of this study was to determine whether spindle reflexes, evoked by mechanical stimulation of an incisor and recorded on the masseter muscle, correlated with different facial patterns.

METHODS

Twenty-eight adult volunteers (16 women; ages, 19-38 years) underwent 2-N tap stimuli to their maxillary left central incisor during simulated mastication. Reflexes were recorded during local anesthesia of the stimulated tooth to eliminate the contribution from periodontal mechanoreceptors. Surface electromyograms of the reflex responses of the jaw muscles to these taps were recorded via bipolar electrodes on the masseter muscle and interpreted by using spike-triggered averaging of the surface electromyograms. Lateral cephalometric analysis was carried out with software (version 10.5, Dolphin, Los Angeles, Calif; and Mona Lisa, Canberra, Australia).

RESULTS

Two-newton tooth taps produced principally excitatory reflex responses beginning at 17 ms poststimulus. Correlation analysis showed a significant relationship between these muscle spindle reflexes and facial heights: specifically, shorter face heights were associated with stronger spindle reflexes. This correlation was strongest between the derived measure of masseter length and the spindle reflex strength during jaw closure (r = -0.49, P = 0.008).

CONCLUSIONS

These results suggest that a similar muscle spindle stimulus will generate a stronger reflex activation in the jaw muscles of patients with shorter faces compared with those with longer faces. This finding might help to explain the higher incidence of clenching or bruxism in those with short faces and also might, in the future, influence the design of orthodontic appliances and dental prostheses.

Reflex control of human jaw muscles

The aim of this review is to discuss what is known about the reflex control of the human masticatory system and to propose a method for standardized investigation. Literature regarding the current knowledge of activation of jaw muscles, receptors involved in the feedback control, and reflex pathways is discussed. The reflexes are discussed under the headings of the stimulation conditions. This was deliberately done to remind the reader that under each stimulation condition, several receptor systems are activated, and that it is not yet possible to stimulate only one afferent system in isolation in human mastication experiments. To achieve a method for uniform investigation, we need to set a method for stimulation of the afferent pathway under study with minimal simultaneous activation of other receptor systems. This stimulation should also be done in an efficient and reproducible way. To substantiate our conviction to standardize the stimulus type and parameters, we discuss the advantages and disadvantages of mechanical and electrical stimuli. For mechanical stimulus to be delivered in a reproducible way, the following precautions are suggested: The stimulus delivery system (often a probe attached to a vibrator) should be brought into secure contact with the area of stimulation. To minimize the slack between the probe, the area to be stimulated should be taken up by the application of pre-load, and the delivered force should be recorded in series. Electrical stimulus has advantages in that it can be delivered in a reproducible way, though its physiological relevance can be questioned. It is also necessary to standardize the method for recording and analyzing the responses of the motoneurons to the stimulation. For that, a new technique is introduced, and its advantages over the currently used methods are discussed. The new method can illustrate the synaptic potential that is induced in the motoneurons without the errors that are unavoidable in the current techniques. We believe that once stimulation, recording, and analysis methods are standardized, it will be possible to bring out the real "wiring diagram" that operates in conscious human subjects.

The role of periodontal mechanoreceptors in mastication

The aim of this review is to discuss what is known about the reflex control of the human masticatory system by the periodontal mechanoreceptors and to put forward a method for standardised investigation. To deliver mechanical stimulus in a reproducible way, the following precautions are suggested: the stimulus should be brought into secure contact with the area of stimulation, and slack between the probe and the area to be stimulated should be taken up by the application of a preload. It is also important to ensure that there is minimal simultaneous activation of receptor systems other than the periodontal mechanoreceptors. It is also necessary to standardise the method for recording and analysing the response.

Response of human jaw muscles to axial stimulation of the incisor

The role of periodontal mechanoreceptors (PMRs) in the reflex control of the jaw muscles has thus far been mainly derived from animal studies. To date, the work that has been done on humans has been limited and confined to orthogonal stimulation of the labial surface of the tooth. The purpose of this study was to investigate the response of the masseter and digastric muscles in humans to controlled axial stimulation of the upper left central incisor, both before and during a local anaesthetic block of the PMRs. Ten neurologically normal young adult females were tested, each on two separate occasions to confirm the reproducibility of the results. It was found that the reflex response in the masseter was modulated by the rate of rise of the stimulus used and, to a lesser degree, the level of background muscle activity. There was little detectable change in the activity of the digastric muscle under the tested conditions and what was found could be attributed to cross-talk with the masseter. The reflex responses obtained were significantly different between subjects; however retesting the same subject on a different occasion yielded similar results. The results indicate that the most common response of the masseter muscle to brisk axial stimulation of the incisor is a reflex inhibition at 20 ms, followed by a late excitation at 44 ms. However, it is possible that this late excitation could be due to delayed action potentials and hence be artefactual. As the application of a local anaesthetic block removed or significantly reduced both of these responses, it was concluded that they originated from the PMRs. Unlike during orthogonal stimulation, slowly rising stimuli did not produce any excitatory reflex activity. This indicated a difference in jaw reflexes to forces applied in different directions, possibly due to the activation of different receptor types when stimulating the tooth in either the orthogonal or axial directions.

Influence of methodological parameters on human jaw-stretch reflexes

The influence of methodological parameters and experimental conditions on the human jaw-stretch reflex was studied in healthy subjects in order to develop a reliable tool for investigation of the excitability of motoneuron pool. Short-latency excitatory reflex responses were evoked by a custom-made stretch device with the subjects biting on a jaw-bar with their front teeth. The displacement and ramp time of the stretches were accurately controlled and automatically triggered by a computer. The reflex responses were measured in the surface electromyogram (EMG) of the masseter and anterior temporalis muscles with online monitoring of the clenching level. The peak-to-peak amplitude of the jaw-stretch reflex was shown to be proportional to the level of EMG activity during isometric contractions, to increase proportionally with increasing stretch displacement at a given ramp time, and to decrease proportionally with increasing ramp time at a given stretch displacement. There were no significant differences in the reflex amplitude between repeated recordings within one session or between different sessions. Local anesthetic around the lower incisors as well as the upper incisors had no significant influence on the reflex amplitude. However, different biting positions on the bars of the stretch device significantly influenced the amplitude of the stretch reflex.

Reflex responses induced by tooth unloading

The reflex response of the masseter muscle to the rapid unloading of a single maxillary incisor tooth was studied. Unloading of a static force of 2 N in the horizontal direction resulted in a short-latency excitation, inhibition, and long-latency excitation of masseter muscle activity occurring at latencies of approximately 13, 20, and 40 ms, respectively, with a corresponding change in bite force occurring slightly later in each case. Following the blocking of periodontal input by the injection of local anesthetic around the stimulated tooth, inhibitory responses were abolished. Therefore, it is concluded that the observed masseteric inhibition was caused by the unloading of periodontal mechanoreceptors and thus that these receptors may contribute to the jaw unloading reflex.

Physiological tremor in human jaw-muscle system

Small, quasi-rhythmical tremor of the jaw occurs at rest and during voluntary movements. In peripheral limbs, tremor consists of a component due to mechanical resonance properties of the system, and a neurogenic component mediated by a central pacemaker or neural loops. The present study attempted to determine if these components were present in jaw tremor measured with a position transducer held lightly between the incisors. When weights were suspended from the mandible, the tremor frequency was unaltered; sharp taps delivered to the jaw did not elicit any kind of damped oscillations of the system. These findings indicated the absence of mechanical resonance in the system. No correlation of the tremor signal with the electrocardiogram was found. However, a strong correlation was found between the rectified electromyographic signals recorded over the masseter muscles and the tremor signal, where the electromyographic signal preceded jaw movement by 20-30 msec. Frequency-domain analysis also showed positive peaks in a majority of coherence functions between electromyographic and tremor signals. These results suggested a strong neurogenic component of the tremor.

Conditions for excitatory or inhibitory masseteric reflexes elicited by tooth pressure in man

The reflex responses evoked by slowly rising pressure "push' stimuli to an upper lateral incisor tooth in human masseter muscle were studied. Factors such as the preload (the static force applied to the tooth by the stimulus probe before the start of the push stimulus) and the shape of the stimulus wave affected the outcome of the reflex response. When the stimulating probe did not apply preload to the tooth before the push stimulus took place, the force profile exhibited a large fast component as the probe took up the 'slack' in the periodontium. The fast component in the force profile was found to be responsible for inducing the inhibitory reflex (sole inhibitory reflex). When preload was applied, the size of the fast component in the force profile was reduced and the change in force rate became slower and smoother. This slower stimulus profile induced the sole excitatory reflex significantly more often (58% vs 21%) and the sole inhibitory reflex significantly less often (15% vs 52%) than in the experiments that used no preload. The shape of the stimulus wave that drove the stimulus probe was also of importance. Provided that a 0.5-N preload was applied to the tooth, the smoothest stimulus wave induced the sole excitatory reflex most often. Fitting a rubber attachment to the up of the probe made the push-force profile even smoother and thence more successful in inducing the sole excitatory reflex. Furthermore, the rubber tip reduced the possibility of the probe slipping off the tooth due to small and unavoidable movements of the participant's head. It is concluded that the periodontal mechanoreceptors can induce both excitatory and inhibitory reflexes on the jaw closers. The excitatory reflex becomes dominant when a smooth force is applied with preload. The inhibitory reflex becomes dominant when fast-force changes are applied on the tooth and/or no preload is used.

The effects of orthodontic treatment on isometric bite forces and mandibular motion in patients before orthognathic surgery

PURPOSE

Little is known about the effects of orthodontic treatment on oral motor function. The objective of this report is to evaluate changes in mandibular motion and maximum bite force that occur between the initiation of presurgical orthodontics and its completion before surgery.

PATIENTS AND METHODS

Fifteen patients (9 women, 6 men) with a variety of dentofacial deformities were examined before and after presurgical orthodontics. Mechanical advantage of the muscles and bite points, mandibular range of motion, maximum isometric bite force, and levels of electromyographic (EMG) activity in the anterior and posterior temporalis and masseter muscles during isometric bites were recorded on all subjects over time. Data obtained before and after completion of presurgical orthodontics were statistically compared.

RESULTS

Presurgical orthodontics reduced mandibular mobility somewhat, but the amount was not significant. Statistically significant reductions in bite force were noted after orthodontics for incisor, canine, premolar, and molar bite positions. No significant difference in the EMG/bite force slopes was obtained, nor was there any difference in the moment arms of the bite points or the muscles of mastication from orthodontics.

CONCLUSIONS

This study showed significant changes in measures of oral motor function resulting from orthodontic treatment. A larger study is needed to confirm that these results will be similar in all orthodontic patients. There is no indication that these changes are the result of physiologic alterations of the muscles of mastication. The best current explanation is that these changes result from the pain and discomfort of the orthodontic appliances and the induced malocclusion.

Masseteric facilitation induced by electrical stimulation of rat orofacial tissues

The effects of a conditioning electrical shock applied to the periodontium of the lower incisor or the glabrous area of the lower lip on the jaw-closing reflex in the anesthetized, non-paralysed rat were studied. The masseteric reflex was triggered by stimulation of the mesencephalic nucleus as a test shock and was recorded from the masseter muscle. There was facilitation of the jaw-jerk reflex, which culminated at an interval of 10-15 ms between the conditioning and the test shocks. This facilitation was not suppressed by digastric excision or by blocking a possible rebound closing reflex evoked by jaw opening. No inhibitory influence was observed. This facilitatory effects relies on an A alpha input and on cell bodies making up the mesencephalic nucleus. The direct excitatory electrical events observed in the masseter muscle after periodontal or labial stimulation proved to be due to the diffusion of the bioelectrical activity generated in the neighbouring jaw-opening muscles.

The effect of sectioning the trigeminal sensory root on the periodontally-induced jaw-opening reflex

The experiment was designed to determine the pathway taken to the brain stem by periodontal afferents responsible for the digastric jaw-opening reflex induced by tooth-tapping. Cutting the trigeminal sensory roots of anesthetized decerebrate cats eliminated the ipsilateral periodontally-induced reflex, although the stretch reflexes of the jaw-closing muscles were undiminished. These results suggest that periodontal afferents causing the jaw-opening reflex reach the brainstem through the Vth sensory root, and confirm that muscle spindle afferents travel through the Vth motor root.

Changes in neuromuscular reflexes in the masseter muscles during functional jaw orthopedic treatment in children

Masseteric reflexes following standardized taps on an upper incisor were recorded electromyographically (1) in children with Class II, Division 1 malocclusions at different times before and during the treatment with the Bionator functional appliance and (2) during an observation period in control subjects who were not being treated. In each experimental session, 72 taps were delivered during clenching at 10% of the maximal electromyographic output. Statistical criteria provided by a previously developed computer program allowed the objective determination of inhibitory and excitatory periods in the electromyographic output following the stimulus, which together constitute the poststimulus electromyographic complex (PSEC). The influence of the change in occlusal relations on the PSEC was determined in ten subjects by comparing pretreatment results with those obtained after 1, 3, and 6 months of treatment. Concurrent with an improvement of the sagittal jaw relation in the patients with a pronounced disharmony, a short latency excitatory peak appeared in their PSECs. It was demonstrated by averaging nonrectified electromyographic activity that this peak was brought about through synchronization of motor units. In the control group, however, the morphology of the PSEC was reproducible over time. Successful functional therapy is thus accompanied by specific transient changes in the reflex response of the masseter muscles to standardized tooth taps. The neural pathways contributing to these observations originate in periodontal and muscular receptors, and might provide a clue for the mechanism underlying successful functional treatment.

Post-stimulus EMG complexes in the masseter muscle following gingival electrical stimulation in man

Post-stimulus EMG complexes (PSECs) were elicited by means of electrical stimulation of the gingiva in six subjects during clenching in three biting positions, namely the maximal occlusion, the incisor edge-to-edge and the hyperpropulsive biting position. No interrelationship was found between the mechanically and electrically elicited PSECs. With electrical stimulation the PSEC morphology was not related to the biting position in contrast with the PSECs elicited with mechanical stimulation. Following gingival electrical stimulation, a P wave was not present in the PSEC.

Intracranial stimulation of the trigeminal nerve in man. II. Reflex responses

The reflex responses evoked by direct electrical stimulation of the intracranial portion of the trigeminal nerve have been studied in 16 subjects undergoing percutaneous retrogasserian thermocoagulation for the treatment of trigeminal neuralgia affecting the second or third division. In the obicularis oculi muscle, early and late responses similar to the R1 and R2 components of the blink reflex were recorded. The former could be evoked only by stimulation of the second division and its latency was consistent with intermediately fast afferents. A late reflex (50-70 ms) was occasionally recorded from the anterior belly of the digastric muscle. The response was sometimes followed by a later activity and showed the features of a polysynaptic reflex. No response was obtained in the jaw elevators when fully relaxed. With the subject voluntarily clenching his teeth, both an early "H-like" response and two silent periods in the background EMG were obtained. The second silent period was similar in the muscles ipsi- and contralateral to intracranial stimulation, while the first silent period was longer in the ipsilateral muscles. Possible mechanisms contributing to the inhibition following stimulation of the mixed portion of the nerve are discussed.

Electromyographic recordings of human jaw-jerk reflex characteristics evoked under standardized conditions

A device for imparting reproducible chin taps was employed to evoke monosynaptic jaw-closing reflexes in subjects without (group A) and with (group B) overt muscle-joint pain dysfunction. Latency, duration and amplitude values obtained were consistent within an individual at constant tap force at a single-recording session but varied among subjects. Latency differences between sides were greater in subjects exhibiting mild to moderate dysfunction (group B) than in group A, but there were not corresponding differences in duration and amplitude. Stimuli were delivered in 5 subjects whilst the jaw was firmly held at postural jaw position with the aid of acrylic resin indices secured with adhesive to the maxillary and mandibular teeth. Recordable reflex responses were evoked in the masseter muscles of one subject only, indicating that vibration alone was not an adequate stimulus consistently to evoke a jaw-closing reflex at postural jaw position. Spindle stretch is needed, unless postural motoneurone excitability is at a sufficiently high level. Reproducible jaw-closing reflexes were evoked following standardized stimuli; subtle variations in motoneurone excitability, such as reflected by differences in jaw-jerk latency between sides, may then become apparent.

Short and long-latency responses in human masseter muscle evoked by chin taps

Ten healthy subjects received downward, backward and upward chin taps with standardized force and direction. Taps in all three directions elicited short (M1) and long (M2 and M3)-latency excitatory EMG responses in the masseter muscle. M1 (mean latency 8.9 ms, amplitude 150 microV) occurred more frequently and with shorter latency during clenching than during relaxation or opening. Downward taps were more efficient in evoking jaw-jerks (25 per cent) than upward (14 per cent) and backward ones (16 per cent). M2 was a weak biphasic deflection occurring during the silent period (mean latency 42 ms, amplitude 87 microV) and M3 was an EMG burst following the silent period (mean latency 69.3 ms, amplitude 169 microV). No long-latency evoked responses were obtained from the relaxed masseter. Thus, muscle stretch increases M1 response frequency but is not a necessary prerequisite. The hypothesis that vibratory transmission plays an important role was confirmed. Voluntary activation of the stimulated muscle (clenching) increases M1-response frequency and shortens response latency.

Factors influencing the excitatory-masseteric reflex evoked by gingival electrical stimulation in man

An excitatory-masseteric reflex response was evoked by electrical gingival stimulation in five subjects; the reflex had an onset latency of 7.6 ms with a SD of 0.7 ms. The response occurred just prior to the silent or inhibitory period. There was consistent post-inhibitory synchronization of the EMG activity record in all subjects. Several methods were used to demonstrate this short-latency reflex. The first was to average non-rectified EMG activity. The second was to increase the number of receptors which are simultaneously activated by using multiple electrodes so that a greater area of gingiva could be stimulated. A third was to increase the level of excitability of the motoneurons in the masseteric pool by increasing voluntary bite force. Fourthly, the excitatory reflex was evoked during increased-excitatory drive to the pool, with the additional feature that the reflex appeared at a time when no ongoing EMG activity was present in the masseter muscle. Little doubt should now remain concerning the existence of a short latency excitatory reflex from intra-oral receptors to masseteric motoneurons in man.