Effect of stimulus parameters and contraction level on inhibitory responses in human jaw-closing muscles: Implications for contingent stimulation

Objective

Examine the effect of stimulus duration, intensity and level of muscle contraction on the inhibitory responses evoked by electrical stimuli in human jaw-closing muscles applied to the right mental nerve.

Design

The inhibitory jaw-reflexes, short-latency (ES1) and long-latency (ES2), were recorded in the surface electromyogram (EMG) of masseter and temporalis muscles in 16 healthy subjects. Three stimulus durations (1 ms single square-wave pulse, 10 and 450 ms square-wave pulse train), two stimulus intensities adjusted to perceived intensity of 3 (non-painful) and 7 (distinct painful) on a 0–10 verbal rating scale were applied to the right mental nerve while the subject was biting at 25% and 50% of the maximal voluntary contraction (MVC).

Results

The magnitude of suppression in the ES2 evoked by 1 and 10 ms stimuli was dependent on stimulus intensity (P < 0.002 and P < 0.001, respectively) but not contraction level. However, ES1 could not be observed in most of the recordings. There were significant decreases evoked by the 450 ms stimuli in RMS–EMG values in the 400–500 ms compared with the pre-stimulus interval (P < 0.001) which was dependent on contraction level (P < 0.01) but not on stimulus intensity (P = 0.486).

Conclusions

The present results suggest that the ES2 reflex response is associated with the duration of the electrical stimuli, the intensity level but not the contraction level. In contrast, the inhibitory effects of ultra-long stimuli (450 ms) are not specifically related to the intensity level suggesting that this is a non-nociceptive response.     

Modulation of exteroceptive suppression periods in human jaw-closing muscles by subanaesthetic dose of propofol

Exteroceptive suppression (ES) periods in human jaw-closing muscles can be conditioned by a wide range of somatosensory stimuli and cognitive states. The aim of this study was to examine the effects of subanaesthetic doses of midazolam, ketamine and propofol on the short latency (ES1) and long latency (ES2) reflex in the jaw-closing muscles. First, we tried to evaluate the various methodological criteria for ES recording. We then examined the effect of subanaesthetic doses of midazolam (0·035 mg kg(-1)), ketamine (0·30 mg kg(-1)) and propofol (0·35 mg kg(-1)) on these reflexes of recording left masseter and temporalis muscle. ES duration did not differ greatly in the present study, recorded with the correct adjustment of stimulating and recording conditions. None of the subanaesthetic doses of the agents influenced ES1, and no significant effects on ES2 were observed with midazolam and ketamine. However, significant inhibitory change was observed in ES2 with propofol. ES2 is thought to be mediated by afferents, which descend in the spinal trigeminal tract and connect with a polysynaptic chain of excitatory interneurones located in the lateral reticular formation. Our observations indicate that propofol is uniquely effective not only through involvement of the gamma-aminobutyric acid type A receptor, but also through a range of other effects.     

Normal side-to-side variation of the exteroceptive suppression of masseter muscle in young dentate adults

Previous studies have shown that various stimuli applied in the orofacial region, evoke bilateral inhibitory responses in the jaw elevator muscles. The exteroceptive supression (ES) of the masseter muscle after electrical stimulation of the mental nerve, often appears as a double phase of inhibition interrupting the voluntary sustained contraction of the muscle. The aim of the present study was to investigate the normal bilateral variation of the masseteric ES in a control group of 20 healthy dentate adults, and to determine the range of its boundaries. The reflex was elicited by electrical stimulation of the mental nerves, during maximum intercuspal clenching. Generally, the overall mean values for the latencies and durations were in line with those reported in previous investigations, while no significant differences were found between the left and the right masseter muscles, regardless of the side of stimulation. However, in the intra-individual analysis, significant bilateral variation was occasionally recorded, particularly in the latency and duration of the late exteroceptive suppression. It was concluded that the exteroceptive suppression of the masseter muscle is a repeatable and clear bilateral reflex response, although an increased number of observations is recommended in normal subjects, before its use could be extended for diagnostic purposes.     

Indications for jaw gape‐related control of relative muscle activation in sequent chewing strokes

Summary Jaw muscle activity ratios in unilateral isometric biting differ from ratios of unilateral chewing but approach the latter if the jaw gape in biting is made as small as the minimum interocclusal distance (MID) of chewing. Especially, the masseter working/balancing side ratio (W/B‐ratio) becomes as asymmetric as in chewing, because of reduction in balancing side (BS) masseter activity. This behaviour of ratios might reflect a ‘chewing‐specific’ motor strategy induced when isometric biting is performed with a ‘chewing‐like’ gape. If this hypothesis applies, activity ratios should be associated with MIDs of sequent chewing strokes in a similar manner as with incremented jaw gapes in isometric biting. To test this prediction, bilateral surface electromyograms of masseter and anterior temporalis muscles and incisor movements were recorded during unilateral chewing in 52 subjects. W/B‐ratios of masseter and temporalis activities and temporalis/masseter‐ratios on both sides were calculated. The ratios were related to MIDs of consecutive chewing cycles. Three of the four ratios were associated with masticatory MID in the same manner as with jaw gape in isometric biting. In particular with decreasing MID, the masseter W/B‐ratio increased from 1·5 to 2·2 (P < 0·01). This increase in asymmetry was attributed to a stronger decrease in masseter activity on the BS than on the working side. We conclude that relative jaw muscle activation is associated with interocclusal distance in a similar way in isometric biting and in chewing. This analogy supports the idea of a common jaw gape‐related neuromuscular strategy facilitated by afferent signalling of interocclusal distance.     

The effects of a maximal jaw clench on an inhibitory jaw reflex

Summary  The purpose of this study was to determine whether mimicking symptoms of temporomandibular disorders by experimentally activating deep nociceptors in the oro-facial region, can modulate an inhibitory jaw reflex. In human subjects, electromyograms were recorded from one (eight subjects) or both (16 subjects) active masseter muscles and electrical stimuli were applied to the upper lip. This procedure was performed before and after a 30-s conditioning period in which the subjects maximally clenched the jaw. In all subjects, the electrical stimuli produced an inhibition of masseter activity. Following conditioning, there was a small but not statistically significant decrease in the mean size of this inhibition ( anova : P  = 0·066 and P  = 0·077, for responses recorded ipsi- and contralaterally to the stimulus). There was no relationship between changes in the reflex and the levels of pain induced by the conditioning procedure and recorded on 100 mm visual analogue scales (range = 0–64 mm, median = 11 mm) (Spearman's correlation test: P  = 0·412). These findings suggest that inhibitory jaw reflexes are little if at all affected by this conditioning procedure. The hypothesis that temporomandibular disorders both cause and are sustained by a decrease in protective jaw reflexes is not supported by these findings.     

Mandibular joint control of genioglossus muscle activity in the cat (Felis domesticus) and monkey (Macaca irus).

Jaw position had a profound influence on genioglossus (GG) muscle activity, the amount of activity varying with the amount of jaw opening. Mandibular rotations of 21 from tooth contact position increased the GG activity which was maintained as long as the jaw was opened. The response could be abolished by bilateral local anaesthesia of the mandibular joint (MJ). Bipolar stimulation of the small branches of the auriculotemporal nerve which supply the joint elicited a reproducible reflex response in the ipsilateral GG muscle with a latency of approximately 10 ms. The reflex could be abolished by administering a muscle relaxant or by severing or anaesthetizing the branches of the auriculotemporal nerve central to the stimulating electrode. Anaesthetizing or severing the nerves supplying the masseter or temporalis muscles did not abolish the GG reflex response to the nerve stimulus or the response to jaw opening. A prior threshold glossopharyngeal (IX) or superior laryngeal (SL) nerve stimulus produced an early facilitation of the GG reflex elicited by threshold MJ stimulation. In contrast, a supra-threshold lingual, IX or SL nerve stimulus resulted in a long-lasting inhibition of the GG reflex. A number of functional relationships may explain the significance of these excitatory and inhibitory responses in the GG muscle.     

Ca2+ sensitivities and transient tension responses to step-length stretches in feline mechanically-stripped single-fibre jaw-muscle preparations

At a muscle length, Lo (just taut), isometric tension at constant levels of various Ca2+ activations and transient tension responses to rapid length stretches (less than 1 per cent of Lo within 2 ms) at maximal Ca2+ activation level were measured in temporal, masseter and digastric (anterior belly) muscles (2-3 mm long and 24-48 micron in diameter). Steady isometric tension increased in a sigmoid fashion with increasing Ca2+ concentration from about pCa 7.28 to 4.49 in temporalis, from about pCa 6.18 to 4.40 in masseter and from about pCa 5.82 to 4.40 in digastric. The maximum tension was 75.5 +/- 10.2 g/mm2 in temporalis, 44.7 +/- 14.1 g/mm2 in masseter, and 46.1 +/- 20.1 g/mm2 in digastric. In the resting state, the sarcomere length at Lo was 2.34 +/- 0.06 micron in temporalis, 2.20 +/- 0.08 micron in masseter, and 2.20 +/- 0.00 micron in digastric. When the sarcomere length was stretched from 2.20 to 2.34 micron (the sarcomere length of temporalis at Lo) in the masseter and digastric, the Ca2+ sensitivity increased without significant change of the maximum tension in either muscle. The transient tension responses in all three muscles showed two distinct phases; an immediate tension increase coincident with the length stretch followed by an exponential tension decrease. The mean value of the time constant in the second phase was 58.5 +/- 19.7 ms in temporalis, 58.5 +/- 12.6 ms in masseter, and 362.6 +/- 16.8 ms in digastric. Thus temporalis showed a higher Ca2+ sensitivity at Lo and a greater maximum tension-producing capability than the other muscles and the cross-bridge turnover rate appears to be slower in digastric than in the others.     

SEMG activity of jaw‐closing muscles during biting with different unilateral occlusal supports

Summary The aim of this study was to test the hypothesis that experimental and reversible changes of occlusion affect the levels of surface electromyographic (SEMG) activity in the anterior temporalis and masseter areas during unilateral maximal voluntary biting (MVB) in centric and eccentric position. Changes were achieved by letting 21 healthy subjects bite with and without a cotton roll between the teeth. The placement alternated between sides and between premolar and molar areas. The SEMG activity level was lower when biting in eccentric position without than with a cotton roll between teeth (P < 0·043). It was always lower with premolar than with molar support when biting with a cotton roll (P < 0·013). In the anterior temporalis areas, the SEMG activity was always lower on the balancing than on the working side (P < 0·001). Such a difference was also found in the masseter areas but only during molar‐supported centric biting (P = 0·024). No differences were found when comparing the SEMG levels in masseter areas between centric and eccentric biting (P > 0·05). In the anterior temporalis area, the balancing side SEMG activity was lower in eccentric than in centric but only in molar‐supported biting (P = 0·026). These results support that the masseter and anterior temporalis muscles have different roles in keeping the mandible in balance during unilateral supported MVB. Changes in occlusal stability achieved by biting with versus without a cotton roll were found to affect the SEMG activity levels.     

Influence of psychological symptoms on home-recorded sleep-time masticatory muscle activity in healthy subjects

The present investigation attempts to describe the correlation between sleep-time masticatory muscle activity (MMA) and psychological symptoms by the use of a four-channel electromyography (EMG) home-recording device in a group of 15 healthy volunteers completing a battery of psychometric questionnaires for the assessment of anxiety, depression and anger. The integrated EMG signal was adopted to quantify the work (μV × s) produced by each of the four muscles (bilateral masseter and temporal) during the 5-h recording span and per each 1-h increment. The duration of MMA events and the muscle work during the first hour of sleep was related to trait anxiety scores for both masseter (P = 0·007) and temporalis muscles (P = 0·022). Trait anxiety was also significantly correlated to the total amount of MMA duration (in seconds) of the temporalis muscles (r = 0·558; P = 0·031). The present investigation provides support to the hypothesis that the duration of sleep-time masticatory muscle activity, especially during the early phases of a night's sleep, may be related to anxiety trait and not to anxiety state, depression or anger. These findings may support the view that features related to the individual management of anxiety, viz. trait, are likely to be more important than acute episodes of anxiety, viz. state, in the aetiology of sleep-time masticatory muscle activity. The role of other psychological symptoms is likely to be less important.     

Masseter muscle fibre changes following reduction of masticatory function

This study evaluated histological changes in masseter muscle fibres following reduced masticatory function by injection of botulinum toxin type A (BTX). Sixty 30-day-old Long-Evans male rats were randomly separated into four groups (15 per group): group I BTX masseter, 25U/ml (0.04ml each muscle) BTX was injected in bilateral masseter muscle whilst bilateral temporalis muscles received an equal amount of normal saline; group II BTX temporalis, 25U/ml (0.04ml each muscle) BTX was injected in bilateral temporalis muscle whilst bilateral masseter muscle received an equal amount of normal saline; group III BTX temporalis and masseter, bilateral temporalis and masseter were given 25U/ml (0.04ml each muscle) BTX; group IV normal saline (control), bilateral temporalis and masseter were given normal saline (0.04ml each muscle). After 45 days, the rats were killed, the muscles dissected and mean muscle mass recorded. The superficial masseter muscles were immunohistochemically analysed. Fibre sizes in group III were bigger than those in other groups. There was a small percentage of type IIa fibres in group III. Reduction in muscle fibre size and transition of muscle fibre subtypes from type IIa to IIx or IIb fibres may occur due to reduced masticatory function.     

Mechanical properties of glycerinated guinea-pig temporal and masseter muscles

At a muscle length, L0 (just taut), isometric tension and tension transients in response to rapid step stretches in length (less than 1 per cent of L0 within 2 ms) were measured at constant levels of Ca2+ activation of various magnitudes in glycerinated temporal and masseter muscles (1.7-2.5 mm long, 48-96 micron in diameter) from guinea pigs. The experiments were at 20 and 30 degrees C, and pH 6.8. Steady isometric tension increased in a sigmoid fashion as Ca2+ varied from about pCa 6.6 to 4.4 in both muscles. However, the maximum tension of temporal muscle was about 4.4 times greater than that of masseter muscle at 20 degrees C. The tension transients showed an immediate tension increase coincident with the stretch (the first phase) and an exponential-like tension decrease (the second phase). The time constant of the second phase was about 87.5 ms in temporalis and 3.7 ms in masseter at 30 degrees C. Decreasing temperature from 30 to 20 degrees C markedly increased the time constant of the tension response in the second phase (Q10 of about 3.4 in temporalis and 2.2 in masseter). Although there was a difference in the sarcomere length between the two muscles at L0 (2.14 micron in temporalis and 1.84 micron in masseter), the mechanical characteristics were almost independent of the muscle length. Thus the cross-bridges in temporalis appear to cycle more slowly and produce more isometric tension than those in masseter muscle.     

Effect of a brief episode of experimental muscle pain on jaw movement and jaw‐muscle activity during chewing

The aims of this study were to determine whether: (i) the jaw motor system develops a new pattern of jaw movement and/or jaw‐muscle activity after resolution of an acute episode of jaw‐muscle pain; and (ii) if jaw‐muscle activity and jaw‐movement features change progressively with repetition of a chewing sequence. Jaw movement and jaw muscle (masseter, anterior temporalis, and digastric) activity were recorded during free and rate‐standardized chewing in eight asymptomatic participants (pain infusion group), before and at three time blocks up to 45 min after a single 0.2‐ml bolus infusion of 5% hypertonic saline into the right masseter muscle. The same procedure, without infusion, was performed in another eight participants (control group). There were no significant main effects of group on jaw movement and muscle activity, suggesting that there were no persistent post‐pain effects on chewing. Across groups, repetitions of free and unstandardized chewing movements were associated with progressive increases in velocity and amplitude of jaw movement and masseter and temporalis electromyographic (EMG) activity. These findings suggest that factors unrelated to pain, such as practice effects, may be playing a role in the changes in jaw movement and jaw‐muscle activity observed after resolution of an acute episode of jaw‐muscle pain.     

The influence of occlusion on jaw and neck muscle activity: a surface EMG study in healthy young adults

The electromyographic (EMG) characteristics of masseter, temporalis and sternocleidomastoid (SCM) muscles during maximum voluntary teeth clench were assessed in 27 male and 35 female healthy young adults. Subjects were divided into two groups: (i) 'complete' Angle Class I (bilateral, symmetric canine and molar Class I relationships), and (ii) 'partial' Angle Class I (one to three canine/molar Class I relationships, the remaining relationships were Class II or Class III). On average, standardized muscular symmetry ranged 80.7-87.9%. During maximum voluntary teeth clench, average co-contraction of SCM muscle was 13.7-23.5% of its maximum contraction. On average, all torque coefficients (potential lateral displacing component) were >90%, while all antero-posterior coefficients (relative activities of masseter and temporalis muscles) were >85%. The average integrated areas of the masseter and temporalis EMG potentials over time ranged 87.4-106.8 muV/muV s%. Standardized contractile muscular activities did not differ between 'complete' and 'partial' Angle Class I, and between sexes (two-way analysis of variance). A trend toward a larger intragroup variability in EMG indices was observed in the subjects with 'partial' Angle Class I than in those with 'complete' Angle Class I (significant difference for the temporalis muscle symmetry, P = 0.013, analysis of variance). In conclusion, the presence of a complete or partial Angle occlusal Class I did not seem to influence the standardized contractile activities of masseter, temporalis and SCM muscles during a maximum voluntary clench. Subjects with a 'complete' Angle Class I were somewhat a more homogenous group than subjects with 'partial' Angle Class I.     

Neuromuscular function in healthy occlusion

Summary This study aimed to measure neuromuscular function for the masticatory muscles under a range of occlusal conditions in healthy, dentate adults. Forty‐one subjects conducted maximum voluntary clenches under nine different occlusal loading conditions encompassing bilateral posterior teeth contacts with the mandible in different positions, anterior teeth contacts and unilateral posterior teeth contacts. Surface electromyography was recorded bilaterally from the anterior temporalis, superficial masseter, sternocleidomastoid, anterior digastric and trapezius muscles. Clench condition had a significant effect on muscle function (P = 0·0000) with the maximum function obtained for occlusions with bilateral posterior contacts and the mandible in a stable centric position. The remaining contact points and moving the mandible to a protruded position, whilst keeping posterior contacts, resulted in significantly lower muscle activities. Clench condition also had a significant effect on the per cent overlap, anterior–posterior and torque coefficients (P = 0·0000–0·0024), which describe the degree of symmetry in these muscle activities. Bilateral posterior contact conditions had significantly greater symmetry in muscle activities than anterior contact conditions. Activity in the sternocleidomastoid, anterior digastric and trapezius was consistently low for all clench conditions, i.e. <20% of the maximum voluntary contraction level. In conclusion, during maximum voluntary clenches in a healthy population, maximum masticatory muscle activity requires bilateral posterior contacts and the mandible to be in a stable centric position, whilst with anterior teeth contacts, both the muscle activity and the degree of symmetry in muscle activity are significantly reduced.     

Effect of tongue position on masseter and temporalis electromyographic activity during swallowing and maximal voluntary clenching: a cross‐sectional study

The purpose of this study was to measure and compare the tonic electromyographic (EMG) activity of the temporalis and masseter muscles following placement of the tongue either on the palate or in the floor of the mouth during swallowing and maximal voluntary clenching (MVC). Thirty healthy dental students with natural dentition and bilateral molar support, between the ages of 18 and 22, with no prior history of oro‐facial injury, or current or past pain in the jaw, mouth or tongue participated in the study. Tonic masseter and temporalis EMG activities were recorded using surface electrodes. Subjects were instructed to passively place the tongue either on the anterior hard palate or in the floor of the mouth during swallowing and MVC. At each tongue position, the resulting EMG was recorded. During swallowing, no significant difference in EMG activity was found either for the masseter (P‐value = 0·1592) or the temporalis (P‐value = 0·0546) muscles, regardless of the tongue position. During MVC, there was a statistically significant difference for both the masseter (P‐value = 0·0016) and the temporalis (P‐value = 0·0277) muscles with lower levels recorded with the tongue in the floor of the mouth. This study found that in normal, pain‐free subjects, placing the tongue in the floor of the mouth significantly reduces masticatory muscle activity during MVC. Thus, it may be considered as a possible therapeutic option to decrease masticatory muscle activity; however, further research is needed in patients with oro‐facial pain.     

Trigeminal responses evoked by painful electrical stimulation

Trigeminal sensory and motor responses to high‐intensity electrical stimuli were studied in 10 healthy male subjects. During contraction at 10% of maximal bite force, late exteroceptive suppression periods assessed by EMG and bite force from masseter muscles were recorded together with evoked vertex potentials. Electrical stimuli were either delivered as 1 ms single pulses or as 21 ms (5 × 1 ms, 200 Hz) train pulses at two different intensities above the pain threshold. Three different perioral regions were stimulated in random order. The duration of the masseter suppression period was significantly longer and its latency significantly shorter at higher stimulus intensities whereas the amplitude of the evoked vertex potentials remained unchanged. Both the evoked potentials and the exteroceptive suppression period in the masseter muscle differed significantly between stimulation at the three perioral regions. The present results from combined measurements of sensory and motor responses in the trigeminal system may be explained by the function of trigeminal polysynaptic reflex circuits and the central cerebral processing of afferent inputs.     

Voluntary and reflex control of the human temporalis muscle

Electromyographic recordings (EMGs) were made in 10 human subjects from the anterior and posterior parts of the temporalis muscle using skin surface electrodes. The activities produced by voluntary maximal clenching tasks and the reflex responses to electrical stimulation of the muco-gingival junction were studied. In most subjects, maximum activity in both parts of the muscle occurred when clenching in the intercuspal position (anterior temporalis: 7 of 10 subjects; posterior temporalis: 9 of 10 subjects). Clenching maximally in the retruded position usually resulted in less activity; when this activity was expressed as a percentage of the maximum achieved by each subject for that part of the muscle, the median values were: anterior temporalis, 68% and posterior temporalis, 79%. Clenching in the protruded position produced little or no activity (median values: anterior temporalis, 3%; posterior temporalis, 5%). There were no significant differences between the EMG activities of the anterior and posterior parts of the muscle during these tasks when the activities were normalized to the maximum achieved in each part of the muscle. Application of electrical stimuli at the muco-gingival junction (upper incisor region) produced reflex inhibitions and excitations in both parts of the muscle. There were no significant differences in the thresholds of these reflexes between the anterior and posterior parts of the muscle. Furthermore, there was little difference between the two parts of the muscle in terms of the latencies, durations and magnitudes of the responses. Thus the results of the study suggest that there are similar neural control mechanisms for the anterior and posterior parts of the temporalis muscle despite the common view that these parts of the muscle have different functions.     

Activity of inferior head of human lateral pterygoid muscle during standardized lateral jaw movements

OBJECTIVE: (a) To describe the changes in electromyographic (EMG) activity from selected jaw muscles during a standardized lateral jaw movement with the teeth together, and (b) to investigate the effects on jaw muscle activity of changes in both the rate of lateral jaw movement and the relative magnitude of jaw-closing force. DESIGN: In 16 healthy volunteers, recordings were made using a jaw-tracking system, of mid-incisor point (MIPT) movements, as well as EMG activity from the contralateral inferior head of the lateral pterygoid muscle (IHLP), and bilateral anterior and posterior temporalis, masseter and submandibular muscles, during lateral jaw movement tasks at two speeds and two closing force levels with the teeth together. RESULTS: The IHLP was the only muscle to show a consistent increase in activity in association with the outgoing phase of the task and a decrease during the return phase. Under high closing force at slow speed, the EMG activities of the IHLP and bilateral anterior temporalis and masseter muscles were significantly (p < 0.05) higher than those under a low closing force, while there was no significant change (p > 0.05) in bilateral posterior temporalis and submandibular muscles. The change from slow to fast lateral movement at low force did not significantly (p > 0.05) alter the mean activity except for the IHLP (increase in activity) and the contralateral anterior temporalis (decrease in activity). CONCLUSIONS: The data suggest that the IHLP is one of the principal jaw muscles involved in a lateral jaw movement with the teeth together while the other jaw muscles may play a contributory or facilitatory role.     

Functional subdivision of the human masseter and temporalis muscles as shown by the condylar movement response to electrical muscle stimulation

In previous studies from our laboratory, a functional subdivision of the human temporalis and masseter muscles was demonstrated by means of opto-electronic recordings of the lower incisal point movement responses to electrical muscle stimulation. In the present study, it was examined whether this subdivision was also reflected in different movement responses of the mandibular condyle. To that end, the condylar movement responses to unilateral stimulation of four masseter muscle parts and three temporalis muscle parts were studied in four different jaw positions. The kinematic centre was used for condylar reference point. For both the amplitude and the direction of the movement responses, the effects of stimulation location and jaw position were studied using multivariate ANOVA and contrast analyses. It was found that for both outcome variables, the functional subdivision of the masseter and temporalis muscles was also reflected in some, but not all, of the movement responses of the mandibular condyles. The deep masseter muscle part and the (anterior) temporalis muscle part responded similarly to electrical stimulation.     

Inhibitory effects in the masticatory neuromusculature of human subjects at median occlusal position.

The reflex inhibitory responses evoked in masticatory muscles by contact of the dentition during the open-tap-close and open-close-clench jaw cycles were compared with the duration of inhibition at median occlusal position. Nine patients with normal occlusions were studied. Electromyographic recordings from the bilateral temporalis, masseter and anterior digastric muscles revealed a highly significant difference in the duration of the EMG silent periods elicited. Median occlusal position produced the longest inhibition of elevator muscle activity, and was considered the test parameter to evaluate the harmony of neuromuscular integration.