The refractory period of the masseteric cutaneous silent period

Silent periods were elicited in the electromyographic activity of the masseter and temporalis muscles from six normal subjects, by electrical stimulation of the skin over the contralateral masseter during clenching. Pairs of suprathreshold square pulses with varied time intervals were delivered using surface electrodes. It was found that the silent periods, which were mostly of the long latency (about 53 ms) type, had a refractory period, during which a new silent period could be evoked only by increasing the strength of the second stimulus. This refractory period could last for up to 1.5 s. During the first 200 ms after the first stimulus a second silent period was almost impossible to evoke with the moderately painful stimuli used in the present experiments. It is suggested that the refractory period should be considered as significant for the methodology when recording the silent period duration. It is also suggested that the long latency silent period should be recorded as a separate parameter and differentiated from the short latency (10-12 ms) silent period. This may be of special importance to recognize when a short latency silent period and a long latency silent period merge.     

Jaw muscle silent periods by tooth tap and chin tap

Electromyographic silent periods from jaw closing muscles were elicited during a clench by tapping on the chin and on selected teeth both in asymptomatic subjects and in subjects reporting symptoms of temporomandibular joint dysfunction. The mean duration in response to a chin tap was significantly shorter in the asymptomatic group than in the dysfunction group. No significant differences in mean duration elicited by tooth taps were found between groups. Other comparisons of mean durations such as among muscles, among teeth, or between chin taps and tooth taps failed to achieve statistical significance.     

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.     

Follow-up study of silent periods in complete denture wearers

Electromyographic silent periods in response to chin taps during clench were recorded from the anterior temporalis and masseter muscles. Ten complete denture wearers were observed up to the 1-year stage of denture wear and eight patients up to the 2-year stage. Silent periods were also recorded from patients clenching on paper rolls in place of the dentures. Regarding the patterns of the silent periods, similar observations of double silent periods were made as in our previous study up to the half-year stage of denture wear (McCall, Tallgren & Ash, 1979). The frequency of the double silent periods was 13.8% at the 1-year stage and 7.3% at the 2-year stage. The mean duration of the silent periods did not differ significantly at the 1-year and 2-year stages. The mean silent period duration when clenching on paper rolls without dentures in the mouth was significantly shorter than when clenching with the dentures.     

Jaw reflexes elicited by electrical stimulation of the hard palate of the rat.

Reflex responses of the oral musculature to electrical stimulation of sensory elements in the hard palate were studied in decerebrate rats. Activity in the suprahyoid muscles, the intrinsic and extrinsic tongue muscles and the jaw-closing muscles was recorded electromyographically; at the same time jaw positions were recorded. Muscle group-activity patterns depended on the place of stimulation and the strength of the stimulus.Stimulation of the antemolar region of the palate evoked:—at threshold stimulation, suprahyoid muscle activity (latency 6–7 ms) which did not result in jaw movement; with stimuli about 1.2–1.5 × threshold, enhancement of suprahyoid muscle activity resulting in a transient jaw opening (latency 15 ms, duration 35 ms) sometimes accompanied by tongue reactions (latency 15 ms, duration 5–10 ms); with stimuli about 1.5–3 × threshold, additional jaw-closing muscle reactions (latencies 5–10 ms and 15–20 ms) sometimes changing the transient jaw opening to a transient jaw closing (latency 15 ms, duration 35 ms).Stimulation of the intermolar region evoked:- at threshold stimulation, two bursts of activity in the intrinsic tongue muscles (latencies 15–20 ms and 40–60 ms) and reactions in the jaw-closing muscles (latency 50–80 ms) resulting in a long-lasting jaw closure; with stimuli about 1.2–1.5 × threshold, a gradual shift from the reactions at threshold to a suprahyoid muscle and jaw-closing muscle response of short latency (6–9 ms), causing fast transient jaw opening followed by prolonged jaw closing; with stimuli about 1.5–3 × threshold, only short latency suprahyoid and jaw-closing muscle reactions. The strongest stimuli at this level abolished the jaw-closing reflex and reinforced the jaw-opening reflex. Stimulation of the intermolar region with two equal stimuli (1.2–1.5 × threshold) and several time intervals showed the following reaction pattern:- At very short intervals (1–7 ms), the jaw-closing reflex was suppressed; intervals larger than 6 ms evoked two bursts in the anterior digastric muscles. At about 12-ms intervals, the second reflex disappeared but reappeared at intervals larger than 30–40 ms, being equal to conditioning value at intervals larger than 60 ms. At stimulus intervals up to 200 ms a second stimulus did not produce a second response of the jaw-closing muscles (long-latency closing reflex).     

Influence of auriculotemporal nerve anaesthesia on the masseteric silent period

Silent periods were evoked from the masseter muscles bilaterally in response to chin taps during a clench. Injection of 0.75 ml of 3% Mepivicaine posteromedial to each condyle led to a distribution of skin anaesthesia consistent with the distribution of the auriculotemporal nerve. Silent periods were evoked prior to and during anaesthesia in ten subjects. The silent period latencies (mean +/- standard deviation) prior to anaesthesia were 12.5 +/- 1.8 ms and 11.9 +/- 2.5 ms in the right and left masseters. During anaesthesia the latency decreased by 0.3 +/- 1.3 ms in the right masseter and by 0.3 +/- 2.1 ms on the left masseter. Neither change was statistically significant (P greater than 0.05). The silent period durations prior to anaesthesia were 20.3 +/- 4.0 ms and +/- 22.8 +/- 7.1 ms in the right and left masseters. During anaesthesia, these decreased by 2.74 +/- 5.8 ms on the right and 2.15 +/- 4.4 ms on the left. These decreases were not statistically significant (P greater than 0.05). These results failed to demonstrate an influence of the predominant nerve supply of the TMJ on either the latency or the duration of the masseteric silent period.     

Inhibitory effects on jaw muscle activity of innocuous and noxious stimulation of facial and intraoral sites in man

Controlled mechanical stimulation of the upper lip in seven subjects resulted in a period of complete or partial inhibition of the electromyographic activity of the masseter muscle. When a painful stimulus was delivered to the lip, a later inhibitory period with a more variable duration was produced. Innocuous stimulation at various intraoral sites led to an early inhibitory period with characteristics similar to those of inhibition produced by innocuous lip stimulation. An increase in stimulus intensity to a noxious level resulted in a second period of inhibition comparable to that produced by noxious lip stimuli. The intensity of the stimulus and the level of muscle activity maintained by the subject were found to influence the incidence and duration of the inhibitory periods. Thus, innocuous and noxious stimulation, whether applied intraorally or extraorally, may result in two distinct periods of inhibition in the jaw-closing muscles. The role in orofacial function of the first inhibitory period has yet to be demonstrated, but the second period is probably involved as a protective reflex.     

EMG silent periods in immediate complete denture patients: a longitudinal study.

Jaw muscle silent periods in response to chin taps were recorded from immediate complete denture patients before extraction of a residual anterior dentition, after insertion of the dentures, and three and six months after insertion. After three months' use of the dentures, the mean EMG silent period duration was significantly increased compared to the pre-extraction stage.     

Masseter inhibitory periods and sensations evoked by electrical tooth pulp stimulation in patients with oral-facial pain and mandibular dysfunction

The masseter inhibitory period (silent period) and sensations evoked by tooth-pulp stimulation were examined in 12 healthy subjects and 12 patients with oral-facial pain and mandibular dysfunction (MPD). Trains of 30 pulses were applied to an upper incisor and the threshold intensities for detecting sensation, for detecting pain sensation and for the masseter inhibitory period were determined. Masseter activity was monitored during tooth stimulation by electromyographic recordings from surface electrodes. Electrical tooth stimulation elicited three different configurations of masseter inhibitory periods in both groups: single, double and merged. MPD patients exhibited a greater proportion of single inhibitory periods. The combined average total durations of the three types of configurations were less in MPD. The findings are consistent with the hypothesis that there is an increase in excitability of the central masseter motorneuron pool in MPD, resulting in a reduction in the effective duration of the masseter inhibitory period. The higher incidence of single inhibitory periods in MPD patients also could result from this increased central excitatory state. There was no difference between masseter inhibitory periods evoked in either painful or non-painful muscles, and no particular configuration associated with pain sensation. The findings do not support the hypothesis that nociceptive input contributes to the increase in duration of the silent period in MPD. Although there were no significant differences between masseter inhibitory period threshold, detection thresholds or pain threshold for both groups, MPD patients had detection thresholds higher than their masseter inhibitory thresholds. These effects may be related to differential central neural influences on sensory-discriminative and reflex pathways in the trigeminal system.     

Effects of different stimulus locations on inhibitory responses in human jaw-closing muscles

The aim of this study was to examine the inhibitory responses in bilateral masseter and temporalis muscle activity when electrical stimulation with short or long duration was applied to six oro-facial locations. The exteroceptive suppression period (ES2) and inhibitory responses were recorded in the surface electromyogram (EMG) of bilateral masseter and temporalis muscles in 16 healthy subjects. Two stimulus durations (1 ms single pulse and 450 ms pulse train) adjusted to a perceived intensity of 7 (distinct painful) on a 0-10 Numerical Rating Scale (NRS) were applied to the following six oro-facial locations on the right side while the subject was biting at 50% of the maximal voluntary contraction: masseter muscle, temporalis muscle, temporomandibular joint, infraorbital nerve, supraorbital nerve, and mental nerve. The stimulus intensity required to reach an NRS score of 7 was significantly lower for 450 ms train stimuli than for 1 ms single stimuli (P<0·001). There were no significant differences in the magnitude of ES2 suppression among the six different locations (P>0·876) for the 1 ms single stimuli. There were significant decreases in Root-Mean-Square-EMG values in the 400-500 ms post-stimulus epoch compared with the pre-stimulus epoch (P<0·023) for 450 ms train stimuli, but there were no significant differences in the magnitude of inhibition among the six different locations (P<0·893). Short- and long-lasting electrical stimulation of various oro-facial locations induces similar bilateral inhibitory effects in the jaw-closing muscles but with different propensity which may reflect the somatotopic organisation of these responses.     

Influences of periodontal and mandibular-joint receptors on reflex sensitivity of human jaw-closing muscles

Reflex sensitivity from stimulating the chin or the dental arch of the mandible and the influences of periodontal and mandibular joint receptors on jaw muscles were investigated by experiments with and without anaesthesia. The findings showed that periodontal receptors exert an inhibitory influence on motoneural excitability of jaw-closing muscles and facilitate alpha-motoneurones of the digastric muscle and that the silent period following a jaw jerk is partly caused by inhibition of alpha-motoneurones of jaw-closing muscles due to activity of periodontal receptors. The ratio of reflex activity in temporal and masseter muscles appeared to depend on the position of the applied stimulus. The mandibular joint receptors appear to play a major part in distributing active muscle-force over these muscles.     

The effect of stimulus intensity and gape on electrically-evoked jaw reflexes in man

The changes in masseter EMG and biting force evoked by electrical stimulation of the lip were measured in normal people. Stimulation at noxious intensities elicited a characteristic, biphasic inhibitory response in the masseter EMG and a transient reduction in the biting force. The pattern of masseter inhibition and force change varied both with stimulus intensity and with inter-incisal separation. In general, the decrease in biting force was greater at intermediate jaw positions than in the open and closed positions. The duration of the EMG inhibitory period increased as the stimulus intensity increased but was independent of the jaw separation. Reflex thresholds could not be correlated with pain thresholds because subjects found it difficult to decide at what level an electrical stimulus became painful. Nevertheless this stimulation of high-threshold afferents may be useful in estimating the rate of relaxation of tension in the jaw muscles.     

Reflex activity in the masseter muscle of young individuals

The monosynaptic myotatic (jaw jerk) potential (MSP) evoked in the relaxed masseter by a tap on the chin and recorded electromyographically in this muscle (latency 6-3 ms, duration 5-9 ms) was similar in subjects with, or without TMJ clicking. The latency of the myotatic potential was significantly shorter (5.8 ms) when evoked during isometric contraction. The duration was 5-2 ms. A potential with similar characteristics as the MSP was evoked by TMJ clicking, as we believe by excitation of muscle spindles from tissue transmitted vibrations (latency 5-5 ms, duration 5-6 ms). The TMJ click evoked MSP could in the contracting massester be followed by silent periods (SPs) both of the early (latency about 10 ms) and the late type (latency about 50 ms). A tap on the chin during isometric contraction of the jaw elevators, was followed by an SP of the early type (latency 12-2 ms, duration 17-5 ms) and sometimes by an SP of the late type (latency 58 ms, duration 18 ms). The late SP occurred significantly more often in patients with TMJ-dysfunction. The period of lower activity following an MSP can be divided into two phases: a first phase with a complete electrical silence, denoted SP, and a second phase during which the activity is slowly increasing to reach the control level. This latter phase will be called "depressed activity" (DA) and is introduced as a new parameter. The SP with tooth tapping (latency 10-5 ms, duration 9-4 ms) was similar in groups with or without TMJ clicking. The superimposed MSP (SMSP) preceding the SP at tooth tapping is supposed to be a manifestation of the "load-compensation reflex".     

Influence of chin tap force and bite force parameters on the duration of the electromyographic silent period

Controversy exists as to whether chin tap force and bite force affect the duration of the electromyographic silent period following the jaw jerk reflex during isometric contraction of the elevator muscles of the mandible. This study demonstrates that statistical trends exist showing increased silent period duration for increased tap forces and decreased silent period duration with increased bite forces.     

Parameters of some reflexes of human jaw elevator muscles

Because of the potential diagnostic value in mandibular joint dysfunction, the similarities and differences in 3 open-close-clench (OCC) cycles, at ad libitum, 1/s and 2/s, were investigated. The response to chin taps with the jaw elevator muscles both relaxed (CTRL) and clenched (CTCL) were also studied from surface electromyographs over the masseter and anterior temporalis muscles of 10 normal adults. In OCC cycles, there were positive correlations between total cycle length and the inactive component and the duration of muscle activity (clench) after inhibition induced by tooth contact. Negative correlations were found between the duration of muscle activity before tooth contact and the clench and inactive components. The inactive and clench components changed most with variations in cycle duration and the latency between tooth contact and resultant inhibition varied little. For chin tap responses, there were no significant differences between masseter and temporalis muscles. The latency between mallet contact and inhibition in CTCL responses was the same as the latency in OCC cycles but shorter than the equivalent sum of latency and activity in the CTRL responses.

If a common mechanism exists for the three reflexes, their parameters suggest that OCC and CTCL reflexes are facilitated with respect to CTRL.     

Jaw muscle silent periods before and after rapid palatal expansion.

1. The electromyographic silent period was observed in jaw-closing muscles during a maximal clench following a tap to the mandibular symphysis in eight subjects before and after rapid palatal expansion. 2. No differences in mean silent period duration were observed among the three jaw-closing muscles examined. 3. A large (r = 0.96) and statistically significant (p less than 0.01) correlation coefficient between two sets of silent period durations at one session indicates that these measurements are repeatable and reliable. 4. The mean silent period duration for the eight patients with malocclusions was 33.5 msec. prior to rapid palatal expansion and 51.2 msec. after the completion of palatal expansion. The difference was statistically significant (p less than 0.01) and represents a 61 percent increase. 5. Patients with symptoms of TMJ dysfunction are not the only group showing a prolonged silent period duration.     

The role of cutaneous receptors in the menton tap silent period.

The neuromuscular mechanisms of the electromyographic silent period are poorly understood but clinically significant. Our purpose was to isolate the contribution of cutaneous afferents from the chin to the latency and the duration of the EMG silent period. Under the conditions of our experiment, the afferents of the mental nerve have no observable influence on the latency or the duration of the silent period educed by a menton tap.     

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.     

Masticatory muscles. Part VIII. Reflexes in the masticatory system

A reflex is an involuntary response on a specific nerve stimulus. Several types of receptors are present in the masticatory system which can be involved in reflex activity. A short mechanical tap on an incisor, for example, evokes a specific pattern of several so-called exteroceptive reflexes in the electromyographic (EMG) activity of jaw-closing muscles, which includes periods with decreased activity (inhibitory reflex) as well as increased activity (excitatory reflex). In this case, periodontal mechanoreceptors are detecting the stimulus. The second type of reflex that is dealt with is the jaw-jerk reflex as an example of a proprioceptive reflex. A transient increase in length of a jaw-closing muscle, detected by length sensors (muscle spindles), evokes an excitatory reflex in the EMG. Dependent on the motor task, the amplitude of both reflexes is modified. The various reflex mechanisms play an important role in normal physiological functions such as posture control and chewing.     

A standardized system for evoking masseteric silent periods with decreased variance

Masseteric silent periods have been evoked in a group of normal subjects under a standardized system and under nonstandardized conditions. In the designed system, head position, electrode placement, bite force, tap force, direction of tap, and surface zone of tap force delivery were controlled. Analysis of data confirmed the hypothesis that standardization of the procedure for eliciting the silent period results in measurements of duration with decreased standard deviations and variances. In the present study, under the standardized system the SPD range of standard deviations was between 1.37 and 3.68 (mean 2.97) and the range of variances was between 1.89 and 13.54 (mean 8.85). The recorded low variability suggests a high degree of reproducibility of the system. High reproducibility and low variability of measurements are necessary if an accurate measure of silent period duration is to be achieved and used in the interpretation of diagnostic findings for craniomandibular disorders.