Response of human jaw muscles to axial stimulation of the incisor

Abstract

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.

Figure 1. Experimental set up

The subjects bit into impression material mounted on two bite bars with their upper left central incisor resting on a tooth rest that fitted into a hole in the upper bite bar (inset). Movement of the subject was further minimized by the use of a fixed nosepiece. The nosepiece also counteracted the axial forces applied to the tooth. A computer then produced the desired force profile randomly between 1.5 and 3 s. Two force transducers, one located in the motor arm and the other below the lower bite bar, picked up the stimulus force applied to the tooth and the total force generated by the jaw, respectively. Both the masseter and digastric EMG activity were recorded as well as the outputs of both force transducers; in addition, the SEMG of the masseter was fed into a filter box and used for on-line feedback.

Figure 2. EMG reflex parameters

Characteristics of the masseteric inhibitory reflex (n = 50) elicited from axial stimulation of the upper left central incisor and recorded from the left masseter of a subject biting at 5 % MVC before the application of local anaesthetic (LA); the stimulus was delivered at time 0. The reflex latency was determined from the CUSUM turning point (equivalent to the point at which the EMG crossed the pre-stimulus mean). The strength was the ratio of the CUSUM reflex deviation to the maximum possible reflex (corresponds to the EMG area). The reflex duration was defined as the time until the next CUSUM turning point. Excitatory reflexes were measured in the same way as inhibitions (time the EMG crossed the pre-stimulus mean and the area between the EMG and the pre-stimulus mean), the only difference being that excitations occurred above the pre-stimulus mean while inhibitions were below.

Figure 3. Bite force reflex parameters

Characteristics of the reflex seen in the bite force record elicited from axial stimulation of the upper left central incisor and recorded from the lower jaw of a subject contracting at 20 % MVC before the application of LA (n = 50). The average pre-stimulus bite force was subtracted to clearly illustrate the change. Both the rising and falling edges of the stimulus can be seen in the bite force as well as the overshoots after both the rising and falling edges of the stimulus. The first turning point (first derivative, dF/dt = 0) after the stimulus corresponded to the peak of an overshoot, not the start of a reflex, since it occurred before, or directly after, the corresponding EMG change. The reflex latency was determined from the inflection after the first turning point (second derivative, d²F/dt²= 0); this is likely to correspond to the genuine latency of the bite force reflex since it occurred approximately 15 ms after the reflex inhibition in the EMG. The reflex duration was defined as the time between the latency and the next turning point. The reflex strength was the force change between the start and end of the reflex.

Figure 4. Jaw muscle response to various stimulus rise times

CUSUMs (ordinate; k ms) of the rectified averaged (n = 50) SEMG response of the masseter and digastric muscles from one subject to various rise time stimuli (fast: 12 ms; medium: 20 ms; slow: 90 ms). All stimuli commenced at time 0. The EMG activity of the masseter was set at 20 % MVC. Results from both before (dark trace) and during (light trace) application of LA are shown. The reflex activity seen in these records is an inhibition (downward movement of the CUSUM) followed by a late excitation (upwards movement of the CUSUM) before LA in the masseter in response to a medium or fast push stimulus; no sizable digastric activity was present. This was representative of most results.

Figure 5. Bite force and CUSUM of SEMG before and during application of LA

Effect of LA block on the bite force and CUSUM of the masseter from one subject biting at 10 % MVC. The stimulus force (medium stimulus rate) is included to illustrate its effect on the bite force record. The average pre-stimulus bite force was subtracted from the two bite force recordings so the change due to the activation of PMRs could be clearly seen. The difference traces (contribution due to activation of PMRs) were calculated by subtracting the result obtained during LA application from that found before LA was applied. All reflex activity in both the CUSUM and the bite force records was removed by the application of LA. The delay between the start of the inhibitory reflex and the reflex reduction in the bite force (18 ms) is included for comparative purposes.

Figure 6. Reflex characteristics of the masseter versus stimulus rise times

The effect of altering stimulus rise time on the latency (A), strength (B) and duration (C) of reflexes seen in the masseter both before and during the local anaesthetic block. E1, IN, E2 and LA stand for early excitation, inhibition, late excitation and local anaesthetic, respectively. Columns indicate the average value while the error bars show the s.e.m. Strength is given as an absolute value so inhibition can be easily compared to the excitations. The effects that the stimulus conditions had on the measured reflex parameters are discussed in the relevant sections in the text and shown in Table 1.

Figure 7. Reflex occurrence rate versus stimulus rise times

The effect of altering stimulus rise time on the occurrence of reflexes seen in the masseter (A) and digastric (B) both before and during the local anaesthetic block. E1, IN, E2, BR and LA stand for early excitation, inhibition, late excitation, reflex in bite force and local anaesthetic, respectively. Columns indicate the average occurrence rate over the 10 subjects while the error bars show the s.e.m. Bite level had no effect on the occurrence rate of the reflexes. The inhibition was the most often evoked reflex in the masseter before LA and increasing the stimulus rate increased the detection rate for all three reflexes. Early excitation was the only masseteric reflex to increase in frequency during the LA block. Many of the reflexes detected in the digastric, particularly before the application of LA, were simultaneously recorded in the masseter, indicating that they were the result of cross-talk.