Andrew G. Mason

Remote noxious stimuli modulate jaw reflexes evoked by activation of periodontal ligament mechanoreceptors in man

The purpose of the study was to investigate whether jaw reflexes evoked by selective stimulation of periodontal ligament mechanoreceptors are susceptible to modulation by remote noxious stimulation. Experiments were performed on 10 volunteer subjects. Skin surface recordings were made from the jaw‐closing masseter muscle. The subjects activated the muscle to approximately 10% of maximum by biting on a rubber impression of their molar teeth while they received visual feedback of the electromyogram (EMG) of the muscle. Reflexes were produced by the application of gentle mechanical stimuli to an upper central incisor tooth. The stimuli were in the form of ‘ramp and hold’ forces with a 5 ms rise‐time and a 1.5 N plateau which lasted 350 ms. The resulting reflexes were recorded both under control conditions and while the subjects received a remote noxious stimulus (immersion of a hand in water at 3 °C). In all 10 subjects, the stimuli produced a single period of inhibition of masseteric activity (latency, 12.8 ± 0.4 ms; duration, 18.1 ± 1.3 ms; means ± S.E.M.), which was usually followed by a period of increased masseteric activity. The period of inhibition constituted a downward wave in full‐wave rectified, averaged signals. The integrals of such waves were significantly smaller (by 17 ± 6.5%; P = 0.027; Students t test) when the reflex was evoked during remote noxious stimulation rather than under control conditions. As such reflexes are believed to play a modulatory role during normal oral function, this finding may be relevant to disorders of mastication associated with pain.

Differences between inhibitory jaw reflexes evoked by stimulation of tooth pulp and across the adjacent alveolar process in man

OBJECTIVE In humans, stimulation of nerves in or around teeth can evoke inhibitory jaw reflexes. Previous studies had suggested that there may be subtle differences in the timings of the responses. The aim of the present study was to investigate this by comparing reflexes evoked by electrical stimulation of a tooth and of the adjacent tissues in individual subjects. DESIGN Experiments were performed on 9 volunteers (3 male, 6 female). EMG recordings were made from the masseter muscle ipsilateral to the stimuli, whilst the subjects maintained a steady level of activity in the muscle. Reflexes were evoked by applying stimuli to an incisor tooth (pulpal stimuli) or across the adjacent alveolar process (transalveolar stimuli), using bipolar electrodes. RESULTS Two inhibitory responses were evoked in most (8/9) subjects. The first occurred at a shorter latency after transalveolar than after pulpal stimulation (12.3+/-0.5 ms vs 19.4+/-1.5 ms; P=0.0014, paired t-test). For technical reasons, it was not possible to make such comparisons for the second inhibitory responses in all the subjects. In 5 subjects where such a comparison was possible, the mean latency of the transalveolar-evoked response was again shorter than that of the pulpal-evoked response (56.4+/-2.8 ms and 58.8+/-5.3 ms, respectively), but this difference was not significant (P=0.5). CONCLUSIONS It appears that inhibitory jaw reflexes evoked from around the teeth are faster than those from the dental pulp. This observation could be due to differences between the peripheral afferent and/or the central pathways mediating the reflexes.

Informing a realistic laboratory erosion-testing regime - observations.

OBJECTIVES To measure aspects of fizzy drink consumption in a social environment to inform the development of a laboratory testing regime. METHODS This was an observational study in which participants were invited to attend one of four pizza and soft drink parties. All such foods and drinks were served in an air conditioned room at a temperature of 24°C. All drinks were at a temperature of 4°C and each participant was asked to spit out, into graduated cups, their first and second sips. Both the temperature and volume of these were measured. Upon completion of the party the volume of drinks consumed was determined. Video footage of the experiment was recorded for subsequent analysis to determine sip count and the elapsed time period between first and last sip. These values were compared to the analogous measured value of expectorated volume in order to assess the usefulness of video observation in the context of this work. RESULTS The mean expectorated beverage temperature was 14.9±2.0°C. The mean time spent drinking was 44.2±17.4 minutes with a mean consumption rate of 13.3±6.0 ml/min. Only the sip volume and sip count per can values were significantly different between sexes (P<0.05) with females displaying lower values for volume and a higher sip count. There was close agreement between the sip volume values observed and calculated using video observation derived parameters. CONCLUSIONS Several human drinking behaviour values were reported in this study and these will be of value in the development of more realistic laboratory erosion-testing regimes. It is concluded, within the limitations of this work, that (1) there are differences in the drinking behaviour of males and females with respect to sip volume and count, (2) the intraoral rise in temperature of a 4°C beverage is lower than that used in previous laboratory simulations and (3) the values derived from video observation agree with those measured directly validating this technique for use in further studies. CLINICAL SIGNIFICANCE The work provides valuable quantitative data on which to base simulated laboratory erosion work. Perhaps the most significant finding is that sipped beverages attain a temperature of only 14.9°C.

Selective stimulation of human tooth‐pulp with a new stable method: Responses and validation

Introduction: The aims of this study were to establish a safe technique for selective stimulation of nerves in human tooth‐pulp during long experiments and to validate its use even with stimuli of high intensities. Methods: A custom‐made veneer containing 2 silver wire‐conductive cream electrodes was attached with cement to the labial surface of an upper central incisor tooth. A variety of stimulus intensities were applied, and sensory and reflex responses from jaw‐closing muscles were recorded. Results: In 15 participants, the stimuli evoked predominantly sharp or painful sensations and reflex inhibitions of activity in the jaw muscles. Stimulation of 3 non‐vital teeth evoked no sensations or reflexes, even at intensities that evoked maximal reflexes in vital teeth. The electrodes had reasonably stable resistances throughout experiments lasting up to 90 min. Conclusion: The method described enables responses to low‐ or high‐intensity stimulation of human pulpal nerves to be investigated in long experiments. Muscle Nerve, 48: 256–264, 2013

Quantification of jaw reflexes evoked by natural tooth contact in human subjects

Inhibitory jaw reflexes are believed to be important for protecting the teeth and temporo-mandibular structures from damage during sudden or forceful biting or mastication. Accordingly, alterations in these reflexes are sometimes implicated in aetiologies proposed for oro-facial pain syndromes, although the association is not well-established. We now aim to develop a method for quantifying objectively inhibitory jaw reflexes evoked by natural tooth contact. In the longer term, this may provide a new approach to examining the association of altered reflexes and clinical conditions. Eighteen subjects gave their written, informed consent, and were recruited to participate in this study. They were instructed to clench their teeth together in response to visual cues. They performed two such tasks twenty times: from the jaw postural position and from a more open position with the jaws set 10mm apart. Both tasks produced a rapid rise then stabilisation in electromyographic activity in the masseter muscle. This was always interrupted by a large inhibitory reflex starting 11.1±1.5 ms (mean±SD) after tooth contact. The inhibitions produced during the second task were similar but of significantly longer duration (24.3±6.4 vs 18.4±6.5 ms, P=0.0003, paired t-test) and greater magnitude (measured as an integral of the waveform: 1577±478 vs 1279±425%.ms, P=0.007, paired t-test). Interestingly, in a minority (13%) of the tasks, a second inhibition with a longer latency (50.9±0.9 ms) was also observed. Thus reflex responses in the masseter muscle to natural tooth contact usually consist of single inhibitory periods. In this respect they are like those induced by externally applied tooth pushing although occasionally there is a second inhibition, reminiscent of that seen with externally applied tooth taps.

In vitro simulation of erosive challenges to human enamel using a novel artificial mouth

This in vitro work reports upon the design, build and operation of an artificial environment (Saltus) that sought to simulate the process of in vivo dental erosion upon human enamel. A novel testing environment, housed 8 erosion testing substrate specimens, that on separate occasions were subject to 4 different experimental diets, of increasing erosive challenge, simulating the consumption of an acidic beverage. Each set of specimens was subjected to one of the experimental diets only. These were liquid only and administered the test beverage over a standardized range of volumes and durations. Flow of both artificial unstimulated and stimulated saliva was maintained throughout and the effects upon the substrates were measured by profilometry, surface microhardness determination and chemical analysis of the saliva and beverage mixture for traces of Calcium and Phosphate ions. The overall trend of surface hardness reduction, depth of surface loss and ion loss across the diets increased in proportion to the severity of insult. Accepting the limitations of this study Saltus appeared to perform well as an environment in which to simulate and assess dental erosion using parameters defined by previous in vivo observations of human drinking behaviour. The authors however acknowledge that in vitro testing can never replicate fully the in vivo situation.