Effect of hypnosis on masseter EMG recorded during the 'resting' and a slightly open jaw posture.

The aim of this experimental study was to determine whether minimal levels of electromyographic activity in the masseter muscle are altered when individuals are in a verified hypnotic state. Experiments were performed on 17 volunteer subjects (8 male, 9 female) all of whom gave informed consent. The subjects were dentate and had no symptoms of pain or masticatory dysfunction. Surface electromyograms (EMGs) were made from the masseter muscles and quantified by integration following full-wave rectification and averaging. The EMGs were obtained (i) with the mandible in 'resting' posture; (ii) with the mandible voluntarily lowered (but with the lips closed); (iii) during maximum voluntary clenching (MVC). The first two recordings were made before, during and after the subjects were in a hypnotic state. Susceptibility to hypnosis was assessed with Spiegel's eye-roll test, and the existence of the hypnotic state was verified by changes in ventilatory pattern. On average, EMG levels expressed as percentages of MVC were less: (i) when the jaw was deliberately lowered as opposed to being in the postural position: (ii) during hypnosis compared with during the pre- and post-hypnotic periods. However, analysis of variance followed by post hoc tests with multiple comparison corrections (Bonferroni) revealed that only the differences between the level during hypnosis and those before and after hypnosis were statistically significant (P < 0·05). As the level of masseter EMG when the mandible was in 'resting' posture was reduced by hypnosis, it appears that part of that EMG is of biological origin.

High bone mass phenotype in a cohort of patients with Osteogenesis Imperfecta caused due to BMP1 and C-propeptide cleavage variants in COL1A1.

OBJECTIVES: Osteogenesis Imperfecta (OI) is a heterogeneous condition mainly characterised by bone fragility; extra-skeletal features in OI include blue sclerae, dentinogenesis imperfecta, skin laxity and joint hyper-extensibility. Most patients with OI are thought to have a low bone mass but contrary to expectations there are certain forms of OI with high bone mass which this study explores in further detail. METHOD: A cohort of n = 6 individuals with pathogenic variants in BMP1 and the C-propeptide cleavage variants in COL1A1 were included in this study. Detailed clinical and radiological phenotyping was done and correlated with genotype to identify patterns of clinical presentation and fracture history in this cohort of patients. This data was compared to previously reported literature in this group. RESULTS: 2 patients with BMP1 and 4 patients with pathogenic variants in C-propeptide region in COL1A1 were deep-phenotyped as part of this study and 1 patient with C-propeptide variant in COL1A1, showed low bone mineral density. In those with an elevated bone mineral density, this became even more apparent on bisphosphonate therapy. Patients in this cohort had variable clinical presentation ranging from antenatal presentation to more of an insidious course resulting in later confirmation of genetic diagnosis up to 19 years of age. CONCLUSIONS: Patients with pathogenic variants in the C-propeptide region of COL1A1/A2 and BMP1 appear to have a high bone mass phenotype with increased sensitivity to bisphosphonate therapy. It is important to closely monitor patients with these genotypes to assess their response to therapy and tailor their treatment regime accordingly.

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

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.

Suppression of an inhibitory jaw reflex by the anticipation of pain in man.

Electromyographic recordings (EMGs) were made from the active masseter muscle, of the inhibitory reflex evoked by application of electrical stimuli to the skin of the upper lip in 15 human subjects. In control sequences, the reflex had a mean latency and duration (+/- S.E.M.) of 45.4 +/- 1.3 msec and 47.9 +/- 2.8 msec, respectively. Significant decreases in the reflex as well as increases in heart rate and anxiety levels assessed by a visual analogue scale, occurred when the subjects were stressed by the anticipation of receiving painful electrical stimuli above the ankle (P < 0.00005; Student's t-tests). During such sequences, the magnitude of the reflex measured by integration of the EMG, was reduced by 47.7 +/- 5.6%. This effect involved a reduction in both the duration and depth of the inhibitory wave. It occurred regardless of whether the painful stimuli were applied during or after the recording of the reflex and of whether the baseline activity in the muscle was inadvertently raised or lowered during the stressful sequences. It is concluded that stress induced by the anticipation of pain, can markedly reduce an inhibitory jaw reflex in man by exerting an influence on the reflex pathway prior to the motoneurones.

Mechanisms underlying the effects of remote noxious stimulation and mental activities on exteroceptive jaw reflexes in man.

Successive inhibitory, excitatory, inhibitory and excitatory reflexes (the Q, R, S and T waves of the post-stimulus electromyographic complex (PSEC)), evoked by applying non-painful taps to an incisor tooth, were recorded from the jaw-closing muscles of 15 subjects. The effects on these reflexes of the subjects undertaking mental exercises (MEx) in the form of arithmetic calculations were compared with those of remote noxious stimulation (RNS; application of 3 degrees C to a hand). This was done to investigate whether the previously established effects of RNS were likely to be related to a change in the subject's mental state and/or to direct nociceptive mechanisms. Both MEx and RNS caused increases in EMG activity around the Q-R and S-T transitions of the PSEC, which resulted principally from shortenings of the inhibitory Q and S waves. Reducing the intensity of the tap stimuli, which mimicked condition-induced disinhibition, caused shortenings of the inhibitory waves at latencies similar to the shortenings induced by MEx or RNS. The magnitude of the RNS-induced effect on the ST segment of the PSEC was greater (P<0.01) than that on the QR segment. By contrast, MEx induced similar effects on both segments. Regression analyses were performed for the relationship between condition-induced changes in amplitude of the excitatory waves and their control amplitudes. These analyses were performed to reveal any condition-induced inhibition or facilitation of the tap-induced influences on the motoneurons. Overall, the evidence suggested that: (1) mental exercise induced a similar degree of inhibition of the two tap-induced inhibitory jaw reflexes and a facilitation of the excitatory ones, and (2) remote noxious stimulation induced an inhibition of the second tap-induced inhibitory reflex which was greater than that of the first one, and an inhibition of the first excitatory reflex. Thus, although factors related to altered mental activity could play a role in the modulation of jaw reflexes by RNS, the differences between the effects of MEx and RNS suggest that alternative or complementary mechanisms are also likely to be involved.

Analysis of differences between conditioned and control reflex series in EMG recordings.

Two methods are presented for the data analysis of signals derived by subtracting conditioned from control reflex data in full-wave rectified and averaged electromyographic (EMG) signals. The first method uses the ratios of the mean amplitudes and standard errors of the mean (S.E.M.s) (i.e. Student's t values) of a series of data points in such a difference signal, and deals with the detection of latencies of reflex components which are susceptible to a conditioning stimulus or experimental situation. The second method applies a modified cumulative sum (CUSUM) technique to full-wave rectified difference signals. This modified CUSUM technique determines the magnitude of the effects of the conditioning situation above that expected due to chance fluctuations, taking into account the effect of reflex modulations on such chance fluctuations in a post-stimulus period. The modified CUSUM technique proved particularly useful when various subtle but consistent, opposing changes occurred sequentially in the conditioned series thus yielding a complicated pattern of effects in a difference signal, with a low signal-to-noise ratio.

The ability of inhibitory controls to 'switch-off' activity in dorsal horn convergent neurones in the rat.

Unitary extracellular recordings were made from 51 convergent neurones in the dorsal horn of the lumbar spinal cords of urethane anaesthetized rats. All the cells tested responded to sustained noxious mechanical stimulation of their receptive fields on the ipsilateral hindpaw, but only 26/49 gave tonic responses lasting for more than 5 min. In all 26 cells, these tonic responses were depressed by diffuse noxious inhibitory controls (DNIC) triggered by applying noxious conditioning stimuli elsewhere on the body. In seven cells, the inhibitory effects could involve a complete abolition of activity and in five cells, when this occurred, activity did not return during 2.5-6-min periods of observation following removal of the conditioning stimuli. However, in those cases, activity could be restored to pre-conditioning levels by further manipulations of the receptive field-either removal and re-application of the original stimulus or brief application of an additional stimulus. These results show that inhibitory controls can 'switch-off' activity in at least a small proportion of dorsal horn convergent neurones. One possible explanation would be that in these neurones, responses to sustained noxious stimuli may depend on activity in a positive feedback circuit within the central nervous system, which when interrupted, may be restored only by additional afferent inputs. The existence of such a loop could also explain the finding of convergent convergent neurones which initially were not spontaneously active but which after stimulation of their receptive fields, developed on-going discharges which could be switched-off by DNIC.

The digastric reflex evoked by tooth-pulp stimulation in the cat and its modulation by stimuli applied to the limbs.

The digastric reflex evoked by electrical stimulation of tooth pulp in anaesthetized cats was studied together with the effects on this reflex of stimulating other parts of the body. The threshold for the digastric reflex generally lay in the range of stimulus intensities which would excite a large proportion of the pulpal afferent fibres which suggested that a large amount of central summation was required to evoke the reflex. During the course of 25/27 experiments, the threshold for the reflex increased. It was also found that repeated application of suprathreshold stimuli produced first an increase and then a decrease in the reflex response. The application of noxious but not of non-noxious mechanical conditioning stimuli to the limbs produced strong, long-lasting depressions of the digastric reflex. Electrical conditioning stimuli applied to the limbs also depressed the reflex; this depression had a latency of onset of 20-50 ms and lasted for up to 500 ms. When conditioning stimuli were applied to the saphenous nerve, the depression of the reflex occurred only when the stimuli were of an intensity sufficient to excite fibres conducting at less than 40 m X s-1; it may be assumed that some of these fibres would have been high threshold mechanoreceptors or nociceptors. These results show that noxious stimulation of anatomically remote structures can depress the activity of a population of trigeminal brainstem neurones. The opiate antagonist, naloxone, had no detectable effect on either the digastric reflex or the depression of the reflex produced by stimulating other parts of the body. The serotonin antagonists, methysergide and cinanserin, strongly depressed the digastric reflex but it was not clear whether these drugs also affected the depression of the reflex by the conditioning stimuli.

Effects of visceral distension on the activities of neurones receiving cutaneous inputs in the rat lumbar dorsal horn; comparison with effects of remote noxious somatic stimuli.

(1) Unitary extracellular recordings were made from 92 lumbar dorsal horn neurones in urethane-anaesthetised rats. These neurones were classed as 'noxious-only' (4), 'non-noxious-only' (33) or 'convergent' (55) by their responses to stimulation of their cutaneous receptive fields on the ipsilateral hindpaw. (2) Distension of abdominal viscera (colon, urinary bladder) depressed the activities of the vast majority (93%) of the convergent neurones but of only one other cell (a non-noxious-only neurone). Similarly, noxious stimulation of widespread somatic structures depressed activity in all but one of the convergent neurones but in only 3 other cells (one non-noxious- and two noxious-only neurones). One or other of these procedures also excited 3 cells--one convergent neurone responding to distension of the colon, another to stimulation of widespread somatic structures and one non-noxious-only neurone being excited by stimulation on the contralateral hindpaw. (3) The inhibitory effects of the noxious somatic stimuli were very like those described previously and termed 'diffuse noxious inhibitory controls' (DNIC) and it seems likely that the effects of the visceral stimuli were also manifestations of DNIC, particularly in view of their similar, nearly total, specificity to convergent neurones. There were however, some small differences in the extent and temporal evolution of the inhibitory effects of the visceral and of the somatic stimuli--the visceral stimuli generally producing weaker inhibitions with slower rates of onset and recovery. It is proposed that these differences may have reflected different amounts and patterns of activity in the relevant primary afferent fibres rather than being due to different central neural mechanisms. (4) These results and the likely explanation that the effects of the visceral stimuli were mediated by a diffuse mechanism should be taken into account when interpreting the results of other studies in which inhibitory effects are produced by visceral stimulation.

The roles of periodontal ligament mechanoreceptors in the reflex control of human jaw-closing muscles.

Controlled mechanical stimuli were applied to an upper central incisor tooth in 19 human subjects and the resulting reflexes in the ipsilateral masseter muscle were examined electromyographically. In most cases the force profile of the stimuli consisted of a ramp leading to a sustained plateau at an intensity of 1 N. In addition 1 N tap stimuli were employed in some subjects. The 1 N ramp stimuli with a rise time of < or = 20 ms consistently evoked a single, short-latency (approximately 12 ms), inhibitory reflex which was often followed by an excitatory wave. The ramp stimuli with shorter rise times produced larger responses than those produced by ramp stimuli with longer rise times. By contrast the tap stimuli elicited a sequence of responses consisting of inhibitory-excitatory-inhibitory-excitatory components. The first inhibitory and excitatory responses evoked by tap stimuli had latencies similar to those of the responses evoked by the ramp stimuli. The latencies of the inhibitory responses evoked by 1 N ramps with rise times ranging between 2.5 and 20 ms did not vary significantly with the rise time. Consistent with this observation it was found that the median threshold force for evoking the short-latency inhibitory reflex was only 0.25 N. This was significantly less than the threshold for the excitatory response (median: 0.75 N). The responses to ramp-plateau forces were not dependent on the level of preloading of the tooth (at least within the range tested: < 0.25 to 1 N). These findings provide evidence that mechanoreceptors in the periodontal ligament contribute to the control of human jaw-closing muscles, notably to short-latency reflex responses. It may be concluded that the additional reflex responses produced by tapping stimuli result from the activation of receptors elsewhere due to vibration.

Diffuse noxious inhibitory controls (DNIC): evidence for post-synaptic inhibition of trigeminal nucleus caudalis convergent neurones.

Activity produced by direct microelectrophoretic application of glutamate onto 19 convergent neurones in trigeminal nucleus caudalis, was strongly depressed during and after the application of heterotopic noxious conditioning stimuli: noxious heat (52 degrees C) applied to the tail, noxious pinches applied to the tail or hindpaws and intraperitoneal injections of bradykinin produced mean reductions in activity of 80-90%. The same noxious conditioning stimuli had no effect on the activities of any of 5 noxious-only or 5-non-noxious-only neurones. These effects were similar to those previously reported to influence peripherally evoked activities of nucleus caudalis convergent neurones and which have been termed diffuse noxious inhibitory controls (DNIC). It is therefore proposed that DNIC act on nucleus caudalis convergent neurones by a final post-synaptic inhibitory mechanism involving hyperpolarization of the neuronal membrane. Consistent with this hypothesis, it was also found that the noxious conditioning stimuli could restore firing of convergent neurones which had been excessively depolarised by large doses of glutamate.

Evidence that diffuse noxious inhibitory controls (DNIC) are medicated by a final post-synaptic inhibitory mechanism.

Nineteen convergent neurones, 19 'non-noxious only' neurones and 13 'proprioceptive' neurones all with peripheral excitatory receptive fields on the ipsilateral hindpaw, were recorded in the lumbar dorsal horn of non-spinalized, anaesthetized rats. These neurones were all excited by the electrophoretic application of glutamate; using 20 s applications of appropriate electrophoretic currents, almost identical levels of activity (around 30 spikes/s) were produced for each of the 3 types of neurones. The application of heterotopic noxious stimuli resulted in strong inhibitions of the glutamate-evoked activity of the convergent neurones. During the application of noxious heat (52 degrees C) to the tail and noxious pinches to the tail, contralateral hindpaw and muzzle, the glutamate-evoked activity was depressed by 78%, 84%, 63% and 59%, respectively. It was also found that these inhibitions outlasted the period of conditioning stimulation by several minutes (post-effects). Twenty seconds after i.p. injection of bradykinin (20 micrograms) the glutamate-evoked activity was depressed by 56% and this effect also lasted for several minutes. The application of identical heterotopic noxious stimuli did not affect the glutamate-evoked activity of the 'non-noxious only' neurones or the 'proprioceptive' neurones. The application to convergent neurones, of doses of glutamate, which were very much larger than the threshold for firing, produced an intense discharge followed by a progressive decrease in spike amplitude and finally a blocking of the spike discharge. During such sequences, which are typical of excessive depolarization, the application of noxious conditioning stimuli (e.g pinch of the contralateral hindpaw or muzzle) resulted in recovery of the spike discharge. In several cases, this recovery long outlasted the period of conditioning noxious stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Thresholds to electrical stimulation of nerves in cat canine tooth-pulp with A beta-, A delta- and C-fibre conduction velocities.

The thresholds to electrical stimulation and the conduction velocities of nerve fibres supplying the pulp of cat canine teeth have been determined. Compound action potentials and the responses of 80 single units were recorded from the inferior alveolar nerve. The properties of 4 types of coronal stimulating electrode were compared. In the single unit studies separate estimates of intradental and extradental conduction velocity of the fibres were obtained by stimulating the pulp in the crown and in the root of the tooth. The units had extradental conduction velocities ranging from 57.7 to 0.9 m . s-1 and therefore included A beta-, A delta- and C-fibres. The thresholds of the units to coronal stimulation ranged from 7 microA, 0.1 ms to 805 microA, 1.0 ms.

Depression of activities of dorsal horn convergent neurones by propriospinal mechanisms triggered by noxious inputs; comparison with diffuse noxious inhibitory controls (DNIC).

The ability of heterotopic noxious stimuli to inhibit the activity of dorsal horn convergent neurones was investigated in both intact anesthetized, and spinal unanesthetized rats. Forty-four convergent neurones in lumbar dorsal horn were recognized by their ability to respond to both noxious and non-noxious natural stimuli and by their characteristic responses corresponding to A- and C-fibre activity following electrical stimulation of their cutaneous excitatory receptive fields on the ipsilateral hindpaw. The application of a sustained pinch to the excitatory receptive field resulted in an initial phasic activation of the neurone, which adapted to a stable tonic level of activity (mean 31.8 +/- 2.2 spikes/s). The levels of activity produced in this fashion were not appreciably different between the two types of preparation. In the intact anesthetized rat, the tonic activity produced by the sustained pinch could be strongly depressed by noxious conditioning stimuli applied to various parts of the body for all 10 neurones studied: heating the tail or pinching the contralateral hindpaw, the tail or a forepaw during 30 s each resulted in comparable inhibitions which had mean values in the order of 80% and which were always marked by post-effects lasting for upwards of 30 s. These inhibitory effects have been called Diffuse Noxious Inhibitory Controls (DNIC). In the spinal unanesthetized rat, the tonic activity was depressed to some extent by the same conditioning stimuli, for only 16/34 neurones studied. By comparison with the intact animals these inhibitions were weak, adapted to base-line levels within 30 s and were more marked for conditioning stimuli applied to structures proximal (tail, contralateral hindpaw) to the excitatory receptive field than for stimuli applied more distally (forepaws). The differences between the inhibitions found in the intact and spinal preparations were subsequently confirmed in a series of experiments in which single convergent neurones were studied before and after the pharmacological blocking of the cervical spinal cord in anaesthetized rats. The results in the spinal preparations provide evidence for the existence of some propriospinal modulatory processes, triggered by the onset of noxious stimulation and acting on convergent neurones. These processes appear to be different from those mediating DNIC, which have been shown to involve supraspinal structures, to concern all convergent neurones, to be very potent and associated with long-lasting post-effects whether the conditioning noxious stimuli are applied to parts of the body proximal or distal to the excitatory receptive field.

Failure of ES 52, a highly potent enkephalinase inhibitor, to affect nociceptive transmission by rat dorsal horn convergent neurones.

The effects of ES 52, a highly potent derivative of the enkephalinase (enkephalin-dipeptidylcarboxypeptidase) inhibitor thiorphan, were studied on nociceptive activities of dorsal horn convergent neurones in the anaesthetized rat. Neither the C-fibre component of the responses elicited by supramaximal electrical stimulation of the hindpaw excitatory receptive fields nor diffuse noxious inhibitory controls triggered by immersion of the tail in 46-48 degrees C waterbaths, were affected by ES 52. Thus we conclude that, in our experimental conditions, modulations of the transmission of nociceptive messages at the spinal level are not greatly modified by specifically blocking the degradation of enkephalins. If a major role for enkephalinase (vs aminopeptidase) in the catabolism of enkephalins at the spinal level can be confirmed, then comparison of the present data with our previous results obtained using the opioid antagonist naloxone, might suggest a predominant role for proenkephalin B products (i.e. dynorphins and/or alpha-neo-endorphin) in modulating nociceptive transmission in the spinal cord.

Effects of remote noxious stimulation on exteroceptive reflexes in human jaw-closing muscles.

Reflexes evoked by applying non-painful taps to an incisor tooth were recorded from the jaw closing masseter and temporal muscles of 21 human subjects. A series of inhibitory, excitatory, inhibitory and excitatory waves (the 'Q, R, S and T' waves of the post-stimulus electromyographic complex (PSEC)) occurred in full-wave rectified and averaged electromyograms. Conditioning by remote noxious stimulation (RNS; application of 3 degrees C water to a hand) usually produced increases in activity at the Q-R and S-T transitions of the PSEC (at mean latencies of 24 and 54 ms respectively), which resulted principally from a shortening of the inhibitory Q and S waves. Changes in the amplitudes of the excitatory R and T waves were also found. The effects of RNS were quantified by integrating records of the difference between conditioned and control PSECs. The RNS-induced effect on the entire PSEC was significantly (P < 0.01) greater when the reflexes were evoked by applying hard (7.4 mN.s) as opposed to soft (3.4 mN.s) taps to the tooth. However the ratio between the effects on the ST and QR segments did not differ significantly between these two intensities of tap stimuli. RNS-induced sensations of pain and increases in systemic arterial blood pressure were not correlated with the RNS-induced effects on the different segments of the PSEC. The results suggest that RNS may affect particularly those elements of the PSEC evoked by higher threshold afferents and that the effects are mediated by mechanisms acting directly at the brainstem level and are not secondary to pain or autonomic responses.

Differentiating condition-induced facilitation, inhibition and disinhibition in a complex series of reflexes in an electromyogram.

In man, the principal exteroceptive reflexes evoked by intra-oral stimulation involve the jaw-closing muscles and include inhibitory and excitatory responses [H.W. van der Glas, A. De Laat, D. van Steenberghe, Oral pressure receptors mediate a series of inhibitory and excitatory periods in the masseteric post-stimulus EMG complex following tapping of a tooth in man, Brain Res. 337 (1985) 117-125.]. These reflexes can be observed in electromyograms (EMGs) recorded with bipolar surface electrodes. The likelihood that these reflexes play important roles in the integrative actions of the jaw has led to interest in the physiological control mechanisms by which they may be modulated. It has been reported recently that the complex series of jaw reflexes evoked by non-painful tapping on human teeth can be modulated by the application of noxious stimulation to the hand [S.W. Cadden, H.W. van der Glas, F. Lobbezoo, A. van der Bilt, Effects of remote noxious stimulation on exteroceptive reflexes in human jaw closing muscles, Brain Res. 726 (1996) 189-197.] or by exercises which produce a change in mental state [S.W. Cadden, H.W. van der Glas, F. Lobbezoo, A. van der Bilt, The influence of attentional factors on short- and long-latency jaw reflexes in man, Arch. Oral Biol. 41 (1996) 995-998.]. The effects of remote noxious stimuli and mental exercises usually involved transient increases in electromyographic (EMG) activity around the interfaces between the successive inhibitory and excitatory reflexes. As the mechanisms underlying the tap-induced inhibitory and excitatory reflexes may show some temporal overlap [H.W. van der Glas, A. De Laat, D. van Steenberghe, Oral pressure receptors mediate a series of inhibitory and excitatory periods in the masseteric post-stimulus EMG complex following tapping of a tooth in man, Brain Res. 337 (1985) 117-125.], these condition-induced increases in EMG activity could, in the simplest hypothesis, have been due to either (i) a condition-induced inhibition of the tap-induced inhibitory influences on the motoneurones (i.e., disinhibition) and/or (ii) a condition-induced facilitation of the tap-induced excitatory influences underlying the subsequent excitatory reflexes. In the present protocol, we describe how it is possible to differentiate between these different underlying mechanisms. The method includes a regression analysis of the relationship between condition-induced changes in amplitude of a reflex and the reflex amplitude under control conditions after taking account of the effect of chance. The analysis is applied on reflex data pooled from various subjects. Although this method of data analysis is illustrated with trigeminal reflexes, it is potentially of use for other complex extracellular recordings including those in other fields of motor control (e.g., EMGs from muscles other than jaw ones).

The effects of inhibitory controls triggered by heterotopic noxious stimuli on a jaw reflex evoked by perioral stimuli in man.

Electromyographic recordings (EMGs) were made from the active masseter muscle of the inhibitory reflex evoked by application of electrical stimuli to the skin of the upper lip in 11 human subjects. In control sequences, the reflex had a mean latency and duration of 41 +/- 1.7 and 44 +/- 2.6 ms, respectively. The magnitude of the reflex (measured by integration of the EMG) was significantly reduced by the application of cold (3 degrees C) or hot (47-48 degrees C) but not warm (38-46 degrees C) water to a hand or foot. The strongest stimuli (3 or 48 degrees C) produced mean reductions of the reflex magnitude in the range of 62-85%. These effects occurred regardless of whether the background activity in the masseter was raised or lowered during the application of the thermal stimuli. Thus, activity in nociceptive nerves from widespread areas of the body can modulate jaw reflexes in man by exerting an influence on the reflex pathway at a point before the motor neurones. This may involve the system of 'diffuse noxious inhibitory controls', which have been shown to depress limb flexion reflexes and neuronal activity in the spinal dorsal horn and trigeminal nuclear complex.

A comparison of reflex depressions of activity in jaw-closing muscles evoked by intra- and peri-oral stimuli in man.

The effects on activity in the masseter muscle of applying electrical stimuli to discrete areas within the mouth or on peri-oral skin were studied electromyographically in 8 subjects. In all subjects, the intra-oral stimuli produced two phases of depressed masseteric activity with mean latencies of 14 ms and 47 ms. By contrast, this shorter latency response was generally not obtained with peri-oral stimuli: responses evoked from vermilion-border skin had an intermediate latency (mean 25 ms) and consisted of either one or two periods of depressed activity, while the responses to stimulation of the hairy skin of the lip generally consisted of a single phase of depressed activity, the time course of which was similar to the later phase produced by intra-oral stimuli (mean latency, 48 ms). These findings were consistently obtained with both single and short trains of electrical stimuli and contrast with reports that intra- and peri-oral stimuli produce similar reflexes. The finding that the short-latency reflex depression of masseteric activity could be evoked only by stimulating nerves supplying intra-oral tissues is consistent with the notion that this reflex may play a role in functions such as mastication, and indeed may indicate that the reflex exists specifically for such a purpose.

Effects of remote deep somatic noxious stimuli on a jaw reflex in man.

The effects on a inhibitory jaw reflex of activating deep somatic afferent nerves in a remote part of the body (the arm) were studied in 13 humans. Electromyographic recordings were made from the active masseter of the long-latency (mean 42.0 +/- 1.1 ms) inhibitory reflex evoked by electrical stimulation of the upper lip. Immediately after a 1-min conditioning period during which the participants compressed a hand-held spring once a second while ischaemia was produced in the arm with an inflated pneumatic cuff, the magnitude of the inhibitory reflex decreased significantly (by 43%). The reflex recovered within 5 min to a magnitude that was not significantly different from its pre-conditioning value. The arm exercise or the ischaemia alone produced no significant changes in the reflex. Furthermore, neither of these last two conditions was reported to be painful, whereas the ischaemic exercise produced pain in all but one participant. It is concluded that activation of remote nociceptive, but not non-nociceptive, deep somatic nerves can modulate jaw reflexes in man.