Nociceptive masseter inhibitory reflexes evoked by laser radiant heat and electrical stimuli

Reduction of sleep bruxism events according to contingent electrical stimulus intensity

BACKGROUND

Although biofeedback with contingent electrical stimulation (CES) has demonstrated the reduction effect on sleep bruxism (SB), the relationship between the actual applied CES intensity and efficacy remains uncertain.

OBJECTIVE

This study aimed to investigate whether the reduction of bruxism events and jaw muscle symptoms could vary according to the intensity of CES and in probable sleep bruxers.

METHODS

Twenty probable sleep bruxers were initially screened for bruxer confirmation based on a 2-week recording of SB events with a portable electromyography recorder (BUTLER®GrindCare®, GC4). A 3-week recording was conducted without CES using a GC4, followed by another 3-week recording with CES. At baseline and before and after the CES (+) session, clinical muscle symptoms were assessed using a 0-10 numerical rating scale (NRS). The relationships between the actual applied CES intensity and the number of SB events/hour, as well as the NRS of clinical muscle symptoms, were analysed.

RESULTS

The actual applied CES intensity was positively correlated with the reduction rate of the number of SB events/hour (R = .643, p = .002), as well as with the reduction rate of NRS for pain, unpleasantness, fatigue, tension and stiffness (R > .500, p < .011).

CONCLUSION

Higher CES elicited a more robust reduction in SB events and clinical muscle symptoms, in probable bruxers. Prior to selecting CES biofeedback as a management option for SB, it would be beneficial to assess the tolerance threshold of CES in each bruxer in order to predict the effectiveness of CES in probable sleep bruxers.

Reevaluation of reflex responses of the human masseter muscle to electrical lip stimulation

We examined the reflex response of the human masseter muscle to electrical stimulation of the lip using both single motor unit and surface electromyogram based methods. Using the classical analysis methods, reflex response to mild electrical stimuli generated two distinct short-lasting inhibitions. This pattern may reflect the development of combinations of short- and long-latency inhibitory postsynaptic potentials as a result of the mildly painful electrical lip stimulation. However, this pattern appearing in the classical analysis methods may have developed as a consequence of earlier responses and may not be genuine. This study examined the genuineness of these responses using both the classical analysis methods and the discharge rate method to uncover the realistic postsynaptic potentials in human trigeminal motor nucleus. Using the discharge rate method, we found that the electrical lip stimulation only generated a long-lasting single or compound inhibitory response that is followed by late, long-lasting excitation. These findings have important implications on the redrawing of the neuronal pathways of the trigeminal nerve that are frequently used to judge neuromuscular disorders of the trigeminal region.NEW & NOTEWORTHY We examined the human masseter reflex response to electrical stimulation of lower lip to uncover realistic postsynaptic potentials in the trigeminal motor nucleus. We found that the stimulation generates a long-lasting single or compound inhibitory response that is followed by a late, long-lasting excitation. These findings have important implications on the redrawing of the neuronal pathways of the trigeminal nerve that are frequently used to judge neuromuscular disorders of the trigeminal region.

[Physiology of pain]

Pain research is based broadly on physiological disciplines and its development follows the methodological progress of the era, from classical psychophysiology to electrophysiological investigations at peripheral and central nociceptive systems, single cells and ion channels to modern imaging of nociceptive processing. Physiological pain research in Germany has long been part of an interdisciplinary research network extending beyond all political boundaries, and this situation has continued since molecular techniques started to dominate all biomedical research. Current scientific questions, such as intracellular nociceptive signal mechanisms, interactions with other physiological systems including the immune system, or the genetic basis of epidemic and chronic pain diseases can only be solved interdisciplinary and with international collaboration.

Exteroceptive Suppression of Masseter Muscle Activity in Juvenile Migraineurs

The second exteroceptive suppression period (ES2) of masseter or temporalis muscle activity may be reduced in adults with chronic tension-type headache. In adults with migraine, ES2 was found normal or tended to be protracted. To date, no studies on exteroceptive suppression in children and adolescents with headaches have been published. We investigated the exteroceptive suppression of masseter muscle activity in 14 migraineurs and 19 controls between 6 and 18 years of age. It was elicited by electrical stimulation at the labial commissure. No differences were found regarding the first suppression period, but ES2 was significantly longer in the migraine group than in controls. The results of the migraine group suggest overactivity of the interneurons of the reflex loop due to impaired inhibitory control from superior antinociceptive systems already at the beginning of this headache disorder.

Heterotopic ischemic pain attenuates somatosensory evoked potentials induced by electrical tooth stimulation: Diffuse noxious inhibitory controls in the trigeminal nerve territory

The purpose of this study was to determine whether the late component of somatosensory evoked potentials (SEP) induced by electrical tooth stimulation and pain intensity are inhibited by heterotopic ischemic stimulation. The tourniquet pressure with 50 mmHg greater than the individual's systolic pressure was applied to the left upper arm for 10 min as ischemic conditioning stimulation. The late component of SEP and visual analogue scale (VAS) were recorded at 4 times and both were significantly decreased when ischemic conditioning stimulation was applied. The maximum reductions in SEP amplitude and the VAS value were 26.1% and 21.2%, respectively, during ischemic conditioning stimulation. After-effect was observed 5 min after removal of the conditioning stimulation. The present study revealed that heterotopic ischemic stimulation attenuated the late component of SEP induced by electrical tooth stimulation, triggering diffuse noxious inhibitory controls (DNIC) and after-effects in the trigeminal nerve territory. It was also suggested that the DNIC effect differs, depending on the intensity, kind, and quality of the test and conditioning stimuli.

Guidelines and recommendations for assessment of somatosensory function in oro-facial pain conditions--a taskforce report

The goals of an international taskforce on somatosensory testing established by the Special Interest Group of Oro-facial Pain (SIG-OFP) under the International Association for the Study of Pain (IASP) were to (i) review the literature concerning assessment of somatosensory function in the oro-facial region in terms of techniques and test performance, (ii) provide guidelines for comprehensive and screening examination procedures, and (iii) give recommendations for future development of somatosensory testing specifically in the oro-facial region. Numerous qualitative and quantitative psychophysical techniques have been proposed and used in the description of oro-facial somatosensory function. The selection of technique includes time considerations because the most reliable and accurate methods require multiple repetitions of stimuli. Multiple-stimulus modalities (mechanical, thermal, electrical, chemical) have been applied to study oro-facial somatosensory function. A battery of different test stimuli is needed to obtain comprehensive information about the functional integrity of the various types of afferent nerve fibres. Based on the available literature, the German Neuropathic Pain Network test battery appears suitable for the study of somatosensory function within the oro-facial area as it is based on a wide variety of both qualitative and quantitative assessments of all cutaneous somatosensory modalities. Furthermore, these protocols have been thoroughly described and tested on multiple sites including the facial skin and intra-oral mucosa. Standardisation of both comprehensive and screening examination techniques is likely to improve the diagnostic accuracy and facilitate the understanding of neural mechanisms and somatosensory changes in different oro-facial pain conditions and may help to guide management.

Trigeminal small-fibre function assessed with contact heat evoked potentials in humans

Contact heat stimuli have been reported to excite mechano-thermal nociceptors and to evoke brain potentials (CHEPs) from the limbs. We investigated whether contact heat evokes reproducible CHEPs from the trigeminal territory and may prove a reliable diagnostic tool in facial neuropathic pain. We applied contact heat stimuli to the perioral and supraorbital regions; CHEPs were recorded from the vertex in 20 controls and 2 patients with facial neuropathic pains, and reflex responses from the orbicularis oculi and masticatory muscles in 5 controls. We studied the correlation between CHEP data and perceptive ratings, site of stimulation, and age. Finally, we compared CHEPs with laser evoked potentials (LEPs). Contact heat stimuli at 51 degrees C evoked vertex potentials consisting of an NP complex similar to that elicited by laser pulses, though with a latency some 100-ms longer. Perioral stimulation yielded higher pain intensity ratings, shorter latency and larger amplitude CHEPs than supraorbital stimulation. CHEP data correlated significantly with age. Contact heat stimuli at 53 degrees C evoked a blink-like response in the relaxed orbicularis oculi muscle and a silent period in the contracted masseter muscle. In patients with facial neuropathic pain the CHEP abnormalities paralleled those seen with LEPs. We were unable to achieve reproducible signals related to C-receptor stimulation by contact heat stimuli at 41 degrees C in the ten subjects in whom they were tested. Contact heat stimulation, as well as laser stimulation, easily yields large-amplitude brain potentials and nociceptive reflexes, both related to the Adelta input. However CHEPs are not suitable for C-fibres potentials recording.

Experimental skin pain and muscle pain induce distinct changes in human trigeminal motoneuronal excitability

Seeking information on the physiological properties of the trigeminal motoneuronal pool we investigated changes in the excitability of trigeminal motor system induced by two types of experimental pain (muscle and skin). In one session, we studied the effect of muscle pain induced by hypertonic saline infusion into the masseter muscle on the recovery cycle of the heteronymous H-reflex in the temporalis muscle and the homonymous silent period (SP) in the masseter muscle, both elicited by stimulation of the masseteric nerve in ten-healthy subjects. In another session, we studied the effect of laser stimuli applied to the perioral region, at conditioning intervals from 20 to 160 ms, on the temporalis H-reflex and masseter SP in nine healthy subjects. Whereas laser-induced skin pain significantly inhibited the temporalis H-reflex and facilitated the masseter SP (P < 0.01), muscle pain left the time course of the temporalis H-reflex and masseter SP unchanged (P > 0.05). The timing of temporalis H-reflex suppression and masseter-SP enhancement induced by laser stimuli indicates that facial skin nociceptors inhibit trigeminal motoneurones via multysynaptic reflex pathways. Hypertonic saline, a stimulus that predominantly activates group III and IV afferents, left both variables reflecting trigeminal motoneuron excitability unchanged. Due to the differences between the two experimental models, we cannot conclude that such inhibitory reflex pathway does not exist from muscle nociceptors to trigeminal motoneurones.

Exteroceptive suppression of temporalis muscle activity in subjects with high and low aggression traits

STUDY AIM

There is evidence that the second exteroceptive suppression period of temporalis muscle activity (ES2) is modulated by the 5-HT neuronal activity in the brainstem, and the aggression trait is also connected with the cerebral 5-HT neuronal innervation. We therefore studied the temporalis ES2 in subjects with high and low aggression traits.

METHODS

Sixty-five subjects with either low or high aggression trends, judged by clinical interview, answered the Zuckerman-Kuhlman Personality Questionnaire (ZKPQ), the Zuckerman Sensation Seeking Scales (SSS) and the Plutchik-van Praag Depression Inventory (PVP). These also underwent a temporalis ES2 test procedure. Twenty-two subjects with a high ZKPQ aggression trait score and 27 with a low score were selected for data analysis.

RESULTS

On average, the high aggression group displayed significantly reduced temporalis ES2 duration, elevated ZKPQ Impulsive Sensation Seeking, Neuroticism-Anxiety and Aggression-Hostility, and PVP scores. The personality traits were not related with either latency or duration of temporalis ES2 in any group. The PVP score, however, was negatively correlated with ES2 duration in all 49 subjects.

CONCLUSIONS

This study demonstrates neurophysiologic signs of brainstem dysfunction in subjects with high aggression traits.

Exteroceptive suppression periods in masseteric EMG: Use of stimulus–response curves

The present study documented quantitative changes in the duration and the depth of the exteroceptive suppression (ES) period in the masseteric electromyogram (EMG) using stimulus-response (SR) curves. From six males and six females (20-41 years), the surface masseteric EMG was recorded bilaterally. Visual feedback from the left masseter muscle (MM) allowed a constant clenching level of 30% or 50% maximal voluntary contraction (MVC). Automatic triggering of the stimulus to the left mental nerve occurred when the background EMG remained constant. Eight to twelve stimulus intensities (range, 0-40 mA) were determined based on the subjects' sensory threshold (T). The late ES (ES2) occurred consistently and could be analyzed using SR curves. ES2 in the right MM appeared at 7.3+/-3.1T at 30% MVC, while it was significantly decreased to 5.2+/-2.2T at 50% MVC (P<0.05). The depth of ES2 suppression in the left MM was 69.7+/-5.7% at 30% MVC, and this increased significantly to 76.6+/-5.5% at 50% MVC (P<0.05). In addition, the depth of ES2 suppression in the left MM was significantly higher than that in the right MM at 50% MVC (70.0+/-7.4%, P<0.05). Consequently, it was confirmed that both the appearance and depth of ES may be affected by the level of background EMG. More importantly, the use of multiple stimulation intensities allowed development of reliable SR curves. These SR curves may be used to determine inter-individual differences in ES.

Cutaneous silent period in hand muscle is evoked by laser stimulation of the palm, but not the hand dorsum

Painful electrical stimulation of the fingers evokes an inhibitory response in hand muscles (cutaneous silent period, CSP). The aim of this study was to determine whether purely nociceptive thermal stimuli applied to the hand evoke a CSP. High-intensity laser pulses (205 +/- 44 mJ) were delivered to the dorsum and palm of the hand in five volunteers. Electromyographic signals were recorded from the ipsilateral first dorsal interosseous muscle. We then compared the laser-evoked CSP with the CSP induced by electrical stimulation. A clear laser CSP (latency 90 +/- 7 ms) was evoked in all subjects when laser pulses were applied to the palm of the hand, whereas no response was recorded after stimulation of the dorsum. Electrical stimulation of both the dorsum and the palm evoked a CSP (latency 65 +/- 5 ms), although the reflex threshold was significantly lower after stimulation of the palm. This study confirms that the CSP is a nociceptive response specific to limbs that grasp. In humans, palm nociceptors are probably more functionally effective than dorsal nociceptors in inducing the hand-muscle inhibition that interrupts hand prehension (so that a potentially noxious source is dropped) before proximal muscles withdraw the limb.

Simultaneous modulation of the exteroceptive suppression periods in the trapezius and temporalis muscles by experimental muscle pain

OBJECTIVE

To investigate whether the exteroceptive suppression (ES) periods in the trapezius and temporalis muscles could be simultaneously modulated by either experimental trapezius muscle pain or temporalis muscle pain.

METHODS

Fourteen healthy volunteers participated in two sessions with randomised injections of either hypertonic or isotonic saline into the right trapezius muscle or right temporalis muscle. The pain intensity was continuously scored on a 10 cm electronic visual analogue scale. During muscle contraction, the early (ES1) and late (ES2) reflex in the temporalis and late (ES2) reflex in the trapezius elicited by electrical stimulation of the right infraorbital nerve were recorded pre-, during-, and post-injection.

RESULTS

Hypertonic saline injection induced local muscle pain and referred pain with the maximal pain intensity of 6.3+/-0.5 cm in the right trapezius and 6.7+/-0.6 cm in the right temporalis muscles. Injection of either hypertonic or isotonic saline into the right trapezius muscle was associated with significantly shortened ES2 duration and offset time in the injected right trapezius muscle. Only injection of hypertonic saline into the right trapezius muscle could significantly reduce the ES2 offset time and decrease the degree of suppression of ES2 in the right temporalis and left trapezius muscles. Injection of either hypertonic or isotonic saline into the right trapezius muscle evoked no changes in ES2 parameters in the left temporalis muscle. Injection of hypertonic saline, but not isotonic saline, into the right temporalis muscle caused a significantly shorter ES2 offset time and shorter ES2 duration bilaterally in the temporalis and trapezius muscles.

CONCLUSIONS

The findings of the present study provide evidence for a bi-directional effects reflecting convergence of muscle afferents from the trigeminal and upper cervical neural systems in humans, which may partly underlie the manifestations of pain referral between these two areas.

Trigeminal responses to laser stimuli

The majority of the studies on laser evoked potentials (LEPs) have been focused on hand and foot stimulations and only lately on the trigeminal system. Because of a high receptor density in the facial skin and the very short conduction distance, LEP recordings after trigeminal stimulation are easier and quicker than those after stimulation of the limb extremities. Laser pulses with a stimulus intensity close to perception threshold can evoke well-defined LEPs. Few trials are sufficient to yield stable and reproducible averages. Even ultralate LEPs related to the C-fibre input are comparatively easily obtained from the trigeminal territory. The brain generators of the main LEP waves are probably very close for the trigeminal and limb stimulations. Trigeminal LEPs have been found absent or delayed in patients with trigeminal neuralgia, trigeminal neuropathies, posterior fossa tumors, and brainstem infarctions or demyelinating plaques. Conversely, trigeminal LEPs appear to be enhanced in patients with migraine. High-intensity pulses directed to any trigeminal division also elicit reflex responses: a blink-like reflex in the orbicularis oculi and a single silent period in the contracting masseter muscle. The availability of a neurophysiological method of assessing function of the trigeminal nociceptive pathways reaching both the cerebral cortex and the brainstem reflex circuits, has provided new opportunities for investigating the pathophysiology of orofacial pain syndromes.

Cutaneous silent periods

The cutaneous silent period (CSP) refers to the brief interruption in voluntary contraction that follows strong electrical stimulation of a cutaneous nerve. The CSP is a protective reflex that is mediated by spinal inhibitory circuits and is reinforced in part by parallel modulation of the motor cortex. This review summarizes current understanding of the afferents and circuits that are responsible for producing CSPs; the utility of the CSP for investigating peripheral and central nervous system disorders; and the relationship between the CSP, other cutaneous reflexes, and peripheral silent periods.

Influence of gender and hemispheric lateralization on heat pain perception in major depression

Increased incidence of clinical pain complaints from patients with major depression, as well as increased experimental pain thresholds have been reported. The basis of this phenomenon remains unclear, as well as its relation to medication, clinical recovery, gender and lateralization of hemispheric function. We aimed to further elucidate heat pain perception in depression applying a testing battery including assessment (on both arms) of warmth perception, heat pain perception and heat pain tolerance, and the jaw opening reflex (duration of ES2 component) as a putative indicator of descending pain inhibition. The battery was applied to 20 patients and 20 age- and sex-matched controls. Patients were assessed: on admission (acutely depressed, off-medication), few days after admission (depressed, on medication), and after clinical recovery (mostly on medication), and controls at corresponding intervals. Significant elevated heat pain thresholds were found off and on medication in the acute stage (mainly in women) and after recovery on the right arm only. Elevated heat pain tolerance (on the right arm only) was seen in medicated patients in the acute and recovered stage. Significant prolongation of ES2 duration was only found in acutely depressed patients off medication. While confirming hypalgesia to heat pain in major depression, our findings demonstrate a close relation to gender and strong influence of lateralization after recovery. Altered pain processing at brain stem level might only partially be responsible for the observed finding.

Recovery cycle of the masseter inhibitory reflex after magnetic stimulation in normal subjects

OBJECTIVE

To evaluate the differences in the recovery cycle of the masseter inhibitory reflex (MIR) obtained with electrical and magnetic stimulation.

METHODS

In 31 healthy subjects we studied the MIR evoked by electrical or magnetic stimulation of the mental territory and the recovery cycle of this reflex with the paired stimuli technique at different interstimulus intervals (ISI), between 100 and 600 ms.

RESULTS

Latency and area of the early and late silent periods (SPs) of the MIR were similar after electrical and magnetic stimulation. The recovery cycle of the test late SP was similar with the two kinds of stimulation, except for short ISIs. The main difference between the two kinds of stimulation was in the painful quality of the stimulus: the magnetic stimuli were always below pain threshold.

CONCLUSIONS

As with electrical stimulation, it is possible to obtain a MIR with magnetic peripheral stimulation. The magnetic paired stimuli are equally effective in the evaluation of the recovery cycle of the MIR. The results demonstrate that magnetic stimulation is a useful tool in the evaluation of excitability of the trigeminal motoneuronal system, with little discomfort for the patient. They also confirm the unlikelihood of nociceptive afferences involvement.

Nociceptive temporalis inhibitory reflexes evoked by CO2-laser stimulation in tension-type headache

The aim of the study was to evaluate the laser-induced suppression periods of the temporalis muscle in patients with tension-type headache, compared with the pattern of temporalis activity suppression induced by electrical stimulation. Fifteen patients with chronic and 10 with episodic tension-type headaches were selected. Suppression periods were recorded simultaneously from both temporalis muscles using both electrical stimuli and CO2-laser stimuli. A significant reduction in the later electrically induced suppression period was found in both tension-type headache groups. Laser stimulation induced a first suppression period (LSP1) with a latency of about 50 ms in all patients. The features of LSP1 were similar across groups. The LSP1 should correspond to the first suppression period induced by electrical stimulus, which is partly a nociceptive response, whereas the second period seemed negligibly linked with the activation of pain-related afferents, though probably their activation may contribute to increase the reflex duration and to emphasize abnormalities in tension-type headache.

Assessment of nociceptive trigeminal pathways by laser-evoked potentials and laser silent periods in patients with painful temporomandibular disorders

We assessed the trigeminal nociceptive pathways in patients with painful temporomandibular disorders (TMD) and control subjects using a CO(2)-laser stimulator which provides a predominant activation of the nociceptive system. Fifteen patients with unilateral pain were examined in accordance with the Research Diagnostic Criteria for TMD and 30 gender- and age-matched individuals were included as a control group. Laser-evoked potentials (LEPs) and laser silent periods (LSPs) after stimulation of the perioral region (V2/V3) on the painful and non-painful sides were recorded in all subjects. LEPs were evoked by low-intensity pulses (1.5 x perception threshold (PTh)) and recorded from scalp electrodes at the vertex. LSPs were evoked by high-intensity pulses (4 x PTh) and recorded bilaterally from masseter muscles with surface electromyogram (EMG) electrodes. Subjects also assessed the stimulus intensity on a 0-10 rating scale. LEPs had normal latency but smaller amplitude in TMD patients compared to the control group (P<0.001). Side-to-side comparison within patients showed that LEP amplitude was even more reduced after stimulation on the painful than the non-painful side (P<0.001). TMD patients showed a significant side-asymmetry of the pre-stimulus EMG activity, with a smaller value in the muscle on the painful side (P<0.001). LSPs were completely absent bilaterally in 12 TMD patients and unilaterally in two patients; only one patient had normal and bilateral LSPs. TMD patients perceived the laser stimulus less intense on the painful than the non-painful side (P<0.05). We found suppression of cortical responses and brainstem reflexes elicited by a predominantly nociceptive input in TMD patients. These findings are consistent with recent experimental pain studies and suggest that chronic craniofacial pain in TMD patients may be associated with a dysfunction of the trigeminal nociceptive system.

Long-term depression of the human masseter inhibitory reflex

Long-term depression (LTD) of synaptic transmission is reliably induced by low-frequency stimulation (LFS) of nociceptive afferents in vitro. LTD can only exceptionally be induced in anesthetized animals. In order to fill the gap between the in vitro cell studies and the in vivo situation, the effects of LFS on the masseter inhibitory reflex (MIR) were investigated in man. Noxious LFS of mental nerve afferents caused a significant depression of the early MIR1 and the late MIR2 components. Whereas the onset latency (+25%), the duration (-42%) and the integral (+68%) of the MIR2 were strongly modulated after LFS, only the integral (+35%) of the MIR1 significantly changed. The results document a long-term depression of trigeminal somatosensory processing in man. The different effects of LFS on the two components of the MIR may point to central mechanisms of LTD.

Modulation of trigeminal laser evoked potentials and laser silent periods by homotopical experimental pain

Cutaneous laser stimulation activates predominantly the A-delta and C mechano-heat nociceptors. Applied to the perioral region, low intensity CO(2)-laser pulses evoke reproducible trigeminal cortical evoked potentials (LEPs). High intensity CO(2)-laser stimuli induce a reflex response in the contracted jaw-closing muscle, the so-called laser silent period (LSP). Both LEPs and LSP provide a useful tool to study the physiology of the trigeminal nociceptive system. In ten healthy subjects we recorded the subjective ratings of the perioral laser stimulation and the trigeminal LEPs and LSP before, during and after homotopic experimental tonic muscle (infusion of hypertonic saline into the masseter muscle) and tonic skin pain (topical application of capsaicin to the cheek). LEPs were recorded from the vertex at two stimulus intensities: low (1.1 x pain threshold, PTh) and high (1.5 x PTh). LSP from masseter and temporalis muscles were recorded bilaterally through surface electromyographic (EMG) electrodes. CO(2)-laser pulses were applied to the perioral region (V2/V3) on the painful and non-painful side. The amplitude of LEPs increased with higher stimulus intensities (P<0.0001), but were suppressed by 42.3+/-5.3% during experimental muscle pain (P<0.0001) and by 41.6+/-3.2% during skin pain (P<0.0001). No pain-related effects were observed for the N and P latency of the LEPs (P> 0.20). The LSP in the masseter and temporalis muscles had similar onset-latency (80+/-5 ms), offset-latency (111+/-5 ms) and duration (31+/-4 ms). Experimental pain had no effect on the onset- and offset-latency (P>0.05). Experimental pain, whether from muscle or from skin, reduced the degree of suppression (P<0.01) and the area under the EMG curve (P< 0.005) of the LSP. The LSP was still suppressed during the post-pain recordings when the skin pain had disappeared (P<0.05). In all experiments experimental tonic pain decreased the subjective ratings of the perioral laser stimulation (P< 0.001). Experimental tonic pain, either from muscle or from skin, induced bilateral inhibitory effects on the trigeminal laser evoked potentials and brainstem reflex responses and on the subjective ratings of the laser pulses. These effects could be mediated through the activation of segmental and suprasegmental inhibitory systems that may function interdependently.

Brainstem reflexes: electrodiagnostic techniques, physiology, normative data, and clinical applications

An overview is provided on the physiological aspects of the brainstem reflexes as they can be examined by use of clinically applicable neurophysiological tests. Brainstem reflex studies provide important information about the afferent and efferent pathways and are excellent physiological tools for the assessment of cranial nerve nuclei and the functional integrity of suprasegmental structures. In this review, the blink reflex after trigeminal and nontrigeminal inputs, corneal reflex, levator palpebrae inhibitory reflex, jaw jerk, masseter inhibitory reflex, and corneomandibular reflex are discussed. Following description of the recording technique, physiology, central pathways, and normative data of these reflexes, including an account of the recording of recovery curves, the application of these reflexes is reviewed in patients with various neurological abnormalities, including trigeminal pain and neuralgia, facial neuropathy, and brainstem and hemispherical lesions. Finally, simultaneous electromyographic recording from the orbicularis oculi and the levator palpebrae muscles is discussed briefly in different eyelid movement disorders.

Human masseter inhibitory reflexes evoked by repetitive electrical stimulation

OBJECTIVES

The relationship between the masseter inhibitory reflex (MIR) and nociceptive processing in the trigeminal region was studied in 10 healthy subjects. Based on the known increase in perceived sensory intensity following repetitive stimulation of the nociceptive system, we examined the hypothesis that the MIR reflects noxious activity in the trigeminal system by determining the possible relation between changes in MIR and perceived sensory intensity.

METHODS

The MIR was quantitated and compared with psychophysical measures following repetitive peri-oral electrical stimulation (5 square wave pulses of 0.5ms repeated at 2Hz). In addition to the early (ES1) and late (ES2) periods of exteroceptive suppression, two periods with apparent excitation could be distinguished from the background electromyografical activity: (a) the inter-suppression period (ISP) between the ES1 and the ES2, and (b) the post-suppression period (PSP) after the ES2. A computer algorithm was used to detect and quantitate ES1, ES2, ISP, and PSP. The response variables were (a) onset latencies and (b) magnitudes of suppression (ES1 and ES2) and excitation (ISP and PSP).

RESULTS

Consistent reduction of the magnitude of ES2 suppression in response to repetitive stimuli was observed below as well as above the pain detection threshold.

CONCLUSIONS

The observed reduction of the magnitude of ES2 suppression is not specifically related to nociceptive processing. Habituation or net inhibitory effects on inhibitory pre-motor neurones (i.e. disinhibition) are possible mechanisms for the observed reduction of the magnitude of ES2 suppression after repetitive stimulation.

Antinociceptive reflex alteration in acute posttraumatic headache following whiplash injury

Brainstem-mediated antinociceptive inhibitory reflexes of the temporalis muscle were investigated in 82 patients (47 F, 35 M, mean age 28.3 years, SD 9.4) with acute posttraumatic headache (PH) following whiplash injury but without neurological deficits, bone injury of the cervical spine or a combined direct head trauma on average 5 days after the acceleration trauma. Latencies and durations of the early and late exteroceptive suppression (ES1 and ES2) and the interposed EMG burst (IE) of the EMG of the voluntarily contracted right temporalis muscle evoked by ipsilateral stimulation of the second and third branches of the trigeminal nerve were analyzed and compared to a cohort of 82 normal subjects (43 F, 39 M, mean age 27.7 years, SD 7.1). Highly significant reflex alterations were found in patients with PH with a shortening of ES2 duration with delayed onset and premature ending as the primary parameter of this study, a moderate prolongation of ES1 and IE duration and a delayed onset of IE. The latency of ES1 was not significantly changed. These findings indicate that acute PH in whiplash injury is accompanied by abnormal antinociceptive brainstem reflexes. We conclude that the abnormality of the trigeminal inhibitory temporalis reflex is based on a transient dysfunction of the brainstem-mediated reflex circuit mainly of the late polysynaptic pathways. The reflex abnormalities are considered as a neurophysiological correlate of the posttraumatic (cervico)-cephalic pain syndrome. They point to an altered central pain control in acute PH due to whiplash injury.

Temporalis exteroceptive suppression in generalized anxiety disorder and major depression

The duration abnormality of the exteroceptive suppressions, or silent periods, of peripheral or jaw-closing muscle activities induced by transcranial magnetic or trigeminal electrical stimuli in patients suffering from anxiety or depression is ill-defined. We therefore studied the exteroceptive suppression periods of the temporalis muscle electromyography elicited by trigeminal territory electrical stimuli, Zuckerman-Kuhlman's Personality Questionnaire, and Plutchik-van Praag's Depression Inventory (PVP) in 12 patients suffering from generalized anxiety disorder (GAD) and 16 from major depression (MD) as well as in 17 healthy volunteers. Durations of the second suppression period (ES2) sociability scores were decreased in GAD patients, while PVP and neuroticism-anxiety scores were elevated in both GAD and MD patients. There was a positive correlation between ES2 duration and sociability score in the GAD group. This study indicates that anxiety can modify the temporalis ES2 duration through cortical descending inhibitory controls.

Brain Stem Reflexes: Probing Human Trigeminal Nociception

Although many people suffer from orofacial pain and headache, objective methods for investigation of trigeminal nociception in humans have been lacking. Trigeminal brainstem reflexes such as the masseter inhibitory reflex and the blink reflex are mediated by central multireceptive neurons that are also involved in trigeminal nociception. Therefore, these trigeminal reflexes are suitable models for probing pontine and medullary pain processing.

The clinical use of brainstem reflexes and hand-muscle reflexes

Brainstem reflexes and hand-muscle reflexes can be elicited and recorded with routine EMG equipment. Not all these reflexes are useful in clinical neurology. But those that are - the subject of this review - exhibit distinct patterns of abnormality that have clinical diagnostic and localizing value in various diseases, including cranial neuropathies, focal lesions within the cervical cord, brainstem, and brain, movement disorders, and pain.

Modulation of exteroceptive suppression periods in human jaw-closing muscles by local and remote experimental muscle pain

The exteroceptive suppression periods (ES) 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 tonic experimental jaw-muscle pain versus remote muscle pain on the short-latency (ES1) and long-latency (ES2) reflex in the jaw-closing muscles. Twelve healthy subjects participated in the first experiment with jaw-muscle pain. In random order 5% hypertonic or 0.9% isotonic saline was infused into the left masseter muscle for 15 min. The pain intensity was scored continuously by the subjects on a 10-cm visual analogue scale (VAS). Electromyographic (EMG) activity was recorded bilaterally from the masseter and temporalis muscles during the pre-infusion, early phase of infusion (from 120 to 480 s), late phase of infusion (from 540 to 900 s) and post-infusion. An electrical stimulus was delivered to the skin above the left mental nerve (ipsilateral to the painful muscle) to evoke the ES in the contracting jaw-closing muscles. Ten healthy subjects participated in experiment 2 which was as identical to experiment 1 except that the electrical stimulus was delivered to the right mental nerve (contralateral to the painful muscle). Nine healthy subjects participated experiment 3 where remote muscle pain was induced in the left tibialis anterior muscle. In experiment 1 painful infusion of hypertonic saline caused a significantly later onset latency of ES2 in the left masseter muscle during the late phase of infusion compared to pre-infusion values (P < 0.05). The duration of ES2 in the same muscle was significantly shorter during the late infusion phase compared to pre- and post-infusion values (P < 0.05) and the degree of suppression was significantly reduced during the early infusion compared to the pre-infusion values (P < 0.05). Isotonic saline did not influence the ES1 or ES2. In experiment 2, similar significant inhibitory changes were found in the ES2 on the painful side. In experiment 3, no significant effects on ES1 and ES2 were observed during painful infusion of hypertonic saline into the leg muscle. These results indicate that the effects of tonic jaw-muscle pain on ES2 can be distinguished from a generalized effect of muscle pain. Furthermore, there seems to be a differential and lateralized effect of jaw-muscle pain on the brain stem reflex circuits involved in the generation of ES1 and ES2 probably through a presynaptic mechanism.

Convergence of meningeal and facial afferents onto trigeminal brainstem neurons: an electrophysiological study in rat and man

Headache is often accompanied by referred pain in the face. This phenomenon is probably due to a convergence of afferent inputs from the meninges and the face onto central trigeminal neurons within the medullary dorsal horn (MDH). The possible existence and extent of this convergence was examined in rat and man. MDH neurons activated by stimulation of the parietal meninges were tested for convergent tactile and noxious mechanical input from all three facial branches of the trigeminal nerve. All 21 units with meningeal input could also be activated by facial stimuli. Brush stimuli applied to the supraorbital nerve area activated 86%, to the infraorbital nerve area 29%, and to the mental nerve area none of the units. Pinch stimuli applied to the supraorbital nerve area activated 95%, to the infraorbital nerve area 86%, and to the mental nerve area 52% of the units. The results suggest convergence of meningeal and facial inputs concentrated on the supraorbital nerve in rat. In man convergence was examined by probing neuronal excitability of MDH applying the blink reflex (BR) during Valsalva maneuver which probably increases intracranial pressure. The BR evoked by supraorbital nerve stimulation remained unchanged, while the BR evoked by mental nerve stimulation was significantly facilitated. This facilitation may be due to convergence of meningeal and facial inputs onto trigeminal neurons in man.

Relation between perceived stimulus intensity and exteroceptive reflex responses in the human masseter muscles

OBJECTIVES

Two periods of exteroceptive suppression are elicited in human masseter muscle following perioral electrical stimulation: ES 1 and ES2. Furthermore, in the masseter EMG, two periods of apparent excitation can be distinguished from the background activity: intersuppression period (ISP) and post-suppression period (PSP). In the present study, the relationship between nociception and ES1, ISP and PSP was investigated.

METHODS

The reflex responses were electrically elicited in 15 volunteers. A novel computer algorithm was used to detect and quantitate ES1, ES2, ISP and PSP.

RESULTS

(a) ES1 and ES2 were more susceptible to changes in magnitude of suppression and excitation, respectively, than ISP and PSP; (b) the on-set latencies were almost unaffected; (c) both ES1 and ES2 occurred at stimulus intensities well below pain threshold; (d) magnitude of ES1 and ES2 suppression increased with increasing stimulus intensity, but supra pain threshold stimulation did not result in further suppression; and (e) no correlation was found between perceived stimulus intensity and ES1, ISP or PSP.

CONCLUSIONS

The increase in ES1 and ES2 suppression seen in our study at increasing stimulus intensity is suggested to be mediated by activity in both nociceptive and non-nociceptive fibres converging onto central interneurones in the reflex circuits. We suggest that neither ISP nor PSP are reflex responses. It is unlikely that ES1, ES2, ISP and PSP evoked by electrical stimuli are directly linked to noxious activity.

Assessment of trigeminal small-fiber function: brain and reflex responses evoked by CO2-laser stimulation

Laser pulses selectively excite mechano-thermal nociceptors and evoke brain potentials that may reveal small-fiber dysfunction. We applied CO2-laser pulses to the perioral and supraorbital regions and recorded the scalp laser-evoked potentials (LEPs) and reflex responses in the orbicularis oculi, masticatory, and neck muscles in 30 controls and 10 patients with facial sensory disturbances. Low-intensity pulses readily evoked scalp potentials consisting of a negative component with a latency of 165 ms followed by a positive component at 250 ms. In vertex recordings, the amplitude of LEPs exceeded 30 microV. Although only high-intensity pulses evoked reflex responses, some subjects showed--even to low-intensity pulses--an orbicularis oculi (blink-like) response that markedly contaminated the scalp recording. Scalp LEPs were abnormal in patients with hypalgesia and normal trigeminal reflexes and normal in patients with normal pain sensitivity and abnormal trigeminal reflexes. Possibly because of the high receptor density in this area and the short conduction distance, laser stimulation of the trigeminal territory yields low-threshold and large LEPs, which are useful for detecting dysfunction in peripheral and central pain pathways.

Conditioning of the masseter inhibitory reflex by homotopically applied painful heat in humans

During contraction of the jaw-closing muscles, afferent input from the intraoral and perioral region can elicit two bilateral suppression periods (SP1 and SP2, respectively) in the masseter electromyogram (EMG). Non-painful electrical stimulation 2 cm from the left labial commissure was used in the present study to evoke these trigeminal inhibitory reflexes. The subjects maintained a level of 50% of their maximum masseter EMG. The degree of suppression was quantified as the percentage suppression of the mean EMG activity in a fixed post-stimulus interval (SP2, 40-90 ms). Further, brief (200 ms) painful radiant heat conditioning stimuli were delivered to the ipsilateral cheek, in order to investigate the influence of nociceptive input on the (non-nociceptive) trigeminal masseter inhibitory reflex. Nine different conditions combining radiant heat and electrical stimuli were used. Twelve stimuli were presented for each condition. The radiant heat preceded the electrical test stimuli by fixed inter-stimulus intervals (ISI), ranging from 100 ms to 500 ms. At 250-350 ms ISIs, the bilateral SP2 suppression was significantly reduced to less than 10%, in comparison to an average suppression degree of 32.5% without conditioning stimuli. The subjects perceived the heat stimulus before the electrical stimulus for a majority of the 12 pairs of stimuli at these ISIs. No differences were found in the VAS ratings for the different conditions. For the contralateral SP1, larger suppression was seen for the 300 ms ISI compared with stimulation without conditioning heat stimuli. Onset and offset for the SP1 was, however, only detected in three subjects using a criteria of 20% suppression of the pre-stimulus activity. A pre-pulse inhibitory effect onto inter-neurons in the SP2 pathways or habituation of the same inter-neurons by the heat stimuli are suggested as possible explanations for the interaction between the non-nociceptive and nociceptive input in the present study.

Convergence of nociceptive and non-nociceptive input onto the medullary dorsal horn in man

Referred pain arising in orofacial pain states is probably due to convergence of different somatosensory input onto the medullary dorsal horn (MDH). To examine convergence between nociceptive and non-nociceptive input onto the MDH, the blink reflex (BR) was applied. R1- and R2-components can be evoked by innocuous stimuli, but only the R2 is elicited by painful heat. The BR was elicited by innocuous electrical stimuli applied to the supraorbital nerve. A conditioning painful heat pulse which did not evoke any BR was homotopically applied to the left forehead preceding the electrical stimulus by 75 ms. While R1 remained unchanged, the R2 was facilitated by about 30%. This study demonstrates a convergence of low-threshold mechanoreceptive and nociceptive inputs onto interneurons of the MDH in man.

Exteroceptive suppression of temporalis muscle activity in patients with fibromyalgia, tension-type headache, and normal controls

Changes of the second suppressive period (ES2) of the exteroceptive suppression of the temporalis muscle activity are found in patients with chronic tension-type headache (TTH) and are suggested to reflect an abnormal endogenous pain control system. We investigated whether similar changes are found in patients with the fibromyalgia syndrome (FMS) that is also believed to result from disturbed central pain processing. The ES2 values of 27 patients with FMS were compared with those of 18 patients with TTH and 40 healthy volunteers. The duration of ES2 (+/-SD) in FMS patients was 30.6+/-7.5 ms and was not significantly different from the control group (33.1+/-7.8 ms), whereas it was significantly shortened in TTH patients (22.9+/-11.5 ms). Our results indicate that, despite similar concepts on the pathophysiology of the two chronic pain disorders, there are no comparable changes of this brain stem reflex activity in FMS.

Characterization of blink reflex interneurons by activation of diffuse noxious inhibitory controls in man

The blink reflex consists of an early, pontine R1-component and a late, medullary R2-component. R1 and R2 can be evoked by innocuous stimuli, but only the R2 also by painful heat, suggesting that the R2 is mediated by wide dynamic range neurons (WDR) of the spinal trigeminal nucleus. Remote noxious stimuli suppress the activity in WDR neurons via activation of diffuse noxious inhibitory controls (DNIC), whereas low-threshold mechanoreceptive neurons (LTM) are unaffected. In order to characterize the trigeminal interneurons of R1 and R2 we investigated the modulation of the blink reflex by remote painful heat. The blink reflex was elicited in 11 healthy subjects by innocuous electrical pulses applied to the left supraorbital nerve. The remote, painful heat stimuli were applied by a Peltier type thermode to the left volar forearm. Remote painful heat of 44 to 46 degreesC significantly suppressed the R2 by 15% (p<0.01), while the R1 remained unchanged. These results provide further evidence that the R2 is mediated by medullary WDR neurons and the R1 by pontine LTM neurons.