Modulation of an inhibitory reflex in single motor units in human masseter by tonic painful stimulation

Perioral electrical stimuli cause inhibitory reflex responses in single motor-units (SMU) and surface electromyographic (EMG) recordings from voluntary contracted human jaw-closing muscles. Tonic experimental masseter pain has recently been shown to reduce the inhibitory reflex response in surface EMG recordings but the effect on SMU activity has not been described. In this study, motor-unit action potentials were recorded with wire electrodes inserted into the left masseter in eleven subjects. The subjects kept the SMU firing rate around 10 Hz by feedback. Ninety-nine electrical stimuli were applied sequentially to the left mental nerve with increasing stimulus delays in steps of 1 ms after the preceding motor unit action potential. The inhibitory reflex in SMU was recorded before, during and after infusion of hypertonic saline (5%) into the ipsilateral masseter muscle. Spike train data were used to calculate (1) the mean pre- and post-stimulus inter-spike-intervals (ISI) in all of the 99 trials, (2) cumulative changes in firing probability, and (3) estimation of the compound inhibitory post-synaptic potential (IPSP) in the masseter motoneuron. Tonic masseter pain did not change pre-stimulus SMU firing characteristics but the mean ISI for the first post-stimulus discharge (158.2+/-9.2 ms) was significantly decreased compared to the pre-pain (175.8+/-11.3 ms, P<0.05) and post-pain conditions (172. 6+/-11.6 ms, P<0.05). The post-stimulus firing probability was significantly increased and the relative amplitude of the estimated IPSP significantly decreased during tonic masseter pain compared to pre-pain and post-pain conditions. In conclusion, this study indicates that tonic masseter pain has a net excitatory effect on the inhibitory jaw-reflexes, which could be mediated by presynaptic mechanisms on the involved motoneurons.     

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.     

Effect of muscle relaxants on experimental jaw-muscle pain and jaw-stretch reflexes: a double-blind and placebo-controlled trial.

A randomised, double-blind, placebo-controlled three-way cross-over study was performed to investigate the effect of two muscle relaxants (tolperisone hydrochloride and pridinol mesilate) on experimental jaw-muscle pain and jaw-stretch reflexes. Fifteen healthy men participated in three randomised sessions separated by at least 1 week. In each session 300 mg tolperisone, 8 mg pridinol mesilate or placebo was administered orally as a single dose. One hour after drug administration 0.3 ml hypertonic saline (5.8%) was injected into the right masseter to produce muscle pain. Subjects continuously rated their perceived pain intensity on an electronic 10-cm visual analogue scale (VAS). The pressure pain threshold (PPT) was measured and short-latency reflex responses were evoked in the pre-contracted (15% maximal voluntary contraction) masseter and temporalis muscles by a standardised stretch device (1 mm displacement, 10 ms ramp time) before (baseline), 1 h after medication (post-drug), during ongoing experimental muscle pain (pain-post-drug), and 15 min after pain had vanished (post-pain). Analysis of variance demonstrated significantly lower VAS peak pain scores (5.9 +/- 0.4 cm) after administration of tolperisone hydrochloride compared with pridinol mesilate (6.8 +/- 0.4 cm) and placebo (6.6 +/- 0.4 cm) (P=0.020). Administration of pridinol mesilate was associated with a significant decrease in PPTs compared with tolperisone hydrochloride and placebo (P=0.002) after medication, but not after experimental jaw-muscle pain. The normalised peak-to-peak amplitude of the stretch reflexes were not significantly influenced by the test medication (P=0.762), but were in all sessions significantly facilitated during ongoing experimental jaw-muscle pain (P=0.034). In conclusion, tolperisone hydrochloride provides a small, albeit significant reduction in the perceived intensity of experimental jaw-muscle pain whereas the present dose had no effect on the short-latency jaw-stretch reflex.     

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.     

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.     

Stretch reflex and pressure pain thresholds in chronic tension-type headache patients and healthy controls

To compare the jaw-stretch reflex and pressure pain thresholds (PPT) in chronic tension-type headache (CTTH) patients and healthy controls, 30 patients (15 male and 15 female) and 30 age- and sex-matched healthy subjects were investigated. Stretch reflexes were recorded in the temporalis and masseter muscles and PPT was determined in the anterior temporalis, splenius capitis and masseter muscles. The results showed that the amplitude of the stretch reflex in CTTH patients was higher compared with control subjects ( P  < 0.045), and higher in women compared with men in the right and left anterior temporalis muscles ( P  < 0.009). There were no differences in the PPT value between CTTH and control subjects ( P  > 0.509), whereas women showed significantly lower PPT measurements ( P  < 0.046). The results demonstrated a facilitation of the stretch reflex pathways in CTTH patients that is unrelated to measures of pericranial sensitivity.     

Induction of non-painful and painful intestinal sensations by hypertonic saline: a new human experimental model.

BACKGROUND AND AIMS: To develop a pain model for chemical stimulation of the human gut. METHODS: In a double-blind experimental study 10 subjects with a previously performed sigmoidostomy were randomised to receive injections with either isotonic or hypertonic saline in the colonic mucosa. In the hypertonic experimental arm, 0.1 ml of 0.9%, 2%, 4%, and 6% and 0.2 ml of 2% and 4% saline were given. In the placebo arm, six 0.9% saline injections of the same quantities were given. In a separate experiment 0.8 ml 4% saline was infused into the mucosa by a pump over a period of 2min. The pain intensity was rated on a 0-10 visual analogue scale with 5 as the pain threshold. RESULTS: The hypertonic saline injections resulted in local as well as referred non-painful and painful sensations in 9 out of the 10 subjects. The evoked sensations were mostly described as a smarting sensation with an intensity of median 1 (range 0-5.6) for 0.1 ml 2% saline to median 2.9 (range 0-6.2) for 0.2 ml 4% saline. Seven subjects reported referred sensations to the abdominal skin. Continuous infusion of 4% saline resulted in a consistent sensory response in all subjects, with a median intensity of 4.1 (range 2.1-8.1). This sensory intensity was reproducible in 70% in a retest experiment after median 7 weeks. In the placebo arm a total of 70 isotonic saline injections only resulted in inconsistent, short-lasting non-painful sensations in three subjects. CONCLUSION: The model represents a safe method for direct chemical activation of the sensory endings in the human gut. The model may be used for pharmacological screening of analgesics and for basic investigations in patients suffering from gastrointestinal diseases.     

Affective associative learning modifies the sensory perception of nociceptive stimuli without participant's awareness

The present experiment examined the possibility to change the sensory and/or the affective perception of thermal stimuli by an emotional associative learning procedure known to operate without participants' awareness (evaluative conditioning). In a mixed design, an aversive conditioning procedure was compared between subjects to an appetitive conditioning procedure. Both groups were also compared within-subject to a control condition (neutral conditioning). The aversive conditioning was induced by associating non-painful and painful thermal stimuli - delivered on the right forearm - with unpleasant slides. The appetitive conditioning consisted in an association between thermal stimuli - also delivered on the right forearm - and pleasant slides. The control condition consisted in an association between thermal stimuli - delivered for all participants on the left forearm - and neutral slides. The effects of the conditioning procedures on the sensory and affective dimensions were evaluated with visual analogue scale (VAS)-intensity and VAS-unpleasantness. Startle reflex was used as a physiological index of emotional valence disposition. Results confirmed that no participants were aware of the conditioning procedure. After unpleasant slides (aversive conditioning), non-painful and painful thermal stimuli were judged more intense and more unpleasant than when preceded by neutral slides (control condition) or pleasant slides (appetitive conditioning). Despite a strong correlation between the intensity and the unpleasantness scales, effects were weaker for the affective scale and, became statistically non-significant when VAS-intensity was used as covariate. This experiment shows that it is possible to modify the perception of intensity of thermal stimuli by a non-conscious learning procedure based on the transfer of the valence of the unconditioned stimuli (pleasant or unpleasant slides) towards the conditioned stimuli (non-painful and painful thermal stimuli). These results plead for a conception of pain as a conscious output of complex informational processes all of which are not accessible to participants' awareness. Mechanisms by which affective input may influence sensory experience and clinical implications of the present study are discussed.     

The effect of delayed onset of muscle soreness on habitual trapezius activity

The aim of the study was to investigate the effect of acute trapezius pain, induced by delayed onset of muscle soreness (DOMS), on habitual trapezius activity. Long‐term (5 h) surface electromyographic (sEMG) activity was recorded bilaterally from the clavicular, descending, transverse, and ascending trapezius on two consecutive weekdays in eleven female subjects (mean age 22 years, range 20–24 years). Body and arm posture were recorded by inclinometers. Immediately after the first long‐term recording, the subjects performed eccentric depression exercise of the left shoulder to induce DOMS. From day 1 to day 2, pressure pain threshold (PPT) decreased and pain scores on visual analog scale (VAS) increased for the left upper trapezius (P <.004 for all comparisons). Habitual sEMG activity (median sEMG level, μV) of the clavicular and descending part of the exercised left trapezius increased from first to second long‐term recording during periods with seated posture (P <.05 for both comparisons). In contrast, trapezius sEMG activity remained unchanged for all other trapezius parts and postures. This study indicates that acute trapezius pain induces elevated habitual trapezius activity during periods with low biomechanical loading of the shoulder/neck muscles with the elevated sEMG activity being restricted to the painful part of the muscle. In contrast to the pain‐adaption model, the current study indicates a relation between acute muscle pain and elevated low‐level muscle activity; however, it remains unknown if development of chronic muscle pain can be preceded by an initial stage with elevated muscle activity.     

表面肌电在腰椎间盘突出症患者功能评定中的应用

目的探讨表面肌电检测技术在腰椎间盘突出症中的临床应用。方法腰椎间盘突出症患者44例,按照腰痛视觉模拟评分（VAS）分为轻度疼痛组（A组）和中重度疼痛组（B组）,分别采集两组患者在完成自身重量的腰部竖脊肌等长负荷测试和踝跖屈最大角度自身重量的腓肠肌等长负荷测试时健、患侧L₅～S₁水平竖脊肌和腓肠肌内侧头的表面肌电信号,分析平均肌电值(AEMG)和中位频率的斜率(MFs)。结果两组患者患侧腰部竖脊肌和腓肠肌内侧头AEMG较健侧均有降低(P＜0.05),患侧MFs绝对值较健侧升高(P＜0.05); 组间比较,B组患侧两组肌肉MFs绝对值均较A组增大 (P＜0.05);B组健/患侧肌肉AEMG比均大于A组(P＜0.05),但MFs比均未见显著性差异(P＞0.05)。结论表面肌电图可以作为一种无创检测腰椎间盘突出症腰部和下肢神经肌肉功能状态的工具;踝跖屈最大角度自身重量的腓肠肌等长负荷测试可以作为评价腰椎间盘突出症患者下肢神经肌肉功能状态的方法之一;腰椎间盘突出症患者腰部主观疼痛感程度不同时,双侧腰部竖脊肌和腓肠肌表面肌电信号的失衡程度也不同。     

Gender-specific adaptations of upper trapezius muscle activity to acute nociceptive stimulation

This study examined gender differences in the effect of experimental muscle pain on changes in the relative activation of regions of the upper trapezius muscle during a sustained contraction. Surface electromyographic (EMG) signals were recorded from multiple locations over the upper trapezius muscle with a 10 × 5 grid of electrodes from nine women and nine men during 90° shoulder abduction sustained for 60 s. Measurements were performed before and after the injection of 0.4 ml hypertonic (painful) and isotonic (control) saline into the cranial region of the upper trapezius muscle. The EMG root mean square (RMS) was computed for each location of the grid to form a map of the EMG amplitude distribution. The peak pain intensity following the injection of hypertonic saline was greater for women (numerical rating scale 0–10: women 6.0 ± 2.1, men 4.2 ± 0.9; P < 0.01). For both genders, upper trapezius RMS averaged across the grid decreased following the injection of hypertonic saline (P < 0.0001). Moreover, there was a relatively larger pain-induced decrease in RMS in the cranial region compared to the caudal region of the muscle for both genders. During the non-painful sustained contractions, the EMG RMS progressively increased more in the cranial than the caudal region, for both men and women, due to fatigue. This mechanism was maintained in men but not in women during the painful condition. The results demonstrate that muscle pain alters the normal adaptation of upper trapezius muscle activity to fatigue in women but not in men.     

The perception of pain in others suppresses somatosensory oscillations: a magnetoencephalography study

Accumulating evidence demonstrates that similar neural circuits are activated during the first-hand experience of pain and the observation of pain in others. However, most functional MRI studies did not detect signal change in the primary somatosensory cortex during pain empathy. To test if the perception of pain in others involves the primary somatosensory cortex, neuromagnetic oscillatory activity was recorded from the primary somatosensory cortex in 16 participants while they observed static pictures depicting body parts in painful and non-painful situations. The left median nerve was stimulated at the wrist, and the poststimulus rebounds of the approximately 10-Hz somatosensory cortical oscillations were quantified. Compared to the baseline condition, the level of the approximately 10-Hz oscillations was suppressed during both of the observational situations, indicating the activation of the primary somatosensory cortex. Importantly, watching painful compared to non-painful situations suppressed somatosensory oscillations to a significant stronger degree. In addition, the suppression caused by perceiving others in the painful relative to the non-painful situations correlated with the perspective taking subscale of the interpersonal reaction index. These results, consistent with the mirror-neuron system, demonstrate that the perception of pain in others modulates neural activity in primary somatosensory cortex and supports the idea that the perception of pain in others elicits subtle somatosensory activity that may be difficult to detect by fMRI techniques.     

Effect of local anesthesia on atypical odontalgia--a randomized controlled trial

The aim of the study was to evaluate the analgesic effect of lidocaine in a double-blind, controlled multi-center study on patients with atypical odontalgia (AO)--a possible orofacial neuropathic pain condition. Thirty-five consecutive AO patients (range 31-81 years) with a mean pain duration of 7.2 years (range 1-30 years) were recruited from four different orofacial pain clinics in Sweden. In a randomized cross-over design, 1.5 ml local anesthesia (20mg/ml lidocaine and 12.5 microg/ml adrenaline) or 1.5 ml saline (9 mg/ml NaCl solution) (placebo) was injected to block the painful area. The VAS pain scores showed an overall effect of time (ANOVA: P<0.001) and treatment (ANOVA: P=0.018) with a significant interaction between the factors (ANOVA: P<0.001). Overall, VAS pain relief was significantly greater at 15-120 min following the lidocaine injections compared to the placebo injections (Tukey: P<0.05). All patients demonstrated significant disturbances in somatosensory function on the painful side compared to the non-painful side as revealed by quantitative sensory tests, however, only one significant inverse correlation was found between percentage pain relief and the magnitude of brush-evoked allodynia (Spearman: P<0.01). In conclusion, AO patients experienced significant, but not complete, pain relief from administration of local anesthetics compared with placebo. The findings indicate that the spontaneous pain in AO patients only to some extent is dependent on peripheral afferent inputs and that sensitization of higher order neurons may be involved in the pathophysiology of AO.     

Associations between pain and neuromuscular activity in the human jaw and neck muscles

The aim of this study was to test the effects of glutamate-evoked jaw or neck muscle pain on electromyographic (EMG) activity of jaw and neck muscles in humans. EMG recordings were made from left (MAL) and right (MAR) masseter muscles, and right sternocleidomastoid (SCM) and splenius (SP) muscles in three different head positions (head rest, head back, head right) or during maximal jaw clenching in 19 men. Glutamate (1 M) or isotonic saline was injected into MAR or SP, and induced pain was recorded on visual analogue scales. EMG activity in MAL and MAR was increased in the head back position compared to head rest and head right positions, whereas EMG activity in SCM and SP was progressively increased as the head was moved from rest position to head back to head right positions. Glutamate-evoked MAR pain was associated with increases in EMG activity in MAR, SCM and SP at rest but not in the head back or head right positions. Glutamate-evoked SP pain was associated with an increase in SP EMG activity at rest and a decrease in SCM EMG activity in the head right position. Decreases in jaw clench-related EMG activity were observed in MAL, MAR and SCM muscles only during glutamate-evoked MAR pain. Isotonic saline injections induced no pain or EMG changes. In conclusion, experimental neck pain is not associated with tonic increases in jaw EMG activity although jaw muscle pain can be linked to increases in neck EMG activity with the head and jaw at rest.     

Activity of masticatory muscles in subjects with different orofacial pain conditions

The existence of a pathophysiological link between tonic muscle activity and chronic muscle pain is still being debated. The purpose of this retrospective, controlled study was to evaluate the electromyographic (EMG) activity of masticatory muscles in subjects with different orofacial pain conditions. The temporal and masseter EMG activity at rest and the masseteric reflex were recorded in two groups of patients with either myofascial pain (n=33) or neuropathic pain (n=20), one group of non-pain patients with disc derangement disorders (n=27) and one control group of healthy, asymptomatic subjects (n=32). The EMG activities of both muscles at rest were significantly higher in the pain patient groups compared to the asymptomatic control group. There was no significant difference between the disc derangement disorder group and the control group. The masseteric reflex amplitude was reduced in all patient groups when compared with the control group. In pain patient groups, the increased EMG activity at rest and the reduction of the masseteric reflex amplitude were equally distributed in the pain and non-pain sides. In addition, subjects presenting with bilateral pain showed higher EMG activity at rest than those with unilateral pain. These results suggested that the modulation of muscle activity was not the direct consequence of a peripheral nociceptive mechanism and seemed to indicate that a central mechanism was at work. The contrast between the increased EMG activity at rest and the reduction of the masseteric reflex amplitude may reflect modulations of motoneurones that differed in tonic versus phasic conditions in chronic pain patients.     

Interaction between cutaneous and muscle afferent activity in polysynaptic reflex pathways: a human experimental study

Interactions between the input from cutaneous and nociceptive muscle afferents in polysynaptic reflex pathways were investigated in man. Interaction was tested by evoking reflexes before, during, and after a period of muscle pain induced by intramuscular injection of hypertonic saline. Muscle pain was induced either in the ankle flexor (tibialis anterior, TA) or in the extensor (soleus, SOL) muscles by injection of 1 ml hypertonic saline. Electrical skin stimulation (1.1 x initial reflex threshold) at the dorsum of the foot over the tarsal joint was used to elicit cutaneo-muscular polysynaptic reflexes in the knee flexor semitendinosus (ST). The injected hypertonic saline evoked a robust muscle pain (the subjects made a continuous score of the muscle pain on a 0-10 cm VAS scale, and the mean VAS area was 1229+/-251 cm x s and lasting 390+/-30 s). In five of 12 subjects, the infusion of hypertonic saline into TA evoked referred pain to the dorsal aspect of the ankle. A significant inhibition (17+/-8.2%, P<0.05) of the ST-reflex by pain in SOL was observed. Pain in TA facilitated (92+/-36%, P<0.05) the short-latency part (50-70 ms post stimulation) of the reflex. The muscle pain did not modulate the perceived sensory intensity of the electrical stimuli. The findings indicate an interaction of input from thin muscle afferents and cutaneous group A-fibre afferents in polysynaptic segmental reflex pathways, which seems to depend on the location of the muscle pain.     

Should Exercises be Painful or not? Effects on Clinical and Experimental Pain in Individuals with Shoulder Pain

Exercise can reduce pain, however the effect of painful versus non-painful exercises is uncertain. The primary aim of this randomized crossover study was to compare the effect of painful versus nonpainful isometric shoulder exercises on pain intensity after exercise in individuals with rotator cuff-related shoulder pain. Secondary exploratory aims were to describe the effects on pressure pain thresholds (PPTs), conditioned pain modulation (CPM) and muscle strength. On separate days, 35 individuals performed painful isometric shoulder exercises (external rotation; 20% above pain threshold), nonpainful isometric shoulder exercises (external rotation; 20% below pain threshold), and a rest condition, in randomised order. Shoulder pain intensity, PPTs, CPM, and external rotation strength were assessed before, immediately after and 45 minutes after conditions. No significant differences were observed between painful and nonpainful exercises. Visual analogue scale scores increased immediately after both painful and non-painful exercises compared with rest (P = .047, partial ƞ² = .07), but were similar to preexercise levels after 45 minutes. No changes in PPTs, CPM, or muscle strength after exercises compared with rest were observed. Painful and non-painful isometric exercises caused a moderate but short-lasting increase in shoulder pain in individuals with RCRSP. Isometric exercises had no effect on pain sensitivity and shoulder muscle strength or CPM.PerspectiveThis study evaluated for the first time in individuals with rotator cuff-related shoulder pain the effects of painful versus non-painful isometric exercises on different pain-related outcome measures. Both painful and non-painful isometric exercises caused a moderate but relatively short-lasting increase in shoulder pain in individuals with rotator cuff-related shoulder pain.Trial registration number: (ClinicalTrials.gov) NCT03675399     

Inhibitory effects do not depend on the subjective experience of pain during heterotopic noxious conditioning stimulation (HNCS): a contribution to the psychophysics of pain inhibition.

Heterotopic noxious conditioning stimulation (HNCS) has been thought to give access to the diffuse noxious inhibitory controls (DNIC) in man, which can be activated in wide-dynamic-range neurons by noxious stimulation from remote areas of the body and form the neurophysiological basis of the phenomenon 'pain inhibits pain'. The latter phenomenon suggests that the subjective experience of pain is a prerequisite for an inhibitory action. The necessity of using painful stimuli as conditioning and as test stimuli to produce inhibitory effects was investigated in the present study, using a HNCS paradigm. Twenty young men received conditioning stimuli created by tonic heat at painful and non-painful levels, using either hot water (hand) or thermode (forearm). The test stimuli were phasic heat stimuli (thermode) at painful and non-painful levels applied to the cheek. Only painful but not non-painful heat as conditioning stimulus increased the heat pain threshold and decreased the ability to discriminate between painful heat of different intensities. These two findings are in accord with an inhibitory effect depending on a painful conditioning stimulus. However, the intensity ratings of the test stimuli indicated inhibitory effects of the conditioning stimuli also upon non-painful levels. Furthermore, non-painful heat as conditioning stimulus also appeared to be capable of decreasing the ratings of the test stimuli at painful levels. The latter two findings suggest: (i) that very strong but subjectively still non-painful stimulation can trigger pain inhibitory effects and (ii) that also subjectively non-painful stimuli are affected by inhibitory influences during HNCS.     

Stress-related electromyographic responses in patients with chronic temporomandibular pain.

Surface electromyographic (EMG) recordings from the right and left masseter and the left biceps muscle during stress and non-stress imagery were obtained from patients with temporomandibular myofascial pain and dysfunction syndrome (MPDS), temporomandibular joint disorder (TMJD), chronic low back pain (CBP) and healthy controls (HC). Both the MPDS and the TMJD groups displayed significantly more masseter EMG reactivity to the stressful imagery than the CBP and HC groups. The 2 dental groups did not differ significantly from each other. The MPDS patients indicated more life stress and gave higher aversiveness ratings during the experiment. These findings are discussed with respect to the validity of the TMJD and MPDS distinction.     

Somatosensory event-related potentials to painful and non-painful stimuli: effects of attention

In order to determine the effects of attention and distraction on painful and non-painful stimuli, the amplitude changes of 3 components (N150, P200, P300) of the somatosensory event-related potential (SERP) elicited by painful and non-painful electrical stimuli were investigated. Painful and non-painful stimuli were determined using a visual analog scale. SERPs were recorded from 16 healthy volunteers at 5 midline and 4 left and 4 right hemispheric sites. The differences between the amplitudes of attended and ignored stimuli were quantified with a baseline-to-peak measure. ANOVA results revealed no significant attention or stimulus intensity effects for N150 but highly significant differences in P200 and P300 amplitudes between attended and ignored stimuli. In addition, P200 and P300 amplitudes were larger for strong stimuli than for weak stimuli, with no significant differences between non-painful and painful stimuli. These findings are consistent with the existence of a relative, rather than an absolute, relationship between SERP component amplitudes and subjective pain reports. Furthermore, the data give evidence that attentional manipulations represent a powerful method to decrease the perception of pain and that, when used with subjective and behavioral measures, the SERP represents a valuable asset in the multidimensional approach to pain measurement and assessment.