Changes in Ib heteronymous inhibition to soleus motoneurones during cutaneous and muscle nociceptive stimulation in humans

Is Thumb Proprioception Decreased in Patients With Trapeziometacarpal Joint Osteoarthritis?

Proprioception is thought to be essential for normal joint homeostasis, and its decreased function has been associated with an increased risk of joint diseases. However, only a few studies have been performed on the association between proprioceptive function in the trapeziometacarpal joint (TMCJ) and osteoarthritis. The purpose of this study was to compare TMCJ proprioceptive function in elderly women with radiographic TMCJ osteoarthritis relative to age-matched control women without osteoarthritis. We enrolled 19 women (mean age, 66 years) with symptomatic, radiographic Eaton and Littler grade 2, 3, and 4 TMCJ osteoarthritis and 19 age-matched control women without osteoarthritis. We evaluated thumb proprioception by using a joint-position reproduction test and compared the reposition error (RE) between the groups. We carried out a multivariate analysis for factors potentially associated with increased RE, such as age, body mass index, hand dominance, the presence of diabetes, pain level, and the presence of osteoarthritis. Also, a logistic regression analysis was performed for factors associated with the occurrence of TMCJ osteoarthritis. Patients with TMCJ osteoarthritis had greater RE than did the control patients in the joint-position reproduction test at 20°, 30°, and 40° of thumb palmar abduction. The multivariate analysis indicated that increased RE was associated with the presence of osteoarthritis, but not with the other factors assessed. The occurrence of TMCJ osteoarthritis was associated with increased RE at 20°, 30°, and 40° of thumb palmar abduction. This study showed that decreased proprioceptive function was associated with the presence of osteoarthritis in the TMCJ, although the causality remains unknown. Further studies on the role of proprioception in the pathogenesis of TMCJ osteoarthritis and the potential role of its training for disease prevention or treatment are required.

Promising Effect of Visually-Assisted Motor Imagery Against Arthrogenic Muscle Inhibition - A Human Experimental Pain Study

Purpose

Clinically, arthrogenic muscle inhibition (AMI) has a negative impact on functional recovery in musculoskeletal disorders. One possible technique to relieve AMI is motor imagery, which is widely used in neurological rehabilitation to enhance motor neuron excitability. The purpose of this study was to verify the efficacy of visually-assisted motor imagery against AMI using a human experimental pain model.

Methods

Ten healthy volunteers were included. Experimental ankle pain was induced by hypertonic saline infusion into unilateral Kager’s fat pad. Isotonic saline was used as control. Subjects were instructed to imagine while watching a movie in which repetitive motion of their own ankle or fingers was shown. H-reflex normalized by the motor response (H/M ratio) on soleus muscle, maximal voluntary contraction (MVC) force of ankle flexion, and contractile activities of the calf muscles during MVC were recorded at baseline, pre-intervention, post-intervention, and 10 minutes after the pain had subsided.

Results

Hypertonic saline produced continuous and constant peri-ankle pain (VAS peak [median]= 6.7 [2.1–8.4] cm) compared to isotonic saline (0 [0–0.8] cm). In response to pain, there were significant decreases in the H/M ratio, MVC and contractile activities (P<0.01), all of which were successfully reversed after the ankle motion imagery. In contrast, no significant changes were observed with the finger motion imagery.

Conclusion

Visually-assisted motor imagery improved the pain-induced AMI. Motor imagery of the painful joint itself would efficiently work for relieving AMI. This investigation possibly shows the potential of a novel and versatile approach against AMI for patients with musculoskeletal pain.

The effects of 6 weeks of constant-angle muscle stretching training on flexibility and muscle function in men with limited hamstrings' flexibility

PURPOSE

The aim of the present study was to evaluate the effects of 6 weeks of a constant-angle hamstring muscle flexibility training on muscle-tendon stiffness and the range of motion (ROM) in young men with limited hamstring ROM.

METHODS

13 participants performed unilateral stretching training (EL), while the contralateral limb acted as control (CL). ROM, relative and peak passive torque, passive stiffness, dynamic knee flexion strength, and active optimum joint angle were assessed before and after the last training session. In addition, participants were tested during the first and last training sessions for first stretch sensation during the stretching procedure only in the EL.

RESULTS

Straight-leg raise and isokinetic knee ROM tests (both p < 0.0001; from 59.4 ± 8.1 to 70.3 ± 9.8, from 28.3 ± 7.6 to 18.5 ± 5.2, respectively) and peak passive torque (p = 0.001; from 53.1 ± 11.7 to 64.9 ± 12.3) increased only in EL and no changes in relative passive torque, passive stiffness, dynamic knee flexion strength, and active optimum joint angle (p > 0.05) were observed. At the point of first stretch sensation, significant increases in passive torque (p = 0.004) and angle (p < 0.001) were found from pre- to post-training.

CONCLUSION

The flexibility training induced significant increases in ROM alongside increases in peak passive torque (stretch tolerance) and the ROM at which stretch was first perceived. However, this occurred without changes in muscle-tendon mechanical properties or transfer to the untrained limb (CL). These results suggest that limb-specific ROM increases were underpinned by neural adaptations.

Effect of experimental muscle pain on the acquisition and retention of locomotor adaptation: different motor strategies for a similar performance

As individuals with musculoskeletal disorders often experience motor impairments, contemporary rehabilitation relies heavily on the use of motor learning principles. However, motor impairments are often associated with pain. Although there is substantial evidence that muscle pain interferes with motor control, much less is known on its impact on motor learning. The objective of the present study was to assess the effects of muscle pain on locomotor learning. Two groups (Pain and Control) of healthy participants performed a locomotor adaptation task (robotized ankle-foot orthosis perturbing ankle movements during swing) on two consecutive days. On day 1 (acquisition), hypertonic saline was injected in the tibialis anterior (TA) muscle of the Pain group participants, while Control group participants were pain free. All participants were pain free on day 2 (retention). Changes in movement errors caused by the perturbation were assessed as an indicator of motor performance. Detailed analysis of kinematic and electromyographic data provided information about motor strategies. No between-group differences were observed on motor performance measured during the acquisition and retention phases. However, Pain group participants had a residual movement error later in the swing phase and smaller early TA activation than Control group participants, thereby suggesting a reduction in the use of anticipatory motor strategies to overcome the perturbation. Muscle pain did not interfere with global motor performance during locomotor adaptation. The different motor strategies used in the presence of muscle pain may reflect a diminished ability to anticipate the consequences of a perturbation. NEW & NOTEWORTHY This study shows that experimental muscle pain does not influence global motor performance during the acquisition or next-day retention phases of locomotor learning. This contrasts with previous results obtained with cutaneous pain, emphasizing the risk of directly extrapolating from one pain modality to another. Muscle pain affected motor strategies used when performing the task, however: it reduced the ability to use increased feedforward control to overcome the force field.

Ranking brain areas encoding the perceived level of pain from fMRI data

Pain perception is thought to emerge from the integrated activity of a distributed brain system, but the relative contribution of the different network nodes is still incompletely understood. In the present functional magnetic resonance imaging (fMRI) study, we aimed to identify the more relevant brain regions to explain the time profile of the perceived pain intensity in healthy volunteers, during noxious chemical stimulation (ascorbic acid injection) of the left hand. To this end, we performed multi-way partial least squares regression of fMRI data from twenty-two a-priori defined brain regions of interest (ROI) in each hemisphere, to build a model that could efficiently reproduce the psychophysical pain profiles in the same individuals; moreover, we applied a novel three-way extension of the variable importance in projection (VIP) method to summarize each ROI contribution to the model. Brain regions showing the highest VIP scores included the bilateral mid-cingulate, anterior and posterior insular, and parietal operculum cortices, the contralateral paracentral lobule, bilateral putamen and ipsilateral medial thalamus. Most of these regions, with the exception of medial thalamus, were also identified by a statistical analysis on mean ROI beta values estimated using the time course of the psychophysical rating as a regressor at the voxel level. Our results provide the first rank-ordering of brain regions involved in coding the perceived level of pain. These findings in a model of acute prolonged pain confirm and extend previous data, suggesting that a bilateral array of cortical areas and subcortical structures is involved in pain perception.

Effects of experimentally induced pain of the plantar soles on centre of foot pressure displacements during unperturbed upright stance

BACKGROUND

Although impaired balance control during quiet standing has repeatedly been reported in persons suffering from foot pain, a better understanding of the effect of foot pain on unperturbed postural control is needed in order to propose and implement efficient podiatry treatments/interventions into clinical practice. The present study was hence designed to address this issue.

METHODS

Ten young healthy adults were asked to stand upright, eyes closed, as still as possible in three experimental conditions: (1) a no-pain condition, (2) a condition when a painful stimulation was applied to the plantar surfaces of both feet, and (3) a condition in which painful stimulation was applied to another body part, the palms of both hands. The centre of foot pressure displacements was recorded using a force platform.

FINDINGS

For the same perceived intensity of the pain, the severe painful stimulation applied to the plantar surfaces of both feet increased centre of foot pressure displacements, whereas the severe painful stimulation applied to the palms of both hands did not.

INTERPRETATION

These results reveal the deleterious effect of experimentally induced pain on the plantar soles on unperturbed bipedal postural control. At this point, it is conceivable that these effects of experimental pain could generalise to the effects of pain in patients. Accordingly, the present findings suggest that clinical and/or instrumental interventions designed to mitigate pain in patients suffering from plantar foot pain (e.g., podiatry treatments/interventions) could improve postural control.

Pain induced by injection of hypertonic saline into the infrapatellar fat pad and effect on coordination of the quadriceps muscles

OBJECTIVE

Musculoskeletal conditions of the knee involve changes in sensorimotor function, but it is unclear whether these changes are a cause or result of pain. Induction of experimental pain may help solve this issue. Although this is commonly achieved by injection of hypertonic saline into muscle, muscle is commonly not the source of pain. This study investigated whether pain induced by injection of saline into the infrapatellar fat pad changes motor control of the quadriceps muscles of the knee.

METHODS

Ten participants performed a standardized task involving ascending and descending a series of steps. Electromyographic activity (EMG) of vastus medialis obliquus (VMO) and vastus lateralis (VL) was recorded with surface electrodes. Trials were conducted without pain, with anterior knee pain induced by injection (0.25 ml) of hypertonic saline (5%) into the infrapatellar fat pad, with anticipation of pain associated with unpredictable electrical shocks to the knee, and 20 minutes after pain cessation. EMG onset and amplitude were analyzed.

RESULTS

When participants ascended the steps with pain, the onset of VMO EMG was delayed relative to that of VL, in contrast to simultaneous or earlier activation of VMO EMG in the pre- and postpain trials. VL EMG amplitude was decreased significantly from the control condition.

CONCLUSION

These data show that alterations in coordination of knee muscle activity can be caused by pain, even when it is of nonmuscle origin. Treatment of pain is therefore important to facilitate performance of the quadriceps muscles, which are essential for locomotor and functional tasks as well as for knee stability.

The effects of experimental muscle and skin pain on the static stretch sensitivity of human muscle spindles in relaxed leg muscles

Animal studies have shown that noxious inputs onto gamma-motoneurons can cause an increase in the activity of muscle spindles, and it has been proposed that this causes a fusimotor-driven increase in muscle stiffness that is believed to underlie many chronic pain syndromes. To test whether experimental pain also acts on the fusimotor system in humans, unitary recordings were made from 19 spindle afferents (12 Ia, 7 II) located in the ankle and toe extensors or peronei muscles of awake human subjects. Muscle pain was induced by bolus intramuscular injection of 0.5 ml 5% hypertonic saline into tibialis anterior (TA); skin pain was induced by 0.2 ml injection into the overlying skin. Changes in fusimotor drive to the muscle spindles were inferred from changes in the mean discharge frequency and discharge variability of spindle endings in relaxed muscle. During muscle pain no afferents increased their discharge activity: seven afferents (5 Ia, 2 II) showed a decrease and six (4 Ia, 2 II) afferents were not affected. During skin pain of 13 afferents discharge rate increased in one (Ia) and decreased in two (1 Ia, 1 II). On average, the overall discharge rate decreased during muscle pain by 6.1% (P < 0.05; Wilcoxon), but remained essentially the same during skin pain. There was no detectable correlation between subjective pain level and the small change in discharge rate of muscle spindles. Irrespective of the type of pain, discharge variability parameters were not influenced (P > 0.05; Wilcoxon). We conclude that, contrary to the 'vicious cycle' hypothesis, acute activation of muscle or skin nociceptors does not cause a reflex increase in fusimotor drive in humans. Rather, our results are more aligned with the pain adaptation model, based on clinical studies predicting pain-induced reductions of agonist muscle activity.

Induction of muscle cramps by nociceptive stimulation of latent myofascial trigger points

The aim of this present study is to test the hypothesis that nociceptive stimulation of latent myofascial trigger points (MTrPs) increases the occurrence of local muscle cramps. Nociceptive muscle stimulation was obtained by a bolus injection of glutamate (0.1 ml, 0.5 M) into a latent MTrP and a control point (a non-MTrP) located in the right or left gastrocnemius medialis muscles in 14 healthy subjects. A bolus of isotonic saline (0.9%, 0.1 ml) injection served as a control. The injections were guided by intramuscular electromyography (EMG) showing resting spontaneous electrical activity at a latent MTrP and no such activity at a non-MTrP. Intramuscular and surface EMG activities in the gastrocnemius medialis muscle were recorded pre-, during-, and post-injection for a period of 8 min to monitor the occurrence of muscle cramps, which are characterized by a brief episodic burst of high levels of EMG activity. The results showed that glutamate and isotonic saline injections into the latent MTrPs induced higher peak pain intensity than into the non-MTrPs (both P < 0.05). Glutamate injection induced higher peak pain intensity than isotonic saline injection into either latent MTrPs or non-MTrPs (both P < 0.05). Muscle camps were observed in 92.86% of the subjects following glutamate injection into the latent MTrPs, but not into the non-MTrPs (P < 0.001). No muscle cramps were recorded following isotonic saline injection into either the latent MTrPs or the non-MTrPs. These results suggest that latent MTrPs could be involved in the genesis of muscle cramps. Focal increase in nociceptive sensitivity at MTrPs constitutes one of the mechanisms underlying muscle cramps.

Experimental muscle pain decreases the frequency threshold of electrically elicited muscle cramps

This study in humans tested the hypothesis that nociceptive muscle afferent input facilitates the occurrence of muscle cramps. In 13 healthy adults, muscle cramps were experimentally induced in the foot by stimulating the tibialis posterior nerve at the ankle with 2-s bursts of stimuli separated by 30 s, with stimulation frequency increasing by 2-Hz increments from 10 Hz until the cramp appeared. The minimum stimulation frequency that induced the cramp was defined "cramp frequency threshold". In 2 days, elicitation of the cramp was performed in the two-feet with and without (baseline condition) injection of hypertonic (painful condition) or isotonic (control condition) saline into the deep midportion of the flexor hallucis brevis muscle, from where surface EMG signals were recorded. The cramp frequency threshold was lower for the painful condition with respect to its baseline (mean +/- SE, hypertonic saline: 25.7 +/- 2.1 Hz, corresponding baseline: 31.2 +/- 2.8 Hz; P < 0.01) while there was no difference between the threshold with isotonic injection with respect to baseline. EMG average rectified value and power spectral frequency were higher during the cramp than immediately before the stimulation that elicited the cramp (pre-cramp: 13.9 +/- 1.6 muV and 75.4 +/- 3.8 Hz, respectively; post-cramp: 19.9 +/- 3.2 muV and 101.6 +/- 6.0 Hz; P < 0.05). The results suggest that nociceptive muscle afferent activity induced by injection of hypertonic saline facilitates the generation of electrically elicited muscle cramps.

The effect of experimental muscle pain on the amplitude and velocity sensitivity of jaw closing muscle spindle afferents

The effect of experimental muscle pain on the amplitude and velocity sensitivity of muscle spindle primary afferent neurons in the trigeminal mesencephalic nucleus (Vmes) was examined. Extracellular recordings were made from 45 neurons designated as spindle primary- or secondary-like on the basis of their response to ramp-and-hold jaw movements. Velocity sensitivity was assessed in spindle primary-like afferents by calculating the mean dynamic index of each unit in response to three different velocities of jaw opening before and after intramuscular injection with hypertonic saline (HS, 5%, 100 microl). The amplitude sensitivity of all jaw muscle spindle afferents was assessed by calculating the mean firing rate of each unit in response to three different amplitudes of jaw openings during both the open and hold phases of the movement and with best-fit lines obtained, using linear regression analysis, before and after HS injection. The variance of the two regression lines obtained for each unit before and after the injection was compared using the coincidence test, and changes in intercept and slope were determined. Seventy-five percent of the primary-like units and 80% of the secondary-like units presented with changes in static behavior after HS injection. Thirty-six percent of the primary-like units showed changes in dynamic behavior. Injection of isotonic saline (control) did not alter the responses of the spindle afferent to jaw opening. Thus, our results demonstrate that the predominant effect of noxious stimulation was a shift in the amplitude sensitivity of both spindle primary-like and secondary-like afferents and, to a lesser extent, the velocity sensitivity of the spindle primary-like unit. In accordance with earlier studies in the cat hindlimb and neck muscles, these results suggest that the activation of masseter muscle nociceptor alters spindle afferent responses to stretch acting primarily through static gamma motor neurons.

Effects in feline gastrocnemius-soleus motoneurones induced by muscle fatigue

Responses of gastrocnemius-soleus (G-S) motoneurones to stretches of the homonymous muscles were recorded intracellularly in decerebrate cats before, during and after fatiguing stimulation (FST) of G-S muscles. Ventral roots (VR) L7 and S1 were cut, and FST was applied to VR S1, a single FST session including 4 to 5 repetitions of 12-s periods of regular 40 s(-1) stimulation. Muscle stretches consisted of several phases of slow sinusoidal shortening-lengthening cycles and intermediate constant lengths. The maximal stretch of the muscles was 8.8 mm above the rest length. Effects of FST on excitatory postsynaptic potentials (EPSPs) and spikes evoked by the muscle stretches were studied in 12 motoneurones from ten experiments. Stretch-evoked EPSPs and firing were predominantly suppressed after FST, with the exception of a post-contraction increase of the first EPSP after FST, which was most likely due to after-effects in the activity of muscle spindle afferents. The post-fatigue suppression of EPSPs and spike activity was followed by restoration within 60-100 s. Additional bouts of FST augmented the intensity of post-fatigue suppression of EPSPs, with the spike activity sometimes disappearing completely. FST itself elicited EPSPs at latencies suggesting activation of muscle spindle group Ia afferents via stimulation of beta-fibres. The suppression of the stretch-evoked responses most likely resulted from fatigue-evoked activity of group III and IV muscle afferents. Presynaptic inhibition could be one of the mechanisms involved, but homosynaptic depression in the FST-activated group Ia afferents may also have contributed.

Influences of chemically-induced muscle pain on power output of ballistic upper limb movements

OBJECTIVE

To analyse the conditioning effects of localised acute muscle pain on power output during ipsi- and contra-lateral ballistic arm extensions.

METHODS

Eight male subjects performed right arm (ipsilateral) and left arm (contra-lateral) bench press movements. The power output (and force and velocity) of the concentric phase of movement was measured before and during muscle pain induced by a standardised intramuscular injection of levo-ascorbic acid in the right pectoralis major muscle (prime mover muscle) and in the lateral head of the right triceps brachii muscle (synergist).

RESULTS

The power output of ipsi- and contra-lateral arm bench press movements was significantly decreased during pain of the right pectoralis major muscle, but not during pain of the right lateral triceps muscle. The velocity and force were both affected and contributed to the decrease in power output.

CONCLUSIONS

Acute muscle pain of a prime mover muscle reduces ipsi- and contra-lateral motor performance of ballistic upper limb extension. This is not a generalised or non-specific inhibitory effect on the motor system, since pain failed to modify motor performance when applied to a synergist, non-prime mover, muscle. A basically bilateral executive cortical plan for ballistic upper limb extension could explain the conditioning effect of muscle pain on both ipsi- and contra-lateral arm.

SIGNIFICANCE

The results presented here contributes to the understanding of the conditioning effects of muscle pain on movement performance.

Effects of intensity and locus of painful stimulation on postural stability

Stimulation of small diameter afferents can influence motor behavior. Little is known about how a prolonged painful stimulation of these small afferents may affect essential motor behavior such as the maintenance of an erect stance. The present study documents the effects of 10-s weak, moderate and extreme painful stimulations applied to the dorsum of the feet on the postural stability. Also, the moderate painful stimulation was applied to the metacarpal heads to determine if a painful stimulation to a limb not involved in the maintenance of the erect stance affects the postural control mechanisms. Increasing the intensity of the painful stimulation applied to the feet yielded larger postural oscillations whereas stimulation to the hands did not affect the control of posture. This suggests that the painful stimulation mainly affected the postural control mechanisms via sensorimotor processes rather than via cognitive resources related to the perception of pain.

Effect of experimental muscle pain on motor unit firing rate and conduction velocity

The aim of this human study was to investigate the relationship between experimentally induced muscle pain intensity (i.e., amount of nociceptive activity) and motor unit (MU) firing decrease and MU conduction velocity (CV). In 12 healthy subjects, nociceptive afferents were stimulated in the right tibialis anterior muscle by three intramuscular injections of hypertonic saline (0.2, 0.5, and 0.9 ml) separated by 140 s. The subjects performed six isometric contractions (20 s long) at 10% of the maximal voluntary contraction during the experimental muscle pain. The same set of six contractions was performed without any infusion before the painful condition on the right leg. The procedure was repeated for the left leg with infusion of isotonic (nonpainful) saline. Intramuscular and surface electromyographic (EMG) signals were collected to assess MU firing rate and CV. The firing rate of the active MUs [range: 7.4-14.8 pulses/s (pps)] did not change significantly in the three control conditions (without infusion for the right and left leg and with infusion of isotonic saline in the left leg). There was, on the contrary, a significant decrease (on average, mean +/- SE, 1.03 +/- 0.21 pps) of the firing rates during the painful condition. Moreover, MU firing rates were inversely significantly correlated with the subjective scores of pain intensity. Single MU CV was 3.88 +/- 0.03 m/s (mean +/- SE, over all the MUs) with no statistical difference among any condition, i.e., the injection of hypertonic saline did not alter the muscle fiber membrane properties of the observed MUs. Progressively increased muscle pain intensity causes a gradual decrease of MU firing rates. This decrease is not associated with a change in MU membrane properties, indirectly assessed by CV. This study demonstrates a central inhibitory motor control mechanism with an efficacy correlated to the nociceptive activity.

Postural stability is altered by the stimulation of pain but not warm receptors in humans

BACKGROUND

It is now recognized that large diameter myelinated afferents provide the primary source of lower limb proprioceptive information for maintaining an upright standing position. Small diameter afferents transmitting noxious stimuli, however, can also influence motor behaviors. Despite the possible influence of pain on motor behaviors, the effects of pain on the postural control system have not been well documented.

METHODS

Two cutaneous heat stimulations (experiment 1: non-noxious 40 degrees C; experiment 2: noxious 45 degrees C) were applied bilaterally on the calves of the subject with two thermal grills to stimulate A delta and C warm receptors and nociceptors in order to examine their effects on postural stability. The non-noxious stimulation induced a gentle sensation of warmth and the noxious stimulation induced a perception of heat pain (visual analogue scores of 0 and 46 mm, respectively). For both experiments, ten healthy young adults were tested with and without heat stimulations of the lower limbs while standing upright on a force platform with eyes open, eyes closed and eyes closed with tendon co-vibration of tibialis anterior and triceps surae muscles. The center of pressure displacements were analyzed to examine how both stimulations affected the regulation of quiet standing and if the effects were exacerbated when vision was removed or ankle proprioception perturbed.

RESULTS

The stimulation of the warm receptors (40 degrees C) did not induce any postural deterioration. With pain (45 degrees C), subjects showed a significant increase in standard deviation, range and mean velocity of postural oscillations as well as standard deviation of the center of pressure velocity. The effects of heat pain were exacerbated when subjects had both their eyes closed and ankle tendons vibrated (increased standard deviation of the center of pressure velocity and mean velocity of the center of pressure).

CONCLUSIONS

A non-noxious stimulation (40 degrees C) of the small diameter afferents is not a sufficiently intense sensory stimulation to alter the control of posture. A painful stimulation (45 degrees C) of the skin thermoreceptors, however, yielded a deterioration of the postural control system. The observed deteriorating effects of the combined stimulation of nociceptors and Ia afferents (when ankle tendons were vibrated) could result from the convergence of these afferents at the spinal level. This could certainly lead to the hypothesis that individuals suffering from lower limb pain present alterations of the postural control mechanisms; especially populations already at risk of falling (for example, frail elderly) or populations suffering from concomitant lower limb pain and sensory deficits (for example, diabetic polyneuropathy).

Early somatosensory processing during tonic muscle pain in humans: relation to loss of proprioception and motor 'defensive' strategies

OBJECTIVE

It is known that tonic muscle pain induced by a Levo-Ascorbic (L-AS) solution injected in a foot muscle can transiently modify both regional proprioception and stimulus perception. These findings are paralleled by changes of middle-latency lower-limb somatosensory evoked potentials (SEPs). However, little is known on the behaviourally relevant aspect whether eventual SEP pain-induced changes could be partly due to a sort of 'motor strategy' of subjects in the frame of a self-protective reaction towards the noxious stimulus. Movement and imagery of movements are in fact known to reduce mainly pre-central SEP amplitude (i.e. gating effect).

METHODS

Low-threshold afferents ulnar SEPs, psychophysical pain ratings and fingers' position sense were monitored in the time-course during L-AS injection in the right first dorsal interosseous muscle. Control experiments included SEPs (either following prevalent ulnar nerve low-threshold afferent stimulation or more conventional mixed nerve stimulation) during actual movements execution and imagery of movements of the right hand.

RESULTS

Tonic pain induced a significant reduction of the post-central N(20)-P(25)-N(33) complex and a significant increase of the N(18) wave. These changes, that were paralleled by distortion of the finger position sense, were delayed 2-5 min with respect to the maximal subjective pain sensation. Conversely, movement imagery tasks lead to a significant, selective, reduction of the pre-central N(30) complex. This wave was even more reduced during actual movements, in combination with a reduction of those post-central components peaking after the first activation of the primary sensory cortex.

CONCLUSIONS

Early sensory processing at cortical level is changed during tonic muscle pain, mainly for those components which may be theoretically involved in proprioceptive afferent elaboration. These changes are likely not due to subconscious or voluntary motor strategies of the subjects in the frame of a self-protective aversive reaction towards the noxious stimulus.

Suppression of motor evoked potentials in a hand muscle following prolonged painful stimulation

Earlier investigations have shown that stimulation of peripheral afferent nerves induces prolonged changes in the excitability of the human motor cortex. The present study compared the effect of experimental pain and non-painful conditioning stimulation on motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) in the relaxed first dorsal interosseous (FDI) and flexor carpi ulnaris (FCU) muscles. The MEPs were measured in 10 healthy subjects, and stimulus-response curves were generated before and after each of four stimulation paradigms conducted in random order on separate occasions: (a) control; (b) "dual stimulation" consisting of electrical stimulation of the FDI motor point paired with TMS; (c) painful infusion of hypertonic saline in the FDI muscle; and (d) pain combined with dual stimulation. There were no significant changes in FDI MEPs following the control paradigm, and dual stimulation induced an increase in the FDI MEPs only inconsistently. In contrast, the painful stimulation and the combined pain and dual stimulation paradigms were followed by significant suppression of the FDI MEPs at higher stimulus intensities. No changes were observed in the FCU MEPs following the four paradigms. In two additional subjects, the responses evoked in FDI by direct stimulation of the descending corticospinal tracts were significantly depressed following painful stimulation of the FDI, although the ulnar-evoked M-waves remained constant. It is concluded that muscle pain is followed by a period with profound depression of MEPs amplitudes in the resting muscle, but that these changes are at least in part due to a lasting depression of the excitability of the motoneurones in the spinal cord. Hence, painful stimulation differs from non-painful, repetitive stimulation, which facilitates the corticomotor pathway.

Pain-related modulation of the human motor cortex

Pain is a complex multi-dimensional phenomenon that influences a wide variety of nervous system functions, including sensory--discriminative, affective--motivational and cognitive--evaluative components. So far, these components have been studied in both patients with chronic pain and in normal subjects in whom pain was induced experimentally. The interaction between pain and motor function is not fully understood, although from everyday life it is known that pain affects movements. The effects of pain on motor control are typically seen as a limited or impaired ability to perform movements. Most studies have dealt with the effects of pain on the spinal cord reflexes, but in recent years, several lines of evidence suggest that the interaction between motor and pain systems in conditions of pain induced experimentally, rather than a simple spinal reflex, is a more complex process that involves also supraspinal brain areas. Although pain-motor interaction shows different features and time course depending on different pain variables, such as duration (tonic versus phasic pain), submodalities (deep versus superficial pain) and location (distal versus proximal pain), a common finding is that pain is able to inhibit the motor cortex. This motor cortex inhibition may act as a sort of motor 'decerebration' so as to allow the spinal motor system to freely develop protective responses to noxious stimulation. Further studies are required to assess the effects of pain on the motor system in patients suffering from chronic pain, in order to develop innovative rational therapeutic strategies to reduce both pain and motor disability.

Effect of chemically activated fine muscle afferents on spinal recurrent inhibition in humans

OBJECTIVE

To test the hypothesis that 'metabolites released during fatiguing muscle contractions excite group III-IV muscle nociceptive afferents, inhibiting homonymous motoneurones via Renshaw cells,' by recording changes in recurrent inhibition of soleus motoneurones when high-threshold, small-diameter afferents (group III-IV fibres) from the same muscle were tonically activated.

METHODS

Experiments were performed in 7 healthy subjects at rest and during weak isometric voluntary contraction of the soleus muscle. Muscle nociceptive afferents were activated by local standardized injection of levo-ascorbic acid. Renshaw cells were orthodromically activated by a conditioning H reflex and the resulting recurrent inhibition of the soleus motoneurones was assessed by a subsequent test H reflex. An additional H reflex of the same size as the test reflex was used to assess motoneurone excitability.

RESULTS

At rest, muscle nociceptive stimulation produced transient facilitation of both test H and reference H reflexes. Under weak voluntary contraction, muscle nociceptive stimulation produced long-lasting extra-inhibition and extra-facilitation of the test reflex and reference reflex respectively, the time course of which closely resembled that of the subjective muscle pain curve.

CONCLUSIONS

Discharge of putative group III-IV muscle afferents facilitated homonymous recurrent inhibition. The filtering property of recurrent inhibition may contribute to limit motoneurone activity during muscle pain and/or adapt motoneurone firing rate to the modified contractile properties of motor units as muscle fatigue developed.

Capsaicin-induced muscle pain alters the excitability of the human jaw-stretch reflex

The pathophysiology of painful temporomandibular disorders is not fully understood, but evidence suggests that muscle pain modulates motor function in characteristic ways. This study tested the hypothesis that activation of nociceptive muscle afferent fibers would be linked to an increased excitability of the human jaw-stretch reflex and whether this process would be sensitive to length and velocity of the stretch. Capsaicin (10 micro g) was injected into the masseter muscle to induce pain in 11 healthy volunteers. Short-latency reflex responses were evoked in the masseter and temporalis muscles by a stretch device with different velocities and displacements before, during, and after the pain. The normalized reflex amplitude increased with an increase in velocity at a given displacement, but remained constant with different displacements at a given velocity. The normalized reflex amplitude was significantly higher during pain, but only at faster stretches in the painful muscle. Increased sensitivity of the fusimotor system during acute muscle pain could be one likely mechanism to explain the findings.

Does anticipation of pain affect cortical nociceptive systems?

Anticipation of pain is a complex state that may influence the perception of subsequent noxious stimuli. We used functional magnetic resonance imaging (fMRI) to study changes of activity of cortical nociceptive networks in healthy volunteers while they expected the somatosensory stimulation of one foot, which might be painful (subcutaneous injection of ascorbic acid) or not. Subjects had no previous experience of the noxious stimulus. Mean fMRI signal intensity increased over baseline values during anticipation and during actual stimulation in the putative foot representation area of the contralateral primary somatosensory cortex (SI). Mean fMRI signals decreased during anticipation in other portions of the contralateral and ipsilateral SI, as well as in the anteroventral cingulate cortex. The activity of cortical clusters whose signal time courses showed positive or negative correlations with the individual psychophysical pain intensity curve was also significantly affected during the waiting period. Positively correlated clusters were found in the contralateral SI and bilaterally in the anterior cingulate, anterior insula, and medial prefrontal cortex. Negatively correlated clusters were found in the anteroventral cingulate bilaterally. In all of these areas, changes during anticipation were of the same sign as those observed during pain but less intense ( approximately 30-40% as large as peak changes during actual noxious stimulation). These results provide evidence for top-down mechanisms, triggered by anticipation, modulating cortical systems involved in sensory and affective components of pain even in the absence of actual noxious input and suggest that the activity of cortical nociceptive networks may be directly influenced by cognitive factors.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

Inhibition of motor system excitability at cortical and spinal level by tonic muscle pain

OBJECTIVE

To assess whether the motor system excitability can be modified by experimental tonic pain induced either in muscles or in subcutis.

METHODS

Transcranial magnetic stimulation of the left primary motor cortex was used to record motor evoked potentials (MEPs) from the right abductor digiti minimi (ADM) muscle. Recordings were made before, during and after experimental pain induced by (1) injection of hypertonic (5%) saline into the right ADM, the right first dorsal interosseum (FDI) and the left ADM muscles, and (2) injection of hypertonic saline in the subcutaneous region of the right ADM. Both MEPs and H-reflex were recorded also from the right flexor carpi radialis (FCR) before, during and after muscle pain.

RESULTS

MEPs recorded from the ADM muscle were significantly reduced in amplitude during pain induced in the right ADM and right FDI muscles, but not during pain in the left ADM muscle or during subcutaneous pain. This inhibitory effect was observed during the peak-pain and persisted also after the disappearance of the pain sensation. In the FCR muscle, the MEP inhibition was observed during the peak-pain, while a significant reduction of the H-reflex's amplitude was observed starting 1 min after the peak-pain.

CONCLUSIONS

Tonic muscle pain can inhibit the motor system. The motor cortex inhibition observed at an early phase is followed by a reduction of the excitability of both cortical and spinal motoneurones.

The peripheral apparatus of muscle pain: evidence from animal and human studies

The peripheral apparatus of muscle pain consists of nociceptors that can be excited by endogenous substances and mechanical stimuli. Histologically, the nociceptors are free nerve endings supplied by group III (thin myelinated) and group IV (nonmyelinated) afferents with conduction velocities less than 30 m/s. At the molecular level, nociceptors have receptors for algesic substances, such as bradykinin, serotonin, and prostagladin E2. The purinergic receptors and tetrodotoxin-resistant sodium channels might be new important targets for the treatment of muscle pain. Algesic substances (capsaicin, bradykinin, serotonin, potassium chloride, and hypertonic saline) and other stimuli (ischemia, strong mechanical stimuli, and electrical stimuli) have been shown to induce nociception from muscle in animals and muscle pain in humans. Muscle nociceptors can be sensitized to chemical and mechanical stimuli. Contrary to a former belief, the sensitization is not an unspecific process; rather, it is caused by endogenous algesic substances binding to highly specific receptor molecules in the membrane of the nociceptive ending. For example, animal studies showed that serotonin sensitizes muscle nociceptors to chemical and mechanical stimuli. Later, human studies showed that serotonin combined with bradykinin induces muscle hyperalgesia to pressure. The sensitization process by endogenous substances that are likely to be released during trauma or inflammatory injury is probably the best established peripheral mechanism for muscle tenderness and hyperalgesia.

Evidence for tonic activation of NK-1 receptors during the second phase of the formalin test in the Rat

Behavioral, electrophysiological, and autoradiographic experiments were done to study the second nociceptive phase in the formalin test. In initial experiments, this second phase was attenuated by 1-10 mg of the NK-1 receptor antagonist CP-99,994, given subcutaneously 10, 30, or 60 min before formalin (n = 8-10) and by 20 microgram given intrathecally 20 min after formalin (n = 13); the inactive isomer CP-100,263 was ineffective. In electrophysiological experiments on single dorsal horn neurons in vivo, the excitatory responses to subcutaneous formalin injection (50 microliter, 2.5%) were attenuated by subsequent intravenously administration of the NK-1 receptor antagonist CP-96,345 (0.5 mg/kg; n = 8), given 35-40 min after formalin, but not by the inactive enantiomer CP-96,344 (0.5 mg/kg; n = 9). Finally, autoradiographic binding of exogenous [(125)I]BH-substance P in the lumbar cord was reduced at 5 and 25 min after formalin (50 microliter, 1 or 5%), with an intermediate level of reduction at 12 min. These data are interpreted as evidence that the second phase of nociceptive scores in the formalin test is attributable at least partially to tonic activation of NK-1 receptors at the spinal level, whether because of a temporally limited release of substance P, for example only during the first phase, but a slow removal or breakdown of substance P, or, more likely, because of tonic release from primary afferents throughout the second phase. Irrespective of the mechanism, it can be concluded that at least some of the persistent nociceptive effects associated with peripheral inflammation, or at least those provoked by subcutaneous injection of formalin, are mediated via continuous activation of NK-1 receptors at the level of the spinal dorsal horn; this may relate directly to mechanisms underlying prolonged nociceptive pains in humans.

Presynaptic excitability changes of group Ia fibres to muscle nociceptive stimulation in humans

Tonic muscle nociceptive discharge evoked chemically from the foot extensor digitorum brevis muscle in man produces a depression of Ia excitation and Ib inhibition of the soleus (Sol) motoneurones (Mns). The possibility that both these changes partly result from presynaptic inhibition of Ia fibres projecting to Sol Mns and to interneurones mediating group I non-reciprocal inhibition is tested. Convergence of Ia fibres on these interneurones was deduced from evidence that reducing the excitatory effect of the extensor Ia fibres (by potentiation of their presynaptic inhibition) resulted in Ib disinhibition. Nociceptive-induced potentiation of Ia presynaptic inhibition was deduced from the following congruent results obtained by two independent methods: (1) depression of heteronymous Ia monosynaptic facilitation of the quadriceps muscle to Sol Mns; (2) potentiation of presynaptic inhibition of Sol Ia fibres evoked by mechanical activation of the tibialis anterior primary spindle afferents. It is concluded that nociceptive volleys arising from dorsal foot muscles facilitate the activity of interneurones intercalated in pathways responsible for presynaptic inhibition of Sol Ia fibres. It is also proposed that the same Ia presynaptic inhibition depresses the excitability of interneurones mediating group I non-reciprocal inhibition, thus resulting in Ib disinhibition of Sol Mns.

On the effect of chemically activated fine muscle afferents on interneurones mediating group I non-reciprocal inhibition of extensor ankle and knee muscles in humans

In a previous paper it was shown that muscle nociceptive discharge depressed the activity of interneurones mediating group I non-reciprocal inhibition (or Ib interneurones) in humans [A. Rossi, B. Decchi, Changes in Ib heteronymous inhibition to soleus motoneurons during cutaneous and muscle nociceptive stimulation in humans, Brain Res. 774 (1997) 55-61.]. However, since nociceptive discharge depressed the size of the soleus H-reflex (by which Ib inhibition was tested) the question arises as to whether modification of motoneurone membrane conductance per se could depress the size of Ib inhibitory post-synaptic potentials. The results of the present study suggest that the contribution of motoneurone hyperpolarization to Ib disinhibition is negligible and that muscle nociceptive discharge actually depresses the activity of these pathways.

Interactions between nociceptive and non-nociceptive afferent projections to cerebral cortex in humans

We investigated the effect of a tonic discharge of muscle nociceptive afferents on somatosensory evoked potentials (SEPs) in humans in response to stimulation of non-nociceptive afferents arising from the same muscle. Conditioning nociceptive muscle stimulation was achieved by local injection of 50 mg levo-ascorbic acid (in a volume of 0.3 ml) in the body of the extensor digitorum brevis muscle (EDB). The test stimulus for SEPs was an electrical pulse applied to the EDB nerve at an intensity below the motor threshold. The main finding was that tonic muscle nociceptive stimulation strongly depressed the middle-latency P60-N75 complex without modifying the size of the early P40-N50 complex of SEPs. Depression of the P60-N75 complex was correlated with the pain-induced loss of proprioception of the foot, making it plausible that this cortical complex reflects neuronal processes leading to perception.