Effect of Experimental Cutaneous Hand Pain on Corticospinal Excitability and Short Afferent Inhibition

The Effect of Acute and Sustained Pain on Corticomotor Excitability: A Systematic Review and Meta-Analysis of Group and Individual Level Data

Pain alters motor function. This is supported by studies showing reduced corticomotor excitability in response to experimental pain lasting <90 minutes. Whether similar reductions in corticomotor excitability are present with pain of longer durations or whether alterations in corticomotor excitability are associated with pain severity is unknown. Here we evaluated the evidence for altered corticomotor excitability in response to experimental pain of differing durations in healthy individuals. Databases were systematically searched for eligible studies. Measures of corticomotor excitability and pain were extracted. Meta-analyses were performed to examine: (1) group-level effect of pain on corticomotor excitability, and (2) individual-level associations between corticomotor excitability and pain severity. 49 studies were included. Corticomotor excitability was reduced when pain lasted milliseconds-seconds (hedges g's = -1.26 to -1.55) and minutes-hours (g's = -0.55 to -0.9). When pain lasted minutes-hours, a greater reduction in corticomotor excitability was associated with lower pain severity (g = -0.24). For pain lasting days-weeks, there were no group level effects (g = -0.18 to 0.27). However, a greater reduction in corticomotor excitability was associated with higher pain severity (g = 0.229). In otherwise healthy individuals, suppression of corticomotor excitability may be a beneficial short-term strategy with long-term consequences. PERSPECTIVE: This systematic review synthesised the evidence for altered corticomotor excitability in response to experimentally induced pain. Reduced corticomotor excitability was associated with lower acute pain severity but higher sustained pain severity, suggesting suppression of corticomotor excitability may be a beneficial short-term adaptation with long-term consequences.

Effects of different modalities of afferent stimuli of the lumbo-sacral area on control of lumbar paravertebral muscles

Somatosensory feedback to the central nervous system is essential to plan, perform and refine spine motor control. However, the influence of somatosensory afferent input from the trunk on the motor output to trunk muscles has received little attention. The objective was to compare the effects of distinct modalities of afferent stimulation on the net motoneuron and corticomotor excitability of paravertebral muscles. Fourteen individuals were recruited. Modulation of corticospinal excitability (motor-evoked potential [MEP]) of paravertebral muscles was measured when afferent stimuli (cutaneous noxious and non-noxious, muscle contraction) were delivered to the trunk at 10 intervals prior to transcranial magnetic stimulation. Each peripheral stimulation was applied alone, and subsequent EMG modulation was measured to control for net motoneuron excitability. MEP modulation and MEP/EMG ratio were used as measures of corticospinal excitability with and without control of net motoneuron excitability, respectively. MEP and EMG modulation were smaller after evoked muscle contraction than after cutaneous noxious and non-noxious stimuli. MEP/EMG ratio was not different between stimulation types. Both MEP and EMG amplitudes were reduced after evoked muscle contraction, but not when expressed as MEP/EMG ratio. Noxious and non-noxious stimulation had limited impact on all variables. Distinct modalities of peripheral afferent stimulation of the lumbo-sacral area differently modulated responses of paravertebral muscles, but without an influence on corticospinal excitability with control of net motoneuron excitability. Muscle stimulation reduced paravertebral activity and was best explained by spinal mechanisms. The impact of afferent stimulation on back muscles differs from the effects reported for limb muscles.

The impact of experimental pain on shoulder movement during an arm elevated reaching task in a virtual reality environment

BACKGROUND

People with chronic shoulder pain have been shown to present with motor adaptations during arm movements. These adaptations may create abnormal physical stress on shoulder tendons and muscles. However, how and why these adaptations develop from the acute stage of pain is still not well-understood.

OBJECTIVE

To investigate motor adaptations following acute experimental shoulder pain during upper limb reaching.

METHODS

Forty participants were assigned to the Control or Pain group. They completed a task consisting of reaching targets in a virtual reality environment at three time points: (1) baseline (both groups pain-free), (2) experimental phase (Pain group experiencing acute shoulder pain induced by injecting hypertonic saline into subacromial space), and (3) Post experimental phase (both groups pain-free). Electromyographic (EMG) activity, kinematics, and performance data were collected.

RESULTS

The Pain group showed altered movement planning and execution as shown by a significant increased delay to reach muscles EMG peak and a loss of accuracy, compared to controls that have decreased their mean delay to reach muscles peak and improved their movement speed through the phases. The Pain group also showed protective kinematic adaptations using less shoulder elevation and elbow flexion, which persisted when they no longer felt the experimental pain.

CONCLUSION

Acute experimental pain altered movement planning and execution, which affected task performance. Kinematic data also suggest that such adaptations may persist over time, which could explain those observed in chronic pain populations.

In anticipation of pain: expectancy modulates corticospinal excitability, autonomic response, and pain perception

ABSTRACT

Pain is a ubiquitous experience encompassing perceptual, autonomic, and motor responses. Expectancy is known to amplify the perceived and autonomic components of pain, but its effects on motor responses are poorly understood. Understanding expectancy modulation of corticospinal excitability has important implications regarding deployment of adaptive and maladaptive protective behaviours in anticipation of pain. We developed a protocol to compare corticospinal excitability to expected high pain, expected low pain, and critically low pain when high pain was expected. Expecting high pain suppressed corticospinal excitability and heightened perceptual and autonomic responses to the low-pain stimulus, as with increased noxious stimulation (ie, expected high pain). Multilevel modelling revealed that perceived pain mediated the effect of both noxious stimulation and this expectancy-modulated pain on autonomic responses, but corticospinal excitability did not. These results demonstrate that merely expecting pain influenced all pain components. Findings shed new light on the aetiology of expectancy-modulated pain, whereby expecting pain mobilises the motor system to protect the body from harm by a protective withdrawal reflex, associated with reduced corticospinal excitability, and activates similar processes as increased nociceptive stimulation. This has significant practical implications for the treatment of pain, particularly in scenarios where avoidance of pain-related movement contributes to its maintenance.

Association of short- and long-latency afferent inhibition with human behavior

Transcranial magnetic stimulation (TMS) paired with nerve stimulation evokes short-latency afferent inhibition (SAI) and long-latency afferent inhibition (LAI), which are non-invasive assessments of the excitability of the sensorimotor system. SAI and LAI are abnormally reduced in various special populations in comparison to healthy controls. However, the relationship between afferent inhibition and human behavior remains unclear. The purpose of this review is to survey the current literature and synthesize observations and patterns that affect the interpretation of SAI and LAI in the context of human behavior. We discuss human behaviour across the motor and cognitive domains, and in special and control populations. Further, we discuss future considerations for research in this field and the potential for clinical applications. By understanding how human behavior is mediated by changes in SAI and LAI, this can allow us to better understand the neurophysiological underpinnings of human motor control.

Impact of Experimental Tonic Pain on Corrective Motor Responses to Mechanical Perturbations

Movement is altered by pain, but the underlying mechanisms remain unclear. Assessing corrective muscle responses following mechanical perturbations can help clarify these underlying mechanisms, as these responses involve spinal (short-latency response, 20-50 ms), transcortical (long-latency response, 50-100 ms), and cortical (early voluntary response, 100-150 ms) mechanisms. Pairing mechanical (proprioceptive) perturbations with different conditions of visual feedback can also offer insight into how pain impacts on sensorimotor integration. The general aim of this study was to examine the impact of experimental tonic pain on corrective muscle responses evoked by mechanical and/or visual perturbations in healthy adults. Two sessions (Pain (induced with capsaicin) and No pain) were performed using a robotic exoskeleton combined with a 2D virtual environment. Participants were instructed to maintain their index in a target despite the application of perturbations under four conditions of sensory feedback: (1) proprioceptive only, (2) visuoproprioceptive congruent, (3) visuoproprioceptive incongruent, and (4) visual only. Perturbations were induced in either flexion or extension, with an amplitude of 2 or 3 Nm. Surface electromyography was recorded from Biceps and Triceps muscles. Results demonstrated no significant effect of the type of sensory feedback on corrective muscle responses, no matter whether pain was present or not. When looking at the effect of pain on corrective responses across muscles, a significant interaction was found, but for the early voluntary responses only. These results suggest that the effect of cutaneous tonic pain on motor control arises mainly at the cortical (rather than spinal) level and that proprioception dominates vision for responses to perturbations, even in the presence of pain. The observation of a muscle-specific modulation using a cutaneous pain model highlights the fact that the impacts of pain on the motor system are not only driven by the need to unload structures from which the nociceptive signal is arising.

Intracortical motor networks are affected in both the contralateral and ipsilateral hemisphere during single limb cold water immersion

NEW FINDINGS

What is the central question of this study? How does single limb cold water immersion affect corticomotor function and intracortical circuitry in the motor cortex of each cerebral hemisphere? What is the main finding and its importance? Immersion of a single limb in very cold water caused an increase in corticomotor excitability and intracortical facilitation, and a decrease in intracortical inhibition, in the motor cortex of both hemispheres. These findings provide evidence that intense sensory stimuli induce widespread changes in motor circuitry in the contralateral, as well as the ipsilateral, hemisphere.

ABSTRACT

Although responses to noxious stimuli have been extensively studied for the contralateral hemisphere, little is known about how the ipsilateral hemisphere may be affected. Therefore, this study examined how exposing a single limb to noxious cold stimuli affects motor output arising from both the contralateral and ipsilateral hemisphere. A total of 17 healthy adults participated in three experiments. Single- and paired-pulse TMS protocols were used to identify how immersing a single upper limb in cold water (4.0 ± 0.5 °C) affects inhibitory and facilitatory circuits in the primary motor cortex (M1) of the contralateral (experiment 1) and ipsilateral (experiment 2) hemisphere. The third experiment used a reaction time task to assess the functional consequences of acute adaptations in the ipsilateral M1. The target muscle in all experiments was the extensor carpi radialis brevis (ECRB). Immersion of a single limb in cold water increased self-perception of pain and temperature, and increased EMG amplitude of the immersed limb. During immersion, motor evoked potentials and intracortical facilitation increased, whereas short interval intracortical inhibition decreased, for both the ipsilateral M1 and contralateral M1. Activity in the ipsilateral hemisphere to the limb immersed in cold water also slowed reaction time for the non-immersed limb. Our findings suggest that altered motor responses from single limb cold water immersion are not restricted to a single hemisphere. Instead, widespread activation of somatosensory systems influences inhibitory and facilitatory circuits in the primary motor cortex of each hemisphere.

Motor corticospinal excitability: a novel facet of pain modulation?

Introduction

Increase in excitability of the primary motor cortex (M1) is associated with pain inhibition by analgesics, which is, in turn, associated with the psychophysical antinociceptive pain modulation profile. However, the relationship between neurophysiological M1 excitability and psychophysical pain modulation has not yet been explored.

Objectives

We aim to study these relationships in healthy subjects.

Methods

Forty-one young healthy subjects (22 women) underwent a wide battery of psychophysical testing that included conditioned pain modulation (CPM) and pain temporal summation, and a transcranial magnetic stimulation neurophysiological assessment of the motor corticospinal excitability, including resting motor threshold, motor-evoked potentials (MEPs), and cortical silent period.

Results

Increased motor corticospinal excitability in 2 parameters was associated with more efficient CPM: (1) higher MEP amplitude (r = -0.574; P _{_Bonferroni} = 0.02) and (2) longer MEP duration (r = -0.543; P _{_Bonferroni} = 0.02). The latter also correlated with the lower temporal summation magnitude (r = -0.421; P = 0.007); however, on multiplicity adjustment, significance was lost.

Conclusions

Increased corticospinal excitability of the primary motor cortex is associated with more efficient inhibitory pain modulation as assessed by CPM, in healthy subjects. Motor-evoked potential amplitude and duration may be considered as an additional, objective and easy to measure parameter to allow for better individual assessment of pain modulation profile.

Effect of movement-related pain on behaviour and corticospinal excitability changes associated with arm movement preparation

KEY POINTS

Experimental pain or its anticipation influence motor preparation processes as well as upcoming movement execution, but the underlying physiological mechanisms remain unknown. Our results showed that movement-related pain modulates corticospinal excitability during motor preparation. In accordance with the pain adaptation theory, corticospinal excitability was higher when the muscle has an antagonist (vs. an agonist) role for the upcoming movement associated with pain. Anticipation of movement-related pain also affects motor initiation and execution, with slower movement initiation (longer reaction times) and faster movement execution compared to movements that do not evoke pain. These results confirm the implementation of protective strategies during motor preparation known to be relevant for acute pain, but which may potentially have detrimental long-term consequences and lead to the development of chronic pain.

ABSTRACT

When a movement repeatedly generates pain, we anticipate movement-related pain and establish self-protective strategies during motor preparation, but the underlying mechanisms remains poorly understood. The current study investigated the effect of movement-related pain anticipation on the modulation of behaviour and corticospinal excitability during the preparation of arm movements. Participants completed an instructed-delay reaction-time (RT) task consisting of elbow flexions and extensions instructed by visual cues. Nociceptive laser stimulations (unconditioned stimuli) were applied to the lateral epicondyle during movement execution in a specific direction (CS+) but not in the other (CS-), depending on experimental group. During motor preparation, transcranial magnetic stimulation was used to measure corticospinal excitability in the biceps brachii (BB). RT and peak end-point velocity were also measured. Neurophysiological results revealed an opposite modulation of corticospinal excitability in BB depending on whether it plays an agonist (i.e. flexion) or antagonist (i.e. extension) role for the CS+ movements (P < 0.001). Moreover, behavioural results showed that for the CS+ movements RT did not change relative to baseline, whereas the CS- movements were initiated more quickly (P = 0.023) and the CS+ flexion movements were faster relative to the CS- flexion movements (P < 0.001). This is consistent with the pain adaptation theory which proposes that in order to protect the body from further pain, agonist muscle activity is reduced and antagonist muscle activity is increased. If these strategies are initially relevant and lead to short-term pain alleviation, they may potentially have detrimental long-term consequences and lead to the development of chronic pain.

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.

Effect of Cutaneous Heat Pain on Corticospinal Excitability of the Tibialis Anterior at Rest and during Submaximal Contraction

Previous studies have shown that pain can interfere with motor control. The neural mechanisms underlying these effects remain largely unknown. At the upper limb, mounting evidence suggests that pain-induced reduction in corticospinal excitability is involved. No equivalent data is currently available at the lower limb. The present study therefore examined the effect of thermal pain on the corticospinal drive to tibialis anterior (TA) at rest and during an isometric submaximal dorsiflexion. Transcranial magnetic stimulation was used to induce motor-evoked potentials (MEPs) in the TA at rest and during contraction in the presence or absence of cutaneous heat pain induced by a thermode positioned above the TA (51°C during 1 s). With similar pain ratings between conditions (3.9/10 at rest and 3.6/10 during contraction), results indicate significant decreases in MEP amplitude during both rest (−9%) and active conditions (−13%) (main effect of pain, p = 0.02). These results therefore suggest that cutaneous heat pain can reduce corticospinal excitability in the TA muscle and that such reduction in corticospinal excitability could contribute to the interference of pain on motor control/motor learning.

Sensory Disturbances, but Not Motor Disturbances, Induced by Sensorimotor Conflicts Are Increased in the Presence of Acute Pain

Incongruence between our motor intention and the sensory feedback of the action (sensorimotor conflict) induces abnormalities in sensory perception in various chronic pain populations, and to a lesser extent in pain-free individuals. The aim of this study was to simultaneously investigate sensory and motor disturbances evoked by sensorimotor conflicts, as well as to assess how they are influenced by the presence of acute pain. It was hypothesized that both sensory and motor disturbances would be increased in presence of pain, which would suggest that pain makes body representations less robust. Thirty healthy participants realized cyclic asymmetric movements of flexion-extension with both upper limbs in a robotized system combined to a 2D virtual environment. The virtual environment provided a visual feedback (VF) about movements that was either congruent or incongruent, while the robotized system precisely measured motor performance (characterized by bilateral amplitude asymmetry and medio-lateral drift). Changes in sensory perception were assessed with a questionnaire after each trial. The effect of pain (induced with capsaicin) was compared to three control conditions (no somatosensory stimulation, tactile distraction and proprioceptive masking). Results showed that while both sensory and motor disturbances were induced by sensorimotor conflicts, only sensory disturbances were enhanced during pain condition comparatively to the three control conditions. This increase did not statistically differ across VF conditions (congruent or incongruent). Interestingly however, the types of sensations evoked by the conflict in the presence of pain (changes in intensity of pain or discomfort, changes in temperature or impression of a missing limb) were different than those evoked by the conflict alone (loss of control, peculiarity and the perception of having an extra limb). Finally, results showed no relationship between the amount of motor and sensory disturbances evoked in a given individual. Contrary to what was hypothesized, acute pain does not appear to make people more sensitive to the conflict itself, but rather impacts on the type and amount of sensory disturbances that they experienced in response to that conflict. Moreover, the results suggest that some sensorimotor integration processes remain intact in presence of acute pain, allowing us to maintain adaptive motor behavior.